JP2017128799A - Rolled material for high strength spring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolled material for high strength spring capable of manufacturing a wire for high strength spring having high strength and excellent in corrosion resistance and ambient air durability with good wire drawing property without need for additional processes in mass production.SOLUTION: The rolled material for high strength spring contains C, Si, Mn, P, S, Al, Cu, Ni, Cr and Ti satisfying regulation ranges and balance iron with inevitable impurities, has ideal critical diameter DCI represented by the regulation formula (1) of 120 or less and decarbonization index CA represented by the regulation formula (2) of 70 or less.SELECTED DRAWING: None

Description

  The present invention relates to a rolled material for high-strength springs. In particular, high strength spring rolling that enables high-strength spring wire with high strength but excellent corrosion resistance and atmospheric durability to be mass-produced with good drawability and without the need for additional processes. Regarding materials.

  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. . In the process of manufacturing a high-strength wire as a material for a high-strength spring, the rolled material is subjected to wire drawing for the purpose of improving the dimensional accuracy of the wire diameter and homogenizing the structure by plastic working, and then tempering, that is, quenching. Tempering is performed. In the production of the high-strength wire, the drawing rate may be increased in order to achieve a more uniform structure. Therefore, good drawing workability is required for the rolled material used for drawing. When wire drawing workability is poor, wire breakage occurs during wire drawing and it cannot be used as an industrial product.

  In addition, since the strength-enhanced spring is poor in toughness, hydrogen embrittlement is likely to occur, and fatigue characteristics under a corrosive environment, that is, corrosion durability is reduced. Therefore, the wire used for manufacturing the spring is also required to have excellent corrosion durability. Further, the spring is also required to have excellent fatigue characteristics under atmospheric atmosphere, that is, air durability. In order to realize a spring excellent in corrosion durability and atmospheric durability, it is necessary that the wire which is the material of the spring has these characteristics.

  As a method of enhancing the wire drawing workability of the rolled material for high-strength springs and the corrosion durability of the wire for high-strength springs obtained using the rolled material, the chemical composition is controlled and the structure of the rolled material is adjusted. It is known to control.

  For example, Patent Document 1 relates to a steel wire for a high-strength spring excellent in wire drawability, and the rolled material has a fine and uniform pearlite structure as well as the chemical composition of the rolled material. The wire drawing property is improved by controlling the average value and the standard deviation thereof to appropriate values. However, since all the proposed steel materials have extremely high hardenability index DI values, it is considered necessary to add a process such as soft annealing. In order to manufacture without adding the above-mentioned process that increases the cost, it is considered that further study is necessary.

  Patent Document 2 relates to a high-strength spring steel having excellent resistance to hydrogen fatigue fracture, and the characteristics are improved by making the wire structure into a layered structure of martensite and ferrite together with chemical components. However, in order to obtain a two-phase structure such as a layered structure of martensite and ferrite, it is necessary to strictly control the heating temperature in the manufacturing process, and it is difficult to say that industrial stability is sufficient. It seems that further study is necessary.

JP 2012-072492 A JP 2003-105485 A

  The present invention has been made in view of the circumstances as described above, and an object of the present invention is to add a high-strength spring wire that has high strength but excellent corrosion durability and atmospheric durability with good wire drawing workability. An object of the present invention is to provide a rolled material for high-strength springs that can be mass-produced and manufactured without requiring a process or the like.

The rolled material for high-strength springs of the present invention that has solved the above problems is in mass%,
C: 0.55-0.63%,
Si: 1.90-2.30%,
Mn: 0.15 to 0.55%,
P: more than 0%, 0.015% or less,
S: more than 0%, 0.015% or less,
Al: 0.001 to 0.1%,
Cu: 0.15-0.45%,
Ni: 0.35 to 0.75%,
Cr: 0.36-0.65%, and Ti: 0.04-0.11%
And the balance is iron and inevitable impurities, the ideal critical diameter DCI represented by the following formula (1) is 120 or less, and the decarburization index CA represented by the following formula (2) is 70 or less. It has features in some places.
DCI = 25.4 × (0.171 + 0.001 × [C] + 0.265 × [C] ² )
× (3.3333 × [Mn] +1) × (1 + 0.7 × [Si])
× (1 + 0.363 × [Ni]) × (1 + 2.16 × [Cr])
× (1 + 0.365 × [Cu]) × (1 + 1.73 × [V]) × (1 + 3 × [Mo])
... (1)
CA = 98 × [Si] −171 × [Mn] −40 × [Cr] −142 × [Ni] −60 × [Cu] (2)
In said formula (1) and formula (2), [element name] means content in steel in the mass% of each element.

  In the rolled material for high-strength springs, in the cross section perpendicular to the rolling direction of the rolled material, the C concentration in the region from the outermost surface of the rolled material to a depth of 20 μm is 50% or more of the C concentration of the parent phase. The total of martensite and bainite in the entire structure is 5% by area or less, and when a region from the outermost surface of the rolled material to a depth of 0.5 mm in the radial direction is observed, the depth is 20 μm or more and the width is 500 μm. It is preferable that the above ferrite single phase does not exist and the tensile strength satisfies 1170 MPa or less.

  The ideal critical diameter DCI of the rolled material for high-strength springs is preferably 70 or more.

