WO2015129403A1 - Rolled material for high strength spring, and wire for high strength spring - Google Patents

Abstract

The purpose of the present invention is to provide: a rolled material, which is a raw material for a high strength spring, capable of exhibiting excellent corrosion fatigue characteristics following hardening and tempering even if the added quantity of alloying elements is suppressed; and a wire for a high-strength spring, which is obtained from such rolled material. This rolled material for a high strength spring contains 0.39-0.65% of C, 1.5-2.5% of Si, 0.15-1.2% of Mn, greater than 0% but not greater than 0.015% of P, greater than 0% but not greater than 0.015% of S, 0.001-0.1% of Al, 0.10-0.80% of Cu, 0.10-0.80% of Ni and greater than 0% but not greater than 0.0010% of O, with the remainder consisting of iron and unavoidable impurities, and is such that the number of oxide-based inclusions having an average diameter of 25 μm or more is not more than 30 per 100 g of steel material, and the quantity of non-diffusible hydrogen is not more than 0.40 ppm by mass.

Description

Rolled material for high strength spring and wire for high strength spring

The present invention relates to a high-strength spring rolled material and a high-strength spring wire using the same. Specifically, it is a rolled material and a high-strength spring wire useful as a raw material for a high-strength spring used in a tempered state, that is, quenched and tempered. The present invention relates to a high-strength spring wire having excellent corrosion fatigue characteristics even when the tensile strength after processing is as high as 1900 MPa or higher.

Coil springs used in automobiles, for example, valve springs and suspension springs used in engines and suspensions, are required to be light in weight to reduce exhaust gas and improve fuel efficiency, and require high strength. . Higher strength springs have poor toughness, tend to cause hydrogen embrittlement, and deteriorate corrosion fatigue characteristics. Therefore, a steel wire for high-strength springs used for manufacturing a spring (hereinafter, the steel wire may be referred to as a wire) is required to have excellent corrosion fatigue characteristics. Corrosion fatigue failure occurs when hydrogen generated by corrosion penetrates into the steel and causes embrittlement of the steel due to the hydrogen. To improve the corrosion fatigue properties, improve the corrosion resistance and hydrogen embrittlement resistance of the steel. It is necessary.

Controlling the chemical composition is known as a method for enhancing the corrosion fatigue characteristics of high-strength spring wires. However, these methods use a large amount of alloy elements, which is not always desirable from the viewpoint of increasing manufacturing costs and saving resources.

By the way, as a manufacturing method of the spring, the steel wire is heated to a quenching temperature and hot-formed into a spring shape, and then oil-cooled and tempered, and the steel wire is quenched and tempered and then cold-formed into a spring shape. The method is known. In the latter cold forming method, it is also known that quenching and tempering before forming is performed by high-frequency heating. For example, Patent Document 1 discloses a structure in which a wire is cold drawn and then quenched and tempered by high-frequency induction heating. A technique for adjusting the above is disclosed. In this technique, the structure fraction of pearlite is 30% or less, the structure fraction composed of martensite and bainite is 70% or more, and then cold drawing is performed at a predetermined area reduction rate, followed by quenching and tempering. Undissolved carbides are reduced and delayed fracture characteristics are improved.

In Patent Document 2, a rolled wire is drawn in an example and subjected to quenching and tempering treatment by induction heating. This technology focuses on achieving both high strength and moldability such as coiling properties, and does not take into account any corrosion fatigue properties.

Patent Document 3 focuses on the amount of hydrogen in steel evaluated by the total amount of hydrogen released when the temperature is raised from room temperature to 350 ° C., and is hot rolled with excellent wire drawing workability under strong wire drawing conditions. Proposes wire rods. However, Patent Document 3 pays attention only to the wire drawing property in a special process called strong wire drawing, and does not consider any corrosion fatigue characteristics after quenching and tempering which are most important in a suspension spring or the like.

JP 2004-143482 A JP 2006-183137 A JP 2007-231347 A

The present invention has been made in view of the above circumstances, and its purpose is a material for hot-winding and cold-winding high-strength springs, which is quenched even if the amount of alloying elements is suppressed. An object of the present invention is to provide a rolled material that can exhibit excellent corrosion fatigue characteristics after tempering, and a high-strength spring wire obtained from such a rolled material.

