JP2021050402A - Steel for high-strength spring and method for producing the same - Google Patents

Abstract

To provide a steel for a high-strength spring that has both of quenching properties for increased strength and workability, and a method for producing the same.SOLUTION: The present disclosure provides a method for producing a steel for a high-strength spring that is subjected to a hot rolling process to be fed as a wire. The steel has a chemical composition comprising, in mass%, C: 0.40-0.65%, Si: 1.50-3.00%, Mn: 0.20-1.00%, P: 0.020% or less, S: 0.020% or less, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Nb: 0.010-0.100%, Al: 0.10% or less, N: 0.010% or less, O: 0.0015% or less, with the balance being Fe and inevitable impurities, where, formula 1: [Ti]+[Nb]+[Al]/2≥0.07 holds when the content of element M is [M] mass%. The steel is mainly constituted by two phases of ferrite and perlite.SELECTED DRAWING: Figure 1

Description

The present invention relates to a high-strength spring steel used as a hot-rolled wire rod and a method for manufacturing the same, and in particular, a high-strength spring steel having excellent hardenability and workability for high strength and its manufacturing method. Regarding the manufacturing method.

High-strength spring steels are often prepared so that the content of Si in the component composition can be increased to increase the strength of the finally obtained spring. However, it is known that when the Si content is high, a ferrite decarburized layer is likely to be formed on the surface layer of the wire rod after hot rolling, and quenching is insufficient at such a portion to reduce the surface layer strength. ing. Therefore, various methods for preventing the formation of a ferrite decarburized layer are being studied.

For example, in Patent Document 1, high-Si spring steels (Si-Mn steel, Si-Cr steel, Si-Cr-V steel) in which C is 0.75 to 0.85 and Si is contained in an amount of 1 to 3%. , Si-Cr-Nb steel), conventionally, preheating prior to hot rolling for wire rod processing was generally performed at a low temperature for a short time to suppress decarburization. Disclosed is a method for producing a high-strength spring steel which is preheated to a high temperature of at least 1050 ° C. and preheated at a high temperature so as to supplement carbon decarburized from the surface of the steel material from the central portion. Since the amount of carbon reduction in steel is determined by the relative velocity between carbon diffusion and the decarburization reaction, decarburization from the surface increases due to high-temperature heating, but at the same time, carbon diffusion and supply from the inside becomes easy. Therefore, it is said that the formation of a ferrite decarburized layer can be prevented as a result of heating above a temperature at which austenite grains can stably exist.

By the way, it has been pointed out that while preventing ferrite decarburization, bainite is likely to be generated and the workability in spring processing is lowered.

For example, in Patent Document 2, in a high-strength spring steel containing 0.4 to 0.65% by mass of C and 1.2 to 2.8% by mass of Si, while preventing the occurrence of ferrite decarburization. The alloy component composition capable of suppressing the formation of bainite is disclosed. After obtaining conditional expressions using parameters that quantify the contribution of each element to the depth of the ferrite decarburized layer, the presence or absence of bainite formation, and the hardness, the alloy composition is adjusted so as to satisfy these conditional expressions. It is supposed to be decided. Here, Si is an element that promotes the occurrence of ferrite decarburization, while Mn, Cu, Ni, and Cr are elements that suppress the occurrence of ferrite decarburization.

Japanese Unexamined Patent Publication No. 58-073718 Japanese Unexamined Patent Publication No. 2016-074949

For high-strength spring steel, it is important to improve the hardenability for high strength. On the other hand, as described above, if bainite is generated in the metal structure in exchange for the improvement in hardenability, the workability is lowered and the degree of freedom in machining in the subsequent spring machining process is lowered.

The present invention has been made in view of the above circumstances, and an object of the present invention is to manufacture a high-strength spring steel having both hardenability and workability for high strength. To provide a method.