The high-strength spring rolled material may further include one or more of the following (a) and (b).
(A) By mass%, B: more than 0%, 0.01% or less,
(B) selected from the group consisting of V: more than 0%, 0.3% or less, Nb: more than 0%, 0.3% or less, and Mo: more than 0%, 0.5% or less. At least one

In the rolled material for high-strength spring, the degree of Cr segregation at the position of 2.0 mm in the radial direction from the outermost surface of the rolled material in the horizontal section in the rolling direction including the center of the rolled material is expressed by the following formula (3). It is preferable to satisfy.
Cr segregation degree ≦ 0.1% (3)
Here, the Cr segregation degree is a value of + 2σ of the distribution of Cr content in the rolling direction, measured by line analysis using EPMA.

  The rolled material for high-strength spring has four sides defined by a radial length from the outermost surface of the rolled material: 2 mm × a length in the rolling direction: 0.10 mm in a cross section horizontal to the rolling direction including the center of the rolled material. The number of TiN having a radial size of 8 μm or more in the shape region is preferably 10 or less.

  Since the rolled material of the present invention has an optimized component composition, a wire excellent in corrosion durability and atmospheric durability even when the tensile strength is as high as 1950 MPa or higher, for example, with excellent wire drawing workability and the aforementioned Mass production can be performed on a general production line without requiring additional steps such as soft annealing as in Patent Document 1 and strict control of heating temperature as in Patent Document 2.

  The inventors of the present invention have manufactured a high-strength spring wire that is excellent in corrosion durability and atmospheric durability while having a high tensile strength of, for example, 1950 MPa or more, with good wire drawing workability, and without any additional steps. In order to obtain a high-strength rolled material for springs that can be made, research was conducted intensively. As a result, it was found that if the chemical composition of the steel material is optimized within a manufacturable range, it is possible to secure a particularly excellent wire drawing workability and to obtain a wire having excellent corrosion durability and atmospheric durability. It was. Hereinafter, a specific method for ensuring the above characteristics will be described.

  First, studies were made to obtain excellent wire drawing workability. The drawing workability is strongly influenced by the ductility of the rolled material, and this ductility can be evaluated by the presence or absence of bainite and martensite, which are hardened parts, and the tensile strength of the rolled material. Hereinafter, the bainite and martensite are collectively referred to as “supercooled structure”. As a result of studies by the present inventors, in order to increase the ductility, that is, to improve the wire drawing workability, the tensile strength of the rolled material is suppressed and the rolled material is controlled by controlling the component composition of the rolled material as described later. It has been found that it is effective to sufficiently suppress the supercooled tissue in all the tissues.

  The tensile strength of the rolled material is preferably suppressed to 1170 MPa or less. In order to ensure higher wire drawing workability, it is more preferable to suppress to 1100 MPa or less. From the viewpoint of securing excellent wire drawing workability, the tensile strength is preferably low. However, when the tensile strength is extremely low, spheroidized or coarse carbides are likely to exist. The presence of the carbide is not preferable because the carbide is not sufficiently dissolved in the subsequent austenitization and the quenching hardness may be insufficient. Therefore, the tensile strength is preferably 900 MPa or more. Moreover, it is preferable that the supercooled structure | tissue in all the structures is 5 area% or less. From the viewpoint of ensuring higher wire drawing workability, it is preferably 2 area% or less, and most preferably no supercooled structure, that is, 0 area%.

  Next, atmospheric durability will be described. As a cause of the atmospheric fatigue failure of the spring, decarburization of the surface layer can be mentioned together with the steel material components and the like. When decarburization exists in the surface layer, the site becomes a hardness-decreasing site, which acts as a starting point for fracture, and causes early breakage that becomes a problem in actual use. Therefore, atmospheric durability can be judged by the presence or absence of surface decarburization. In order to improve atmospheric durability, the present inventors suppress the decarburization of the surface layer by controlling the component composition of the rolled material as described later, specifically, suppress the generation of coarse ferrite decarburization. In addition, it was found effective to ensure the surface layer C concentration.

  In the present invention, as the coarse ferrite decarburization, a ferrite single phase having a depth of 20 μm or more and a width of 500 μm or more is not present in a certain region as described in Examples described later. In the case of minute ferrite decarburization, C and the like diffuse by heat treatment and the ferrite decarburization disappears, but the ferrite decarburization of the above size remains without disappearing even in the heat treatment, and adversely affects atmospheric durability. It is because it is thought that it affects.

  Moreover, in this invention, while suppressing the production | generation of the said coarse ferrite decarburization, C density | concentration of the area | region from the outermost surface of a rolling material to the depth of 20 micrometers fills 50% or more of C density | concentration of a parent phase. To do. Thereby, the hardness of the surface layer part of a rolling material and a wire can be ensured, As a result, the outstanding atmospheric durability can be ensured. This “C concentration in the region from the outermost surface of the rolled material to a depth of 20 μm in the radial direction” can be obtained by the method described in “Measurement of surface C concentration” in Examples described later. Hereinafter, the “C concentration in the region from the outermost surface of the rolled material to a depth of 20 μm in the radial direction” may be referred to as “surface layer C concentration”. The surface layer C concentration is preferably 60% or more of the C concentration of the parent phase, more preferably 70% or more of the C concentration of the parent phase.