The rolled material for high-strength springs of the present invention that has solved the above problems is
% By mass
C: 0.39 to 0.65%,
Si: 1.5 to 2.5%,
Mn: 0.15 to 1.2%,
P: more than 0%, 0.015% or less,
S: more than 0%, 0.015% or less,
Al: 0.001 to 0.1%,
Cu: 0.10 to 0.80%,
Ni: 0.10-0.80% and O: more than 0%, 0.0010% or less, respectively, the balance being iron and inevitable impurities,
The number of oxide inclusions having an average diameter of 25 μm or more is 30 or less per 100 g of steel material, and the amount of non-diffusible hydrogen is 0.40 mass ppm or less.

In addition, when calculating | requiring the average diameter of an oxide inclusion, it observes by EPMA (Electron Probe Micro Analyzer: Electron probe microanalyzer), respectively, measures the major axis and the minor axis of an oxide inclusion, and oxide An average value of the major axis and minor axis of the system inclusion, that is, a value obtained by dividing the sum of the major axis and the minor axis by 2 is defined as an average diameter. Inclusions having an average value of 25 μm or more are subject to the number measurement in the present invention.

The rolled material for high-strength springs of the present invention preferably further contains one or more kinds belonging to any of the following (a) to (d) in mass%.
(A) Cr: Over 0%, 1.2% or less (b) Ti: Over 0%, 0.13% or less (c) B: Over 0%, 0.01% or less (d) Nb: Over 0% , 0.1% or less and Mo: more than 0%, at least one of 0.5% or less

The present invention also includes a high-strength spring wire made of any of the chemical components of steel described above, having an area ratio of tempered martensite of 80% or more and a tensile strength of 1900 MPa or more.

According to the present invention, even if a large amount of alloy elements is not added, the oxide inclusions in the rolled material are reduced and the amount of non-diffusible hydrogen is suppressed, so excellent corrosion even after quenching and tempering. Exhibits fatigue properties. In such a rolled material, the corrosion fatigue characteristics of the wire can be improved even if the steel material cost is reduced, so that a high strength spring that is extremely unlikely to cause corrosion fatigue failure, for example, a coil such as a suspension spring that is one of automotive parts The spring can be supplied at low cost.

FIG. 1 is a graph showing the influence of the number of inclusions in the rolled material and the amount of non-diffusible hydrogen on the corrosion fatigue characteristics.

¡When the corrosion of the wire progresses, pits are generated on the surface of the wire, and the wire diameter of the wire is reduced due to thinning due to corrosion. In addition, hydrogen generated by corrosion penetrates into the steel and causes steel material embrittlement due to hydrogen. Corrosion fatigue failure occurs starting from these corrosion pits, thinned portions, and steel material embrittlement. Therefore, corrosion fatigue failure can be improved by improving the resistance to hydrogen embrittlement and corrosion of the wire.

The present inventors examined the factors affecting the hydrogen embrittlement resistance and the corrosion resistance from various angles. As a result, corrosion and fatigue properties can be achieved by quenching and tempering a rolled material in which both the number of oxide inclusions of a given size in steel and the amount of non-diffusible hydrogen in the steel are controlled appropriately. It became clear that improved significantly. When there are many large oxide inclusions in the steel, not only the atmospheric durability is lowered, but also a “strain field” is formed around the steel, and it becomes a hydrogen accumulation point, which makes the surrounding grain boundary particularly brittle. It has been found that the corrosion fatigue characteristics are lowered.

By properly controlling the oxide inclusions and the amount of hydrogen, the corrosion fatigue characteristics can be improved even if the addition amount of the corrosion resistance improving element is reduced. The requirements for the number of oxide inclusions, the amount of non-diffusible hydrogen in steel, and the chemical composition specified in the present invention will be described below.

The number of oxide inclusions When large oxide inclusions are present in the steel, not only the atmospheric durability is lowered, but also a strain field is formed around the inclusions, forming hydrogen accumulation sites, and particularly surrounding grain boundaries. Brittle and reduce corrosion fatigue properties. In order to reduce adverse effects on corrosion fatigue properties, the number of oxide inclusions having an average diameter of 25 μm or more per 30 g of steel material (hereinafter, sometimes referred to as “30/100 g or less”) The number of oxide inclusions is preferably 20 pieces / 100 g or less, more preferably 10 pieces / 100 g or less.In order to improve corrosion fatigue properties, oxide inclusions are required. Although it is not necessary to provide a lower limit of the number of objects, since it takes a manufacturing cost to reduce the number to 0/100 g, it is preferably 2/100 g or more for industrial production. When it is 25 μm or more, it becomes a fracture starting point as a stress concentration source and deteriorates the corrosion fatigue characteristics, but those having an average diameter of less than 25 μm do not adversely affect the corrosion fatigue characteristics.