The high-strength spring steel according to the present invention is a high-strength spring steel used as a hot-rolled wire rod, and has a mass% of C: 0.40 to 0.65% and Si: 1.50. ~ 3.00%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cu: 1.0% or less, Ni: 2.0% or less, Cr : 2.0% or less, Nb: 0.010 to 0.100%, Al: 0.10% or less, N: 0.010% or less, O: 0.0015% or less, balance Fe and unavoidable impurities In addition, it has a component composition of the formula 1: [Ti] + [Nb] + [Al] / 2 ≧ 0.07, where the content of element M is [M] mass%, and is mainly a two-phase of ferrite and pearlite. It is characterized by being composed of an organization.

According to such an invention, it is possible to have both hardenability and workability for increasing the strength of the obtained spring.

In the above-described invention, the two-phase structure may contain 1.0 × 10 ⁴ particles / mm ² or more of carbonitride particles having a particle size of 1 nm or more. According to such an invention, it is easy to secure workability.

In the above-described invention, the component composition may further contain Ti: 0.030 to 0.140% in mass%. Further, the component composition may be characterized in that, in mass%, Mo: 1.0% or less, V: 0.50% or less, and B: 0.0050% or less can be contained. According to such an invention, it is possible to further facilitate the securing of workability.

The method for producing high-strength spring steel according to the present invention is a method for producing high-strength spring steel that is hot-rolled and used as a wire rod, in terms of mass%, C: 0.40 to 0. 65%, Si: 1.50 to 3.00%, Mn: 0.25 to 1.00%, P: 0.020% or less, S: 0.020% or less, Cu: 1.0% or less, Ni : 2.0% or less, Cr: 2.0% or less, Nb: 0.010 to 0.100%, Al: 0.10% or less, N: 0.010% or less, O: 0.0015% or less, It is composed of the balance Fe and unavoidable impurities, and has a component composition of the formula 1: [Ti] + [Nb] + [Al] / 2 ≧ 0.07, where the content of the element M is [M] mass%. It is characterized in that steel in which carbon nitride particles are dispersed is hot-rolled into the wire rod from a heating temperature of 1000 ° C. or higher, and cooled so as to mainly have a two-phase structure of ferrite and pearlite.

According to such an invention, it is possible to have both hardenability and workability for increasing the strength of the obtained spring.

In the above-described invention, the component composition may further contain Ti: 0.030 to 0.140% in mass%. Further, the component composition may be characterized in that, in mass%, Mo: 1.0% or less, V: 0.50% or less, and B: 0.0050% or less can be contained. According to such an invention, it is easy to secure workability.

It is a table of the composition of steel used in an Example and a comparative example. It is a table which shows the number of carbonitride particles of an Example and a comparative example, the occurrence of ferrite decarburization, and the formation state of bainite.

A spring steel as an example of the high-strength spring steel according to the present invention will be described. Here, the steel for spring is a wire rod obtained through hot rolling, and is a material before spring processing.

The spring steel in this embodiment has C: 0.40 to 0.65%, Si: 1.50 to 3.00%, Mn: 0.25 to 1.00%, P: 0. 020% or less, S: 0.020% or less, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 2.0% or less, Nb: 0.010 to 0.100%, Al: 0 It has a component composition containing 10% or less, N: 0.010% or less, and O: 0.0015% or less, and is mainly composed of a two-phase structure of ferrite and pearlite. Further, this component composition further satisfies the following formula 1 when the content of the element M is [M] mass%.
Equation 1: [Ti] + [Nb] + [Al] / 2 ≧ 0.07
Equation 1 is defined as a condition for the presence of a large number of carbonitride particles in the wire rod.

With such a component composition, hardenability for obtaining a predetermined strength as a high-strength spring obtained through spring processing or the like can be ensured. For example, ensuring hardenability can be achieved by relatively increasing the content of alloying elements such as Cr and Ni that suppress the occurrence of ferrite decarburization.