  Furthermore, corrosion durability will be described. This corrosion durability is also called corrosion fatigue characteristics. Corrosion fatigue failure occurs when hydrogen generated by corrosion penetrates into steel and the steel material becomes embrittled by the hydrogen. Therefore, in order to increase the corrosion durability, it is necessary to increase the corrosion resistance and hydrogen embrittlement resistance of the steel material by controlling the composition of the rolled material as described later.

  When mass-producing high-strength spring rolled materials on a general rolling line, the occurrence of supercooled structures that adversely affect the wire drawing workability and the occurrence of ferrite decarburization that adversely affect the atmospheric durability are significant. It becomes a problem. In the present invention, particularly in mass production, the supercooled structure and ferrite decarburization are suppressed to ensure good wire drawing workability, atmospheric durability and corrosion durability, and also to ensure good corrosion durability. The component composition of the rolled material, which is the material of the spring or wire, is controlled as follows.

(C: 0.55-0.63%)
C is an element necessary for securing the strength of the spring wire, and also necessary for producing fine carbides that serve as hydrogen trap sites. From this point of view, the C content is set to 0.55% or more. The minimum with the preferable amount of C is 0.57% or more, More preferably, it is 0.58% or more. However, when the amount of C becomes excessive, coarse retained austenite and undissolved carbides are likely to be generated after quenching and tempering at the time of manufacturing the wire, and the hydrogen embrittlement resistance may decrease instead. Further, C increases the strength of the rolled material and causes a decrease in wire drawing workability, breakage, and the like. From such a viewpoint, the C content is set to 0.63% or less. The upper limit with preferable C amount is 0.62% or less, More preferably, it is 0.61% or less.

(Si: 1.90-2.30%)
Si is an element necessary for ensuring the strength, and has the effect of increasing the corrosion durability by making the carbide fine and ensuring sufficient hydrogen trap sites. In order to exhibit these effects effectively, the Si content is set to 1.90% or more. The minimum with the preferable amount of Si is 1.95% or more, More preferably, it is 2.10% or more. On the other hand, since Si is also an element that promotes decarburization, if the amount of Si is excessive, formation of a decarburized layer on the surface of the steel material is promoted, and excellent atmospheric durability cannot be ensured. In addition, a peeling process for removing the decarburized layer is required, resulting in an increase in manufacturing cost. Further, the amount of undissolved carbides increases, and the corrosion durability decreases. From such a viewpoint, the amount of Si was determined to be 2.30% or less. The upper limit with the preferable amount of Si is 2.25% or less, More preferably, it is 2.20% or less.

(Mn: 0.15 to 0.55%)
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 0.20% or more, More preferably, it is 0.25% or more. However, when the amount of Mn becomes excessive, the hardenability increases, and the toughness decreases, and the steel material is easily embrittled. From such a viewpoint, the amount of Mn was determined to be 0.55% or less. The upper limit with the preferable amount of Mn is 0.50% or less, More preferably, it is 0.45% or less.

(P: more than 0%, 0.015% or less)
P is a harmful element that deteriorates the ductility such as coiling property of the rolled material, that is, the 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 such a viewpoint, it is better that P is as small as possible. In the present invention, P 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. As described above, the smaller the amount of P, the better. However, it can usually be contained at least about 0.001%.

(S: more than 0%, 0.015% or less)
S, like P described above, is a harmful element that degrades ductility such as coiling properties of the rolled material. 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, it is desirable that S is as small as possible. In the present invention, it is determined to be 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 can usually be contained at least 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, More preferably, it is 0.020% or less.

(Cu: 0.15-0.45%)
Cu is an element effective for suppressing surface layer decarburization and improving corrosion resistance. Therefore, in the present invention, the amount of Cu is set to 0.15% or more. The amount of Cu is preferably 0.20% or more, more preferably 0.25% or more. However, if Cu is excessively contained, cracks occur during hot working or the cost increases. Therefore, the amount of Cu is set to 0.45% or less. The amount of Cu is preferably 0.43% or less, more preferably 0.40% or less.

(Ni: 0.35-0.75%)
Ni is an element effective for suppressing surface decarburization and improving corrosion resistance, similarly to Cu. Therefore, in the present invention, the amount of Ni is determined to be 0.35% or more. The amount of Ni is preferably 0.40% or more, more preferably 0.45% or more. However, if Ni is excessively contained, the cost increases. Therefore, the Ni content is set to 0.75% or less. The amount of Ni is preferably 0.70% or less, more preferably 0.65% or less.

(Cr: 0.36-0.65%)
As shown below, Cr is an important element in the component design of suspension spring steel. That is, for example, a phenomenon in which C on the surface layer is decarburized during rolling at a high temperature occurs, but if Cr is contained in the steel, a dense scale is formed on the surface of the steel material due to high-temperature oxidation and suppresses the diffusion of C. To do. As a result, surface decarburization can be suppressed and a decrease in the surface layer C concentration can be suppressed, and excellent atmospheric durability can be ensured. Further, Cr has an effect of further improving the corrosion resistance, and further has an effect of increasing the strength of the wire by showing a temper softening resistance. In order to exert these effects, it is necessary to contain 0.36% or more of Cr. The amount of Cr is preferably 0.38% or more, more preferably 0.40% or more. On the other hand, when Cr is excessively contained, the strength of the rolled material is increased and wire drawing is difficult. There may also be a problem that corrosion progresses locally. Therefore, the Cr amount is set to 0.65% or less. The amount of Cr is preferably 0.60% or less, more preferably 0.55% or less.