Non-diffusible hydrogen content In the rolled material of the present invention, the non-diffusible hydrogen content needs to be 0.40 mass ppm or less. When the amount of non-diffusible hydrogen in the rolled material is large, non-diffusible hydrogen also increases in the wire after quenching and tempering. If there is a lot of non-diffusible hydrogen in the wire, the allowable amount of hydrogen that invades further before the wire becomes brittle, even a small amount of hydrogen that entered during use as a spring will cause wire embrittlement, making it easier to break early, Hydrogen brittleness resistance decreases. The amount of non-diffusible hydrogen is preferably 0.35 mass ppm or less, more preferably 0.30 mass ppm or less. The smaller the amount of non-diffusible hydrogen, the better. However, it is difficult to make it 0 ppm by mass, and the lower limit is about 0.01 ppm by mass.

Non-diffusible hydrogen is the amount of hydrogen measured by the method described in the examples below. Specifically, it is released at 300 to 600 ° C. when the steel material is heated at 100 ° C./hour. Means the total amount of hydrogen.

The rolled material for high-strength springs according to the present invention is a low alloy steel in which the content of alloy elements is suppressed, and its chemical composition is as follows. In addition, this invention also includes the wire which hardened and tempered after drawing the said rolling material, The chemical composition is the same as the chemical composition of a rolling material. In the present specification, the chemical composition means mass%.

C: 0.39 to 0.65%
C is an element necessary for ensuring the strength of the spring wire, and is also necessary for generating fine carbides that serve as hydrogen trap sites. From such a viewpoint, the C content is set to 0.39% or more. The minimum with the preferable amount of C is 0.45% or more, More preferably, it is 0.50% or more. However, when the amount of C becomes excessive, coarse retained austenite and undissolved carbides are likely to be formed even after quenching and tempering, and hydrogen embrittlement resistance may be lowered instead. Further, since C is an element that deteriorates the corrosion resistance, it is necessary to suppress the amount of C in order to enhance the corrosion fatigue characteristics of a spring product such as a suspension spring that is the final product. From such a viewpoint, the C content is set to 0.65% or less. The upper limit with preferable C amount is 0.62% or less, More preferably, it is 0.60% or less.

Si: 1.5-2.5%
Si is an element necessary for ensuring strength and has an effect of making carbide fine. In order to exhibit such an effect effectively, the Si amount was determined to be 1.5% or more. The minimum with the preferable amount of Si is 1.7% or more, More preferably, it is 1.9% or more. On the other hand, since Si is an element that promotes decarburization, when the amount of Si is excessive, formation of a decarburized layer on the surface of the wire is promoted, and a peeling process for removing the decarburized layer is required, resulting in an increase in manufacturing cost. In addition, undissolved carbides increase, and hydrogen embrittlement resistance decreases. From this point of view, the Si amount was determined to be 2.5% or less. The upper limit with preferable Si amount is 2.3% or less, More preferably, it is 2.2% or less, More preferably, it is 2.1% or less.

Mn: 0.15 to 1.2%
Mn is used as a deoxidizing element and reacts with S, which is a harmful element in steel, to form MnS, which is an element useful for detoxification of S. Mn is also an element contributing to strength improvement. In order to exhibit these effects effectively, the amount of Mn was determined to be 0.15% or more. The minimum with the preferable amount of Mn is 0.2% or more, More preferably, it is 0.3% or more. However, when the amount of Mn is excessive, the toughness is lowered and the steel material becomes brittle. From such a viewpoint, the amount of Mn was determined to be 1.2% or less. The upper limit with the preferable amount of Mn is 1.0% or less, More preferably, it is 0.85% or less.

P: more than 0% and not more than 0.015% P is a harmful element that deteriorates the ductility, for example, coiling property, of a rolled material such as a wire. Further, P is easily segregated at the grain boundary and causes embrittlement at the grain boundary, and the grain boundary is easily broken by hydrogen, which adversely affects the resistance to hydrogen embrittlement. From this point of view, the P content is set to 0.015% or less. The upper limit with the preferable amount of P is 0.010% or less, More preferably, it is 0.008% or less. The smaller the amount of P, the better. However, it is usually contained in an amount of about 0.001%.