In particular, with the above-mentioned composition, it is possible ^{to allow 1.0 × 10 4} pieces / mm ² or more of carbonitride particles having a particle size of 1 nm or more to be present in the cross section of the wire rod. That is, a large number of carbonitride particles can be dispersed. This large number of carbonitrides are already present at the time of heating in the hot rolling process, whereby the pinning effect of suppressing the coarsening of austenite grains can be obtained and the crystal grain size can be kept small. Furthermore, carbonitride can promote the formation of ferrite + pearlite structure. It is considered that these can prevent the formation of bainite, and as a result, the workability required for spring machining can be ensured. The carbonitride particles preferably mainly contain Nb and / or Al. Further, since extremely coarse carbonitride particles reduce the strength of the obtained spring, it is preferable that the particle size is 50 μm or less.

Further, in order to prevent the occurrence of ferrite decarburization, it is effective to raise the heating temperature during rolling, but by setting the heating temperature to a temperature of 1000 ° C. or higher, ferrite decarburization can be prevented more reliably. On the other hand, even if the heating temperature is set to a temperature of 1000 ° C. or higher, for example, about 1150 ° C., the austenite particle size can be kept small by the large number of carbonitride particles as described above, and the formation of bainite can be prevented.

The above-mentioned component composition may further contain Ti: 0.030 to 0.140% in mass%. Further, in mass%, Mo: 1.0% or less, V: 0.50% or less, B: 0.0050% or less can be contained. Ti, V and B contribute to forming carbonitride particles and keeping the austenite grain size small while enhancing the hardenability of steel. That is, it also contributes to preventing the formation of bainite and ensuring workability. Mo also contributes to the improvement of hardenability.

[Manufacturing test]
The results of a manufacturing test in which a plurality of test materials imitating a wire rod as a high-strength spring steel are manufactured, carbonitride particles are counted, and the presence or absence of a ferrite decarburized layer and bainite are confirmed will be described.

As shown in FIG. 1, steel having a total of 17 kinds of component compositions of Examples 1 to 15 and Comparative Examples 1 and 2 was used as the test material. Each steel was melted and molded to produce a cylindrical test material having a diameter of 13.5 mm and a length of 100 mm.

As shown in FIG. 2, each test material was heat-treated to imitate hot rolling for producing a wire rod as a spring steel. Specifically, it was kept at the heating temperature shown in the figure for 30 minutes in an atmospheric furnace, and then oil-cooled or air-cooled. The oil-cooled test material was used for counting the carbonitride particles because the number of carbonitride particles can be maintained in the state at the start of cooling by quenching. On the other hand, the air-cooled test material was used for observing the presence or absence of the ferrite decarburized layer and the presence or absence of bainite. Further, the values on the left side of Equation 1 ([Ti] + [Nb] + [Al] / 2) were calculated and recorded.

Regarding the number of carbonitoxide particles, the cross section of the test material was observed at a magnification of 5000 to 100,000 times using SEM and TEM, and the number of carbonitoxide particles having a particle size of 1 nm or more was counted and per ^{1 mm 2.} Converted to the number of. Ten or more visual fields were randomly selected for one test material, and these were averaged and recorded.

In the cross section of the central part in the longitudinal direction of the test material, observe the vicinity of the outer periphery of the cross section (surface layer part of the test material) for the ferrite decarburized layer and record the presence or absence, and for the bainite structure, from the outer circumference to the center of the cross section. The position of the distance of 1/4 of the diameter was observed and the presence or absence was recorded. The presence or absence of the ferrite decarburized layer was determined in accordance with JIS G 0558.

As shown in FIG. 2, in Examples 1 to 15, the number of carbonitride particles satisfying Equation 1 (the value on the left side of Equation 1 is 0.07 or more) and having a particle size of 1 nm or more is 1.0 ×. is 10 ⁴ / mm ² or more, ferrite decarburized layer is not observed, the generation of bainite was observed.