(Ti: 0.04 to 0.11%)
Ti reacts with S to form a sulfide, and is an element useful for detoxifying S. Ti also has the effect of forming a carbonitride to refine the structure. Furthermore, TiC has the harmless effect of hydrogen and improves hydrogen embrittlement resistance and rotational bending properties after corrosion. Therefore, in the present invention, the amount of Ti is set to 0.04% or more. The amount of Ti is preferably 0.05% or more, more preferably 0.06% or more. On the other hand, when the amount of Ti is excessive, coarse Ti sulfide is formed and ductility may deteriorate. Therefore, the Ti amount is set to 0.11% or less. The amount of Ti is preferably 0.10% or less.

  The components of the steel material of the present invention are as described above, and the balance is iron and inevitable impurities. In addition to the above elements, the hardenability and the like can be further improved by adding an appropriate amount of the following elements. Hereinafter, these elements will be described in detail.

(B: more than 0%, 0.01% or less)
B is an element that improves hardenability, has an effect of strengthening the prior austenite grain boundaries, 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 0.0030% or less.

(V: at least one selected from the group consisting of more than 0%, 0.3% or less, Nb: more than 0%, 0.3% or less, and Mo: more than 0%, 0.5% or less)
V, Nb, and Mo are all elements that form precipitates with C and N and contribute to refinement of the structure. These elements may be used alone or in combination of two or more. Hereinafter, each element will be described.

(V: over 0%, 0.3% or less)
V is an element that contributes to strength improvement and crystal grain refinement. In order to effectively exhibit such effects, the V amount is preferably 0.1% or more, more preferably 0.15% or more, and further preferably 0.20% or more. However, when the amount of V becomes excessive, the cost increases. Therefore, the V amount is preferably 0.3% or less, more preferably 0.25% or less.

(Nb: more than 0%, 0.2% or less)
Nb is an element that forms carbonitrides with C and N and contributes mainly to refinement of the structure. From this viewpoint, Nb may be contained preferably in an amount of 0.005% or more, and more preferably 0.010% or more. However, when the amount of Nb becomes excessive, coarse carbonitrides are formed and the ductility of the steel material deteriorates. It also leads to increased costs. From these viewpoints, the Nb content is preferably 0.3% or less, more preferably 0.2% or less, still more preferably 0.10% or less, and particularly 0.07% or less from the viewpoint of cost reduction. More preferably.

(Mo: over 0%, 0.5% or less)
Mo, like Nb, is an element that forms carbonitrides with C and N and 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 set to 0.10% or more. However, when the amount of Mo becomes excessive, coarse carbonitrides are formed, and ductility such as coilability of the steel material deteriorates. Therefore, the Mo amount is preferably 0.5% or less, and more preferably 0.3% or less.

  Further, in the rolled material of the present invention, the ideal critical diameter DCI represented by the following formula (1) and the decarburization index CA represented by the following formula (2) satisfy the following ranges.

The ideal critical diameter DCI represented by the following formula (1) is 120 or less DCI = 25.4 × (0.171 + 0.001 × [C] + 0.265 × [C] ² )
× (3.3333 × [Mn] +1) × (1 + 0.7 × [Si])
× (1 + 0.363 × [Ni]) × (1 + 2.16 × [Cr])
× (1 + 0.365 × [Cu]) × (1 + 1.73 × [V]) × (1 + 3 × [Mo])
... (1)
In said formula (1), [element name] means content in steel in the mass% of each element.

  If the ideal critical diameter DCI represented by the above formula (1) is too high, the hardenability becomes high, and a supercooled structure is likely to be generated in normal rolling, and the wire drawing workability at the time of wire production is lowered as described above. . Therefore, in the present invention, the upper limit of the DCI is 120. The DCI is preferably 115 or less, more preferably 110 or less. On the other hand, when the DCI is low, the hardenability is low, and when the hardening is performed as the heat treatment at the time of manufacturing the wire, the martensite structure may not be formed to the inside. Therefore, the lower limit of the DCI is preferably 70 or more. The DCI is more preferably 75 or more, and still more preferably 80 or more.

Decarburization index CA represented by the following formula (2) is 70 or less CA = 98 × [Si] −171 × [Mn] −40 × [Cr] −142 × [Ni] −60 × [Cu] (2) )
In said formula (2), [element name] means content in the steel in the mass% of each element.

The above-described ferrite decarburization of the surface layer of the rolled material is caused by the ferrite decarburization generated in the steel piece before rolling. If coarse ferritic decarburization is present in the steel slab, the C concentration in that portion is extremely lowered and the A ₃ point is raised. Therefore, it is impossible to enter the austenite single-layer region beyond the _three points A be heated to a high temperature rolling and the like, it remains in the two-phase zone below ₃ points A. As a result, ferrite decarburization does not disappear, and this ferrite decarburization is carried over to the rolled material.