S: more than 0% and not more than 0.015% S is a harmful element that deteriorates ductility such as coiling property of the rolled material in the same manner as P described above. In addition, S is easily segregated at the grain boundary and causes embrittlement of the grain boundary, and the grain boundary is easily broken by hydrogen, which adversely affects the resistance to hydrogen embrittlement. From this point of view, the S content is set to 0.015% or less. The upper limit with the preferable amount of S is 0.010% or less, More preferably, it is 0.008% or less. The smaller the amount of S, the better. However, it is usually contained in an amount of about 0.001%.

Al: 0.001 to 0.1%
Al is mainly added as a deoxidizing element. Moreover, it reacts with N to form AlN to render the solid solution N harmless and contribute to the refinement of the structure. In order to fully exhibit these effects, the Al content is determined to be 0.001% or more. The minimum with preferable Al amount is 0.002% or more, More preferably, it is 0.005% or more. However, since Al is an element that promotes decarburization in the same way as Si, it is necessary to suppress the amount of Al in spring steel containing a large amount of Si. In the present invention, the amount of Al is set to 0.1% or less. The upper limit with preferable Al amount is 0.07% or less, More preferably, it is 0.030% or less, Most preferably, it is 0.020% or less.

Cu: 0.10 to 0.80%
Cu is an element effective for suppressing surface layer decarburization and improving corrosion resistance. Therefore, the Cu amount is determined to be 0.10% or more. The minimum with the preferable amount of Cu is 0.15% or more, More preferably, it is 0.20% or more. However, if Cu is excessively contained, cracks occur during hot working or the cost increases. Therefore, the Cu amount is set to 0.80% or less. The upper limit with preferable Cu amount is 0.70% or less, More preferably, it is 0.60% or less. The amount of Cu is preferably 0.48% or less, 0.35% or less, or 0.30% or less.

Ni: 0.10 to 0.80%
Ni is an element effective for suppressing surface decarburization and improving corrosion resistance, similarly to Cu. Therefore, the amount of Ni is set to 0.10% or more. The minimum with preferable Ni amount is 0.15% or more, More preferably, it is 0.20% or more. However, if Ni is excessively contained, the cost increases. Therefore, the Ni content is set to 0.80% or less. The upper limit with preferable Ni amount is 0.70% or less, More preferably, it is 0.60% or less. The amount of Ni is preferably 0.48% or less, 0.35% or less, or 0.30% or less.

O: more than 0% and 0.0010% or less When oxygen is present in the steel material, oxide inclusions such as Al ₂ O ₃ , SiO ₂ , CaO, MgO, TiO ₂ are formed. The oxide inclusions are hard, and distortion occurs around the oxide inclusions due to the difference in hardness from the surrounding substrate. Hydrogen accumulates in this strain and embrittles surrounding grain boundaries. For this reason, reducing the amount of oxygen is important in improving the corrosion fatigue characteristics. Therefore, the upper limit of the O amount is set to 0.0010% or less. Preferably it is 0.0008% or less, More preferably, it is 0.0006% or less. On the other hand, the lower limit of the amount of O is generally 0.0002% or more in terms of industrial production.

The basic components of the rolled material of the present invention are as described above, and the balance is substantially iron. However, as a matter of course, it is permissible for steel to contain inevitable impurities such as Ca, Mg, and N that are brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like. The rolled material for springs of the present invention has the above-mentioned chemical composition and can achieve high strength and excellent coiling properties and hydrogen embrittlement resistance, but further contains the following elements for the purpose of improving corrosion resistance depending on the application. Also good.

Cr: more than 0% and 1.2% or less Cr is an element effective for improving corrosion resistance. In order to effectively exhibit such effects, the Cr content is preferably 0.05% or more, more preferably 0.08% or more, and still more preferably 0.10% or more. However, Cr has a strong tendency to generate carbides, forms unique carbides in steel, and is an element that easily dissolves in cementite at a high concentration. Although it is effective to contain a small amount of Cr, since the heating time in the quenching process is short in high-frequency heating, austenitization in which carbide, cementite and the like are dissolved in the base material tends to be insufficient. For this reason, if a large amount of Cr is contained, undissolved cementite in which Cr-based carbides and metallic Cr are dissolved in a high concentration is generated, it becomes a stress concentration source and easily breaks, and the hydrogen embrittlement resistance deteriorates. Become. Accordingly, the Cr content is preferably 1.2% or less, more preferably 0.8% or less, and still more preferably 0.6% or less.