On the other hand, in Comparative Examples 1 and 2, the ferrite decarburized layer was not observed, but both produced bainite. Further, neither of them satisfied the formula 1, and the number of carbonitride particles having a particle size of 1 nm or ^{more was less than 1.0 × 10 2} / mm ² , which was an extremely small amount. It is probable that bainite was generated because the number of carbonitride particles was small and the coarsening of austenite particles could not be suppressed. It is probable that in Comparative Examples 1 and 2, neither Nb nor Al was contained, and therefore the carbonitride particles could not be sufficiently dispersed.

As described above, in Comparative Examples 1 and 2, the formation of bainite could not be prevented, whereas in Examples 1 to 15, the formation of bainite could be prevented while preventing the formation of the ferrite decarburized layer. That is, workability could be ensured while ensuring hardenability.

By the way, the composition range of the steel that can give mechanical properties substantially equivalent to those of the spring steel having both hardenability and workability including the above-mentioned examples is defined as follows.

C is an element necessary for ensuring the strength of the obtained spring. On the other hand, excessive content causes a decrease in toughness and fatigue strength. In consideration of these, C is in the range of 0.40 to 0.65%, preferably in the range of 0.45 to 0.60% in terms of mass%.

Si is an element effective for increasing the settling resistance of the obtained spring. On the other hand, if it is contained in an excessive amount, not only the toughness is lowered, but also ferrite decarburization is likely to occur. In consideration of these, Si is in the range of 1.50 to 3.00%, preferably in the range of 1.80 to 2.50% in terms of mass%.

Mn fixes S, which is an element that lowers toughness, as MnS, and improves hardenability. On the other hand, excessive content causes a decrease in toughness. In consideration of these, Mn is in the range of 0.25 to 1.00%, preferably in the range of 0.60 to 1.00% in mass%.

Since P embrittles the grain boundaries, it is desired to minimize its content, but excessive refining causes an increase in manufacturing cost. In consideration of these, P is in the range of 0.020% or less in mass%.

As described above, S is preferably combined with Mn to form non-metallic inclusions such as MnS, which serves as a starting point for stress concentration and lowers fatigue strength, so that the content is preferably lowered, but inevitably in steel. It is an element that exists in. In consideration of these, S is in the range of 0.020% or less in mass%.

Cu is an element that is effective not only for improving corrosion resistance but also for preventing ferrite decarburization. On the other hand, if it is contained in an excessive amount, the manufacturing cost will increase. In consideration of these, Cu is in the range of 1.0% or less, preferably in the range of 0.50% or less in terms of mass%.

Ni is an element effective in improving corrosion resistance and preventing ferrite decarburization. On the other hand, if it is contained in excess, the manufacturing cost will increase. In consideration of these, Ni is in the range of 2.0% or less, preferably in the range of 0.8 to 1.0% in terms of mass%.

Cr is an element effective for improving corrosion resistance and hardenability. On the other hand, if it is contained in an excessive amount, the corrosion pits formed are sharpened and the fatigue strength is lowered. In consideration of these, Cr is in the range of 2.0% or less, preferably in the range of 0.7 to 1.50% in mass%.

Nb is an element necessary for preventing the formation of bainite by forming a carbonitride and suppressing the coarsening of austenite grains. On the other hand, if it is contained in an excessive amount, the manufacturing cost is increased and the processability is lowered. In consideration of these, Nb is in the range of 0.010 to 0.100% in mass%.

Al is an element necessary for preventing the formation of bainite by acting as a deoxidizer during melting and suppressing the coarsening of austenite grains by forming a carbonitride. On the other hand, if it is contained in an excessive amount, non-metal inclusions are increased and the fatigue strength is lowered. In consideration of these, Al is in the range of 0.10% or less, preferably in the range of 0.050% or less in mass%.

N is an element necessary for forming carbonitride particles by binding with Nb or the like. In consideration of this point, N is in the range of 0.010% or less in mass%.

O produces oxide-based non-metallic inclusions. Therefore, O is in the range of 0.0015% or less in mass%.