The boundary temperature between the two-phase region and the austenite region is affected by the components. Therefore, in order to eliminate the coarse ferrite decarburization, it is preferable to control the components so that A ₃ point is low. From this viewpoint, the above formula (2) was derived. In the present invention, if the CA satisfies 70 or less, the austenite single phase can be obtained by heating in a general rolling process, and ferrite decarburization can be eliminated. The CA is preferably 65 or less, more preferably 50 or less.

In a preferred embodiment, the rolled material of the present invention has a Cr segregation degree at a position 2.0 mm from the outermost surface of the rolled material in the radial direction in a cross section horizontal to the rolling direction including the center of the rolled material (this specification). (Sometimes referred to as “Cr segregation degree”) satisfies the following formula (3).
Cr segregation degree ≦ 0.1% (3)
Here, the Cr segregation degree is a value of + 2σ of the distribution of Cr content in the rolling direction measured by line analysis using EPMA.

  By controlling the Cr segregation degree to 0.1% or less, variation in Cr concentration can be made sufficiently small (that is, formation of Cr-enriched portion where Cr is enriched can be suppressed), and this A spring manufactured using a rolled material having a small Cr segregation degree can have excellent impact characteristics. When the degree of segregation of Cr in the rolled material is too large, that is, when many Cr concentrated portions are formed in the rolled material, the Cr concentrated portion is tempered compared to the Cr non-concentrated portion where Cr is not concentrated. Since softening and tough ductility recovery due to heat resistance are difficult to occur, temper brittleness is likely to occur, so that impact characteristics may be deteriorated. Therefore, the rolled material of the present invention preferably has a Cr segregation degree of 0.1% or less.

The Cr segregation degree of the rolled material specified in this specification can be measured as follows.
Embedded polishing is performed in a cross section (also referred to as a longitudinal section in the present specification) horizontal in the rolling direction including the center of the rolled material, and a position of 2.0 mm in the radial direction from the outermost surface of the rolled material in the longitudinal section. Then, line analysis by EPMA is performed in the rolling direction, and the Cr amount is measured. The EPMA beam diameter is 1 μm, the length of the line analysis is 250 μm, and the standard deviation (σ) is calculated by calculating the Cr amount at all 250 points as a population, and the standard deviation is doubled (ie, 2 0.0σ) can be the Cr segregation degree of the rolled material.

  In another preferred embodiment, the rolled material of the present invention has a radial length from the outermost surface of the rolled material: 2 mm × length in the rolling direction: 0. 0 mm in a section horizontal to the rolling direction including the center of the rolled material. In the quadrangular region defined by 10 mm, the number of TiNs having a radial size of 8 μm or more is 10 or less. By limiting the number of TiN having a radial size of 8 μm or more to such a range, it is possible to suppress the crack propagation when an impact is applied, and to further improve the impact characteristics.

  If TiN having a radial size of 8 μm or more exists excessively, the progress of cracks is promoted and the impact characteristics may be deteriorated. In addition, TiN tends to occur in the same location as the Cr-concentrated portion having poor toughness, and the presence of a large amount of TiN having a radial size of 8 μm or more in the Cr-concentrated portion further improves impact characteristics. May decrease. Therefore, the rolled material of the present invention has four sides defined by a radial length from the outermost surface of the rolled material: 2 mm × a length in the rolling direction: 0.10 mm in a horizontal section in the rolling direction including the center of the rolled material. The number of TiN having a radial size of 8 μm or more in the shape region is preferably 10 or less.

The number of TiNs defined in this specification can be measured as follows.
The rolled material is cut to a length of about 10 mm, embedded in a resin, subjected to surface polishing, and a horizontal section in the rolling direction including the center of the rolled material is observed using an optical microscope (for example, 200 times magnification). In the observation cross section, such observation is made with a quadrilateral region (0.20 mm ² ) defined by a radial length from the outermost surface of the rolled material: 2 mm × a rolling direction length: 0.10 mm as one observation region. Select 10 areas. And the number of TiN whose radial direction size is 8 μm or more in each observation region is measured according to JIS G 0555, and the average value in 10 observation regions is measured in the radial direction of the target rolled material. The number of TiNs whose size is 8 μm or more.

(Production method)
The rolled material of the present invention has a defined component composition as described above, and the production method is not particularly limited, and can be produced by a general method. That is, after a steel material is melted and cast by a general method to obtain a steel material, for example, homogenization treatment at 1000 to 1300 ° C., partial rolling, and then heating at 1000 to 1300 ° C., for example, followed by hot rolling For example, a rolled material having a diameter of 9 to 25 mm can be obtained.

  In a preferred embodiment, the rolled material of the present invention is a final rolling stand for hot rolling in the above-described basic manufacturing method, in which the steel material after continuous casting is heated at 1100 to 1300 ° C. for 1 to 5 hours for homogenization. It is preferable that the outlet side temperature (final rolling temperature) is 920 ° C. or higher, and the temperature after hot rolling up to 700 ° C. is cooled at an average cooling rate of 2 ° C./s or higher. With such a manufacturing method, a rolled material having a Cr segregation degree of 0.1% or less at a position of 2.0 mm in the radial direction from the outermost surface of the rolled material can be obtained.