Ti: More than 0% and 0.13% or less Ti is an element useful for detoxifying S by reacting with S to form a sulfide. Ti also has the effect of forming a carbonitride to refine the structure. In order to effectively exhibit such an effect, the Ti content is preferably 0.02% or more, more preferably 0.05% or more, and further preferably 0.06% or more. However, when the amount of Ti becomes excessive, coarse Ti sulfide may be formed and ductility may deteriorate. Therefore, the Ti amount is preferably 0.13% or less. From the viewpoint of cost reduction, the content is preferably 0.10% or less, and more preferably 0.09% or less.

B: More than 0% and 0.01% or less B is an element that improves hardenability, has an effect of strengthening the prior austenite grain boundary, and contributes to suppression of fracture. In order to effectively exhibit such effects, the B content is preferably 0.0005% or more, more preferably 0.0010% or more. However, since the above effect is saturated even if the amount of B becomes excessive, the amount of B is preferably 0.01% or less, more preferably 0.0050% or less, and still more preferably 0.0040% or less.

Nb: more than 0%, less than 0.1% and Mo: more than 0%, less than 0.5% Nb is an element that forms carbonitrides with C and N and contributes mainly to refinement of the structure It is. In order to effectively exhibit such an effect, the Nb content is preferably 0.003% or more, more preferably 0.005% or more, and still more preferably 0.01% or more. However, when the amount of Nb becomes excessive, coarse carbonitrides are formed and the ductility of the steel material is deteriorated. Therefore, the Nb amount is preferably 0.1% or less. From the viewpoint of cost reduction, it is preferably 0.07% or less.

Mo, like Nb, forms carbonitrides with C and N, and is an element that contributes to refinement of the structure. It is also an effective element for securing strength after tempering. In order to effectively exhibit such an effect, the Mo amount is preferably 0.15% or more, more preferably 0.20% or more, and further preferably 0.25% or more. However, when the amount of Mo becomes excessive, coarse carbonitrides are formed, and the ductility of the steel material, for example, coiling properties deteriorates. Therefore, the Mo amount is preferably 0.5% or less, and more preferably 0.4% or less.

Nb and Mo may be contained alone or in combination of two kinds. The rolled material of the present invention contains N as an inevitable impurity, and this amount is preferably adjusted to the following range.

N: more than 0%, 0.007% or less N amount is an element contained in inevitable impurities, but as the amount increases, coarse nitrides are formed together with Ti and Al, which adversely affects fatigue properties. Preferably as little as possible. The N amount may be, for example, 0.007% or less, and more preferably 0.005% or less. On the other hand, if the amount of N is reduced too much, the productivity is significantly reduced. N also forms nitrides with Al and contributes to the refinement of crystal grains. From such a viewpoint, the N content is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.

Next, a method for producing the rolled material of the present invention will be described. In a series of steps in which the steel having the above chemical composition is melted and continuously cast, piece-rolled, and hot-rolled, (A) the amount of hydrogen in the molten steel stage, (B) the homogenization treatment temperature before the piece-rolling and The amount of non-diffusible hydrogen in the rolled material can be controlled by adjusting at least one of the time and (C) the cooling rate from 400 to 100 ° C. after hot rolling.

In order to reduce the hydrogen in the steel after solidification, it is necessary to remove the hydrogen in the steel by diffusion. In order to release the hydrogen from the steel surface, in order to increase the hydrogen diffusion rate, Heating for hours is effective. Specifically, as a method of reducing the amount of hydrogen in the steel, adjustment at the molten steel stage, adjustment at the continuous cast material stage of 1000 ° C. or higher after solidification, adjustment at the heating stage before hot rolling, rolling heating Examples include adjustment in the middle stage and adjustment in the cooling stage after rolling. In particular, it is particularly effective to perform at least one of the following non-diffusible hydrogen reduction treatments (A) to (C).

(A) A degassing process is performed by a molten steel process, and the amount of hydrogen in the molten steel is set to 2.5 mass ppm or less.
For example, in a secondary refining process, a vacuum tank equipped with two dip tubes is installed in the ladle, Ar gas is blown from the side of one dip tube, and the buoyancy is used to circulate the molten steel to the vacuum tank. It is effective to perform degassing. This method is excellent in hydrogen removal capability. The amount of hydrogen in the molten steel is preferably 2.0 mass ppm or less, more preferably 1.5 mass ppm or less, and particularly preferably 1.0 mass ppm or less.