Ti, V, and B contribute to forming carbonitride particles to keep the austenite particle size small while improving the hardenability of steel, and Mo contributes to improving the hardenability of steel. It may be selectively added as long as the characteristics of the high-strength spring steel are not impaired. In consideration of these, in mass%, V is in the range of 0.50% or less, preferably 0.30% or less, and Ti is in the range of 0.030 to 0.140%, preferably. It is in the range of 0.080 to 0.140%, B is in the range of 0.0050% or less, Mo is in the range of 1.0% or less, preferably 0.5% or less. ..

In Formula 1, the relationship between the contents of Ti, Nb, and Al was determined in order to prevent coarsening of austenite particles by allowing a large number of carbonitride particles to be present in the wire rod. Formula 1 is an empirical formula obtained based on the results of Examples 1 to 15, Comparative Examples 1 and 2, and some other production tests described above, and the content of the element M is defined as [M] mass%. [Ti] + [Nb] + [Al] / 2 ≧ 0.07.

Although typical examples of the present invention have been described above, the present invention is not necessarily limited to these, and those skilled in the art will not deviate from the gist of the present invention or the appended claims. , Various alternative and modified examples will be found.

Claims (8)

A high-strength spring steel used as a hot-rolled wire rod.
By mass%
C: 0.40 to 0.65%,
Si: 1.50 to 3.00%,
Mn: 0.25 to 1.00%,
P: 0.020% or less,
S: 0.020% or less,
Cu: 1.0% or less,
Ni: 2.0% or less,
Cr: 2.0% or less,
Nb: 0.010 to 0.100%,
Al: 0.10% or less,
N: 0.010% or less,
O: 0.0015% or less,
It consists of the balance Fe and unavoidable impurities, and
Let the content of element M be [M] mass%
Equation 1: [Ti] + [Nb] + [Al] / 2 ≧ 0.07
A steel for high-strength springs, which has a composition of two components and is mainly composed of a two-phase structure of ferrite and pearlite. The high-strength spring steel according to claim 1 ^{, wherein the two-phase structure contains 1.0 × 10 4} particles / mm ² or more of carbonitride particles having a particle size of 1 nm or more. The composition of the components is mass%.
Ti: 0.030-0.140%,
The high-strength spring steel according to claim 1 or 2, further comprising. The composition of the components is mass%.
Mo: 1.0% or less,
V: 0.50% or less,
B: 0.0050% or less,
The high-strength spring steel according to any one of claims 1 to 3, wherein the steel can be included in the above. A method for manufacturing high-strength spring steel that is hot-rolled and used as a wire rod.
By mass%
C: 0.40 to 0.65%,
Si: 1.50 to 3.00%,
Mn: 0.25 to 1.00%,
P: 0.020% or less,
S: 0.020% or less,
Cu: 1.0% or less,
Ni: 2.0% or less,
Cr: 2.0% or less,
Nb: 0.010 to 0.100%,
Al: 0.10% or less,
N: 0.010% or less,
O: 0.0015% or less,
It consists of the balance Fe and unavoidable impurities, and
Let the content of element M be [M] mass%
Equation 1: [Ti] + [Nb] + [Al] / 2 ≧ 0.07
It is characterized in that a steel having a composition of components and dispersed with carbonitride particles is hot-rolled into the wire rod from a heating temperature of 1000 ° C. or higher, and cooled so as to have a two-phase structure mainly of ferrite and pearlite. A method for manufacturing steel for high-strength springs. The method for producing a high-strength spring steel according to claim 5 ^{, wherein the two-phase structure contains 1.0 × 10 4} particles / mm ² or more of carbonitride particles having a particle size of 1 nm or more. The composition of the components is mass%.
Ti: 0.030-0.140%,
The method for producing a steel for a high-strength spring according to claim 5 or 6, further comprising. The composition of the components is mass%.
Mo: 1.0% or less,
V: 0.50% or less,
B: 0.0050% or less,
The method for producing a steel for a high-strength spring according to any one of claims 5 to 7, wherein the method can be included in the above.

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