  By setting the final rolling temperature to a high temperature of 920 ° C. or higher, the diffusion of Cr is promoted, and the degree of Cr segregation can be reduced. On the other hand, if the average cooling rate up to 700 ° C. after hot rolling is too small, the volume fraction of ferrite produced from the austenite phase increases, excessive concentration of Cr to cementite in the pearlite structure occurs, and the degree of Cr segregation Becomes larger. By setting the average cooling rate up to 700 ° C. after hot rolling to 2 ° C./s or more, excessive generation of ferrite can be suppressed and the degree of Cr segregation can be reduced.

  In a more preferred embodiment, the rolled material of the present invention is obtained by cooling the surface temperature of the steel material after continuous casting to a range of 500 to 800 ° C. in the basic manufacturing method described above, and thereafter the steel material is 1 to 5 at 1100 to 1300 ° C. Homogenization treatment is performed by heating for a period of time, the exit temperature (final rolling temperature) of the final rolling stand of hot rolling is set to 920 ° C. or higher, and after hot rolling up to 700 ° C. at an average cooling rate of 2 ° C./s or higher. It is preferable to cool. With such a manufacturing method, the degree of segregation of Cr is 0.1% or less, and the length in the radial direction from the outermost surface of the rolled material is 2 mm × the length in the rolling direction: 0.10 mm. It is possible to obtain a rolled material in which the number of TiN having a radial size of 8 μm or more in the region is 10 or less.

  By cooling the surface temperature of the steel material after continuous casting to a range of 500 to 800 ° C., N that causes generation of TiN can be precipitated as AlN, so that TiN coarsening can be suppressed. In addition, by cooling the surface temperature of the steel material after continuous casting to a range of 500 to 800 ° C., a large number of fine TiN nuclei can be generated in the steel. It can be held and the degree of Cr segregation can be reduced.

When the surface temperature of the steel material after continuous casting is cooled to less than 500 ° C., cooling of the deep portion of the steel material proceeds, so that there is a possibility that cracking may occur on the surface of the steel material due to heating before partial rolling. For this reason, it is necessary to reduce the heating rate up to the homogenization heat treatment temperature. As a result, the coarsening of TiN proceeds and the impact characteristics deteriorate.
On the other hand, when the surface temperature of the steel material cooled after continuous casting exceeds 800 ° C., generation of AlN is insufficient, and generation of fine TiN is also insufficient, so that coarse TiN is generated after the block rolling, Impact characteristics may deteriorate.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples, and can be implemented with modifications within a range that can meet the purpose described above and below. They are all included in the technical scope of the present invention.

Example 1
Steel materials satisfying the chemical composition shown in Tables 1 and 2 were melted in a converter and homogenized at 1100 to 1300 ° C. after continuous casting. After homogenization, it was subjected to block rolling, then heated at 1100 to 1280 ° C. and then hot rolled to obtain a wire rod having a diameter of 14.3 mm, that is, a rolled material. In Tables 1 and 2, a blank means that no addition is made.

  About the said rolling material, while identifying the structure | tissue in the following ways, tensile strength TS, coarse ferrite decarburization, and surface layer C density | concentration were measured. Further, the rolled material was drawn under the following conditions to evaluate the drawability.

Identification of the structure After buffing the cross-section perpendicular to the rolling direction of the rolled material, etching with a corrosive solution, the microstructure of the entire cross-section is observed with an optical microscope at a magnification of 100 times, and it is a ferrite, pearlite, or supercooled structure Identification of bainite and martensite was performed. Then, the case where the area ratio of the supercooled tissue was 5% or less was evaluated as “OK”, assuming that there was no supercooled structure, and the area ratio exceeding 5% was evaluated as “NG”, assuming that there was a supercooled structure.

Measurement of Tensile Strength (TS) A test piece obtained by cutting a rolled material so that the distance between chucks was 200 mm and the length of each chuck portion was about 50 mm was obtained. Using this test piece, a tensile test was performed according to JIS Z 2241 (2011) at a tensile speed of 5 mm / min, and the tensile strength TS was measured. And the case where said TS was 1170 Mpa or less was evaluated as a pass.

Measurement of coarse ferrite decarburization In a cross section perpendicular to the rolling direction of the rolled material, a region from the outermost surface of the rolled material to a depth of 0.5 mm in the radial direction (this is called the surface layer portion) is magnified with an optical microscope: The entire periphery was confirmed at 400 times, and the depth and width were measured as the size of the ferrite single phase region. The “depth” refers to the distance from the outermost surface in the center direction, that is, the radial direction, and the “width” refers to the distance in the circumferential direction. In Table 3, no. 12 and no. The column of “F decarburization” 18 means that a coarse ferrite single phase having a width exceeding 500 μm and a depth of 20 μm and 30 μm, respectively, was present.

Measurement of surface layer C concentration In a cross section perpendicular to the rolling direction of the rolled material, the amount of C in 20 locations in total is set to 1 μm at regular intervals from the outermost surface of the rolled material to a depth of 20 μm, and the total amount of C is 20 Wire microanalyzer) measured by line analysis. As an EPMA measuring apparatus, an X-ray microanalyzer “JXA-8800 RL” manufactured by JEOL Ltd. was used. And the average value of C amount of 20 places in total was calculated | required. This measurement was performed in four directions in the cross section, that is, at respective positions of 0 degrees, 90 degrees, 180 degrees, and 270 degrees, and the average value of the C amount in these four directions was defined as the surface layer C concentration.