(B) The homogenization treatment (heating) before the bulk rolling is performed at 1100 ° C. or higher, preferably 1200 ° C. or higher for 10 hours or longer.

(C) The average cooling rate from 400 to 100 ° C. after hot rolling is 0.5 ° C./second or less, preferably 0.3 ° C./second or less.

In particular, when the cross-sectional area of the steel material is large, heating for a long time is required. However, heating the steel material for a long time promotes decarburization. In such a case, the amount of hydrogen in the steel is reduced by performing the above (A). It is preferable to reduce.

Further, cooling conditions other than the coil winding temperature TL after hot rolling and the temperature range of 400 to 100 ° C. after winding are not particularly limited.

The coil winding temperature TL can be, for example, 900 ° C. or higher and 1000 ° C. or lower, preferably 910 ° C. or higher, more preferably 930 ° C. or higher. The average cooling rate at the coil winding temperature TL to 650 ° C. can be 2 ° C./second or more and 5 ° C./second or less. The lower limit of the average cooling rate at the coil winding temperature TL to 650 ° C. is preferably 2.3 ° C./second or more, more preferably 2.5 ° C./second or more. The upper limit of the average cooling rate at the coil winding temperature TL to 650 ° C. is preferably 4.5 ° C./second or less, more preferably 4 ° C./second or less. Further, the average cooling rate of 650 to 400 ° C. can be 2 ° C./second or less. The average cooling rate of 650 to 400 ° C. is preferably 1.5 ° C./second or less, more preferably 1 ° C./second or less. Although the minimum of this average cooling rate is not specifically limited, For example, it is about 0.3 degree-C / sec.

Reduction of oxide inclusions In order to reduce oxide inclusions, it is necessary to make the oxygen content of the wire below a specified value. Further, by sufficiently deoxidizing with aluminum or silicon and sufficiently degassing, inclusions can be reduced, high cleaning can be achieved, and oxide inclusions can be reduced.

For example, in order to manufacture a coil spring used in an automobile or the like, it is necessary to manufacture a wire by wire-processing, that is, wire drawing, the above-described rolled material. The wire is quenched and tempered, and such a wire is also included in the present invention.

A high-strength wire having a tensile strength of 1900 MPa or more can be obtained by subjecting a rolled material to wire processing, that is, wire drawing, followed by quenching and tempering by induction heating or the like. Specifically, the rolled material is drawn at a reduction in area of about 5 to 35%, then quenched at about 900 to 1000 ° C., and tempered at about 300 to 520 ° C. The quenching temperature is preferably 900 ° C. or higher in order to sufficiently austenite, and 1000 ° C. or lower is preferable in order to prevent crystal grain coarsening. The tempering heating temperature may be set to an appropriate temperature in the range of 300 to 520 ° C. according to the target value of the wire strength. When quenching and tempering is performed by high frequency heating, the quenching and tempering time is about 10 to 60 seconds, respectively.

The structure after quenching and tempering needs to have a tempered martensite structure of 80 area% or more. When the ratio of undissolved ferrite and retained austenite in the structure increases, the strength decreases. The structure after quenching and tempering preferably has a tempered martensite structure of 85 area% or more. In order to set the ratio of the tempered martensite structure to 80% by area or more, it is preferable to heat to 900 ° C. or higher during quenching and sufficiently austenite, and then cool to 100 ° C. or lower by water cooling or oil cooling.

The wire of the present invention thus obtained can realize a high tensile strength of 1900 MPa or more. The tensile strength may be selected according to the spring design strength, and is usually selected from 1900 MPa to 2200 MPa. The upper limit of the tensile strength is not particularly limited, but is approximately 2500 MPa. Further, since the wire of the present invention uses the rolled material of the present invention, it can exhibit excellent corrosion fatigue characteristics even at a high strength of 1900 MPa or more.

This application claims the benefit of priority based on Japanese Patent Application No. 2014-039368 filed on February 28, 2014. The entire contents of the specification of Japanese Patent Application No. 2014-039368 filed on February 28, 2014 are incorporated herein by reference.

Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can of course be implemented with appropriate modifications within a range that can be adapted to the above-described gist. Included in the range.