  In the measurement of the coarse ferrite decarburization, when the entire circumference is confirmed, the ferrite single phase having the depth of 20 μm or more and the width of 500 μm or more does not exist, and the surface layer C concentration is as shown in Table 3. It was evaluated that the wire was excellent in atmospheric durability when the value was “50% of the C amount of the parent phase” or more. On the other hand, the case where at least one of the above was not satisfied was evaluated as being poor in atmospheric durability.

Evaluation of wire drawing workability The rolled material was drawn to a diameter of 12.5 mm, that is, cold drawn, and the presence or absence of disconnection in the wire drawing work was evaluated. The case without disconnection was evaluated as “OK”, and the case with disconnection was evaluated as “NG”. About the example which disconnected, the following hardening tempering process and corrosion durability evaluation were not performed.

  The wire drawing material obtained by the above wire drawing was quenched and tempered under the following conditions. Except for 10, a wire having a tensile strength (TS) of 2000 MPa was obtained. No. above. In No. 10, the tensile strength of the wire was 1920 MPa, which was below 1950 MPa, and the wire strength could not be secured, so the corrosion durability was not evaluated. Corrosion durability was evaluated using the wire after quenching and tempering. The tensile strength of the wire is JIS Z2241 at a tensile speed of 5 mm / min using a test piece obtained by cutting the wire so that the distance between chucks is 200 mm and the length of each chuck is about 50 mm. It was determined by conducting a tensile test according to (2011).

(Quenching and tempering conditions)
・ High-frequency heating ・ Heating rate: 150 ° C./second ・ Quenching: 930 ° C. for 20 seconds, water cooling / tempering: 420 to 520 ° C., 20 seconds, water cooling

Evaluation of corrosion durability As a test piece, a quenched and tempered wire was cut to prepare 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 conducted without shot peening on the surface. First, the test piece obtained by the above polishing was subjected to corrosion treatment under the following conditions, and then, as shown in the following rotating bending fatigue test, an Ono rotating bending fatigue test was performed to evaluate the corrosion durability. did.

Corrosion treatment Using 5% NaCl aqueous solution at 35 ° C, spraying with salt water for 8 hours, then drying and holding for 16 hours in a humid environment at 35 ° C and a relative humidity of 60% is one cycle. The cycle was repeated and the specimen was subjected to corrosion treatment.

Rotating bending fatigue test A rotating bending test was performed on the test piece after the corrosion treatment, and corrosion durability was evaluated. No. in Table 3 below. Ten test pieces were used for each, an ono type rotating bending fatigue test was performed with the load stress set to 500 MPa, and the fatigue life until each test piece was broken was measured. And the average value of the fatigue life of 10 test pieces was calculated | required, and it evaluated that the case where this average value of fatigue life was 300,000 times or more was excellent in corrosion durability.

  These results are shown in Table 3.

  The following can be understood from Tables 1 to 3. That is, no. The rolled materials 1-5, 17 and 19-33 all satisfy the specified ranges of the component composition, DCI and CA, so that a desired structure is obtained and surface carbon decarburization is suppressed to ensure surface C concentration. did it. As a result, the air durability of the wire obtained using the rolled material could be secured. Moreover, the wire drawing workability could be secured and further excellent corrosion durability was obtained. In addition, according to the present invention, additional processes such as softening annealing as described in Patent Documents 1 and 2 described above and strict control of the heating temperature are not required, and the productivity is excellent in mass production.

  On the other hand, in examples other than the above, at least one of the component composition, DCI, and CA does not satisfy the specified range, and at least one of the above-described atmospheric durability, wire drawing workability, and corrosion durability. One was inferior. Details are as follows.

  No. In No. 6, since the amount of C was excessive, the strength of the rolled material was high and the wire drawing workability was poor. No. No. 7 was insufficient in corrosion resistance because the amount of C was insufficient, so that precipitates serving as hydrogen trap sites could not be secured.

  No. In No. 8, since the amount of Si was excessive, ferrite decarburization proceeded and the surface layer C concentration could not be ensured, and atmospheric durability could not be obtained. No. No. 9 could not secure excellent corrosion durability because the amount of Si was insufficient. The reason may be that precipitates could not be refined and hydrogen trap sites could not be secured sufficiently.

  No. No. 10 had insufficient wire strength due to insufficient amount of Mn.

  No. No. 11 was insufficient in the amount of Cu, and therefore could not suppress surface layer decarburization and could not secure atmospheric durability. Moreover, corrosion durability was also inferior. No. In No. 12, the amount of Ni was insufficient and the CA exceeded the specified range, and as a result, surface decarburization could not be suppressed and the atmospheric durability was poor. Moreover, it was inferior to corrosion durability.

  No. In No. 13, since the Cr amount was excessive and the DCI exceeded the specified range, a supercooled structure was generated, and the tensile strength of the rolled material was increased. As a result, disconnection occurred during wire drawing. No. No. 14 was insufficient in atmospheric durability because the amount of Cr was insufficient, so that surface decarburization could not be suppressed and the surface C concentration was low.