Steel materials having the chemical composition shown in Tables 1 to 3 below were melted by converter melting, and after continuous casting, homogenization was performed at 1100 ° C. or higher. After the homogenization treatment, it was subjected to block rolling, heated at 1000 to 1280 ° C., and then hot rolled to obtain a rolled material having a diameter of 14.3 mm, that is, a wire. The presence or absence of degassing treatment of molten steel by the above-described method, the presence or absence of cooling after winding, that is, whether or not cooling at 400 to 100 ° C. after rolling was performed at an average cooling rate of 0.5 ° C./second or less It is as shown in Tables 4-6. The amounts of O in the molten steel shown in Tables 4 to 6 were adjusted by controlling the degree of deoxidation with aluminum or silicon.

At this time, the coil winding temperature TL after the hot rolling is 950 ° C., and the other cooling after the winding is an average cooling rate of 4 ° C./second from TL to 650 ° C., and 1 ° C. from 650 to 400 ° C. Cooled at an average cooling rate of / sec. In the test example described as “implementation” in the column of homogenization treatment, the homogenization treatment at 1100 ° C. is performed for 10 hours or more, and in the test example described as “−”, the time for the homogenization treatment at 1100 ° C. Is less than 10 hours.

The obtained wire was measured for the amount of non-diffusible hydrogen and the number of oxide inclusions in the following manner. The results are shown in Tables 4-6. In Tables 4 to 6, the number of oxide inclusions having an average diameter of 25 μm or more in the rolled material is expressed as “the number of inclusions having a diameter of 25 μm or more in the rolled material”.

Non-diffusible hydrogen amount A test piece having a width of 20 mm and a length of 40 mm was cut out from the rolled material, that is, the wire. Using a gas chromatography apparatus, the test piece was heated at a temperature increase rate of 100 ° C./hour, and the amount of released hydrogen at 300 to 600 ° C. was measured, which was defined as the amount of non-diffusible hydrogen.

Number of Oxide Inclusions The number of oxide inclusions was calculated by calculating the average value of the results of investigating six 50 g rolled material samples and converting them to the number per 100 g. The number of inclusions was investigated by the acid dissolution method. The 50 g sample is dissolved with an acid, the undissolved inclusions remain on the filter paper, and inclusions having an average diameter of 25 μm or more are selected by EPMA, and EDX (Energy Dispersive X-ray spectroscopy: energy dispersive X (Line analysis), and oxide inclusions were selected. With respect to each of the six samples described above, the number of oxide inclusions having an average diameter of 25 μm or more was measured to obtain an average value thereof and converted into the number per 100 g of steel material. At this time, nitric acid adjusted so that the oxide inclusions were not dissolved was used for dissolution with an acid. The average diameter of the oxide inclusions means an average value of the major axis and the minor axis, that is, a value obtained by dividing the sum of the major axis and the minor axis by 2. In addition, in order to reduce the number of oxide inclusions, sufficient vacuum degassing was performed during the melting of the converter to remove oxygen.

Next, the wire was drawn to a diameter of 12.5 mm, that is, cold drawn and quenched and tempered. The area reduction rate of the wire drawing is about 23.6%, and the conditions for quenching and tempering are as follows.

Quenching and tempering conditions ・ High-frequency heating ・ Heating rate: 200 ° C./second ・ Quenching: 950 ° C., 20 seconds, water cooling / tempering: 300 to 520 ° C., 20 seconds, water cooling

By performing the quenching and tempering described above, a structure in which the area ratio of tempered martensite occupies 80% or more can be obtained. In this test, it was confirmed that the area ratio of all tempered martensite was 80% or more.

The tensile strength and corrosion fatigue characteristics of the wire after wire drawing and quenching and tempering were evaluated. The results are also shown in Tables 4 to 6 below.

Measurement of tensile strength A wire after quenching and tempering was cut into a predetermined length, and a tensile test was performed according to JIS Z2241 (2011) with a distance between chucks of 200 mm and a tensile speed of 5 mm / min.

Evaluation of Corrosion Fatigue Properties Corrosion fatigue properties were evaluated by the Ono rotary bending fatigue test after corrosion treatment and the fracture life. The test piece cut the quenched and tempered wire to produce a No. 1 test piece of JIS Z 2274 (1978). The parallel part of the test piece was polished with # 800 emery paper. The test was performed without performing shot peening on the surface. First, the processed test piece was subjected to corrosion treatment under the following conditions.

Corrosion treatment 35 ° C, 5% NaCl solution sprayed with salt water for 8 hours, then dried and kept in a humid environment at 35 ° C and relative humidity of 60% for 16 hours. The test piece was subjected to corrosion treatment by repeating 10 cycles. The test piece after the corrosion treatment was subjected to a rotating bending test to evaluate the corrosion fatigue characteristics. Ten test pieces were used for each test, and the Ono type rotating bending fatigue test was performed with the load stress set to 500 MPa. The fatigue life until each test piece was broken was measured. The average value of fatigue life in 10 test pieces was measured, and the average fatigue life value of 100,000 times or more was evaluated as being excellent in corrosion fatigue life.

From these results, it can be considered as follows. That is, the test No. shown in Table 4 1 to 16 and the test Nos. Shown in Table 6. Nos. 32 to 48 are steels in which the chemical composition of the steel material is appropriately adjusted according to the above-mentioned preferable production conditions, so that the number of oxide inclusions and the amount of non-diffusible hydrogen are within the ranges specified in the present invention. Is satisfied. The wires after drawing such wire and quenching and tempering all have excellent tensile strength of 1900 MPa or more. Moreover, all of the wires after quenching and tempering exhibit a fatigue life of 100,000 times or more, and have excellent corrosion fatigue characteristics.

In contrast, the test No. shown in Table 5 Nos. 17 to 31 are inferior in corrosion fatigue properties because at least one of the chemical composition of the steel material specified in the present invention, the number of oxide inclusions, and the non-diffusible hydrogen content is inappropriate. ing.

Test No. 17 and 18 are examples using steel types 17 and 18 in which Cu or Ni is not added or less than the specified lower limit, and the corrosion fatigue characteristics are deteriorated. Test No. In Nos. 19 to 24, the deoxidation treatment was insufficient and the amount of O in the steel was excessive, the number of oxide inclusions in the rolled material increased, and the corrosion fatigue characteristics deteriorated.

Test No. 25 to 29, the amount of O in the steel is controlled in an appropriate range, but since none of the non-diffusible hydrogen reduction treatment described above is performed, the amount of non-diffusible hydrogen in the rolled material increases. The fatigue life was less than 100,000 times, and the corrosion fatigue characteristics deteriorated.

Test No. Nos. 30 and 31 have insufficient deoxidation treatment, and the amount of O in the steel is excessive, and none of the non-diffusible hydrogen reduction treatment described above is performed, so oxide inclusions in the rolled material And the amount of non-diffusible hydrogen in the rolled material increased, and in all cases, the fatigue life was less than 100,000 times and the corrosion fatigue characteristics deteriorated.

Based on these results, the influence of the number of oxide inclusions and the amount of non-diffusible hydrogen in the rolled material on the corrosion fatigue characteristics is shown in FIG. In FIG. 1 to 16, and the comparative example marked with x is test No. in Table 5. 19 to 31 are shown, and the number of oxide inclusions in the rolled material was expressed as “inclusion number”. As is clear from this result, it is understood that strictly defining the number of oxide inclusions and the amount of non-diffusible hydrogen is effective in improving the corrosion fatigue characteristics.

The rolled material and wire of the present invention can be suitably used for coil springs used in automobiles and the like, for example, valve springs and suspension springs used in engines and suspensions, etc., and are industrially useful.

Claims (3)

% By mass
C: 0.39 to 0.65%,
Si: 1.5 to 2.5%,
Mn: 0.15 to 1.2%,
P: more than 0%, 0.015% or less,
S: more than 0%, 0.015% or less,
Al: 0.001 to 0.1%,
Cu: 0.10 to 0.80%,
Ni: 0.10-0.80% and O: more than 0%, 0.0010% or less, respectively, the balance being iron and inevitable impurities,
A rolled material for high-strength springs, wherein the number of oxide inclusions having an average diameter of 25 μm or more is 30 or less per 100 g of steel, and the amount of non-diffusible hydrogen is 0.40 mass ppm or less. The rolled material for high-strength springs according to claim 1, further comprising one or more of the following (a) to (d) in mass%.
(A) Cr: Over 0%, 1.2% or less (b) Ti: Over 0%, 0.13% or less (c) B: Over 0%, 0.01% or less (d) Nb: Over 0% , 0.1% or less and Mo: more than 0%, at least one of 0.5% or less A high-strength spring wire comprising the chemical composition of steel according to claim 1 or 2, wherein the area ratio of tempered martensite is 80% or more, and the tensile strength is 1900 MPa or more.

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