  No. No. 15 was inferior in corrosion durability because the amount of Ti was insufficient. The reason may be that precipitates that become hydrogen trap sites could not be sufficiently secured.

  No. In No. 16, the content of each element was within the specified range, but since DCI exceeded the specified range, a supercooled structure was generated and the tensile strength of the rolled material was increased. As a result, disconnection occurred during wire drawing.

  No. In No. 18, the content of each element was within the specified range, but since CA exceeded the specified range, surface decarburization could not be suppressed and the atmospheric durability was poor.

  No. 34 to 36 are examples in which the Cr amount is insufficient. In these examples, the surface layer C concentration could not be secured and the atmospheric durability was poor.

(Example 2)
No. shown in Table 1 and Table 2. 1-5 and no. As shown in Table 4, the rolling material which satisfy | fills the chemical component composition of 26-29 prepared 3 each by changing manufacturing conditions. Specifically, Nos. 1 and 2 shown in Tables 1 and 2 are used. 1-5 and no. Steel materials satisfying the chemical composition of 26 to 29 are melted in a converter, and after continuous casting, the obtained steel materials are cooled to the cooling temperature after casting shown in Table 4, and then the heating temperature before the block rolling shown in Table 4 -Homogenization treatment is performed in time, and after that, rolling is performed, then, after heating to 1100-1280 ° C, hot rolling is performed so that the exit temperature of the final rolling stand becomes the final rolling temperature shown in Table 4 A wire having a diameter of 14.3 mm, that is, a rolled material was obtained.
In addition, No. shown in Table 4 1-A, 1-B and 1-C are Nos. Described in Table 1 and Table 2 described above, respectively. 1 is a rolled material satisfying the same chemical composition as 1 and prepared under different production conditions. The same applies to other rolled materials shown in Table 4.

  As shown in Table 4, no. 1-A to 5-A and 26-A to 29-A are manufactured under basic manufacturing conditions according to the embodiment of the present invention. No. 1-B to 5-B and 26-B to 29-B are rolled materials manufactured under the preferable manufacturing conditions described above, and conditions for reducing the degree of Cr segregation (that is, the heating temperature before block rolling is 1100 to 1300). It is manufactured so that the final rolling temperature is set to 920 ° C. or higher and cooled to 700 ° C. after hot rolling at an average cooling rate of 2 ° C./s or higher. No. 1-C to 5-C and 26-C to 29-C are manufactured under the more preferable manufacturing conditions described above. Conditions for reducing the degree of Cr segregation and conditions for refining TiN (that is, cooling after casting) The temperature is set to 500 to 800 ° C., the heating temperature before split rolling is set to 1100 to 1300 ° C. and heated for 1 to 5 hours, the final rolling temperature is set to 920 ° C. or higher, and the hot rolling up to 700 ° C. is 2 ° C./s or higher. (Cooled at an average cooling rate).

About the said rolling material, the magnitude | size of the radial direction in the quadrilateral area | region prescribed | regulated by Cr segregation degree, radial direction length from the outermost surface of a rolling material: 2 mm x rolling direction length: 0.10 mm in the following ways. The number of TiN of 8 μm or more (indicated as “TiN number of 8 μm or more” in Table 5) and Charpy absorbed energy (indicated as “impact value vE- ₅₀ ” in Table 5) were determined.

(Cr segregation degree)
Embedded polishing is performed with a horizontal section (longitudinal section) in the rolling direction including the center of the rolled material, and in the longitudinal section, from the position of 2.0 mm in the radial direction from the outermost surface of the rolled material, toward the rolling direction. Line analysis by EPMA was performed, and the Cr amount was measured. More specifically, the EPMA beam diameter is 1 μm, the length of the line analysis is 250 μm, and the standard deviation (σ) is calculated by calculation using the Cr amount at all 250 points as a population, and the standard deviation is doubled. The obtained value (that is, 2.0σ) was used as the degree of Cr segregation of the rolled material.
Table 5 shows the measurement results.

(The radial length from the outermost surface of the rolled material: 2 mm × the rolling direction length: the number of TiN having a radial size of 8 μm or more in a quadrilateral region defined by 0.10 mm)
The rolled material was cut into a length of about 10 mm, embedded in a resin, surface-polished, and a cross section horizontal to the rolling direction including the center of the rolled material was observed with an optical microscope at a magnification of 200 times. More specifically, in the observation cross section, a quadrilateral region (0.20 mm ² ) defined by a radial length from the outermost surface of the rolled material: 2 mm × a rolling direction length: 0.10 mm is used as one observation region. Ten such observation regions were selected. And the number of TiN whose radial direction size in each observation region is 8 μm or more is measured according to JIS G 0555, and the average value in 10 observation regions is measured in the radial direction of the rolling material to be measured. The number of TiN was 8 μm or more. Table 5 shows the measurement results.

(Charpy absorbed energy)
An impact test piece with a 5 mmU notch was collected from each rolled material, and Charpy absorbed energy (vE- ₅₀ ) at -50 ° C was determined according to JIS Z 2242. Two tests were performed for each rolled material, and the average value was defined as the Charpy absorbed energy of each rolled material. Table 5 shows the measurement results.

As shown in Table 4 and Table 5, the rolling material No. manufactured by satisfying the Cr segregation degree reduction condition. 1-B to 5-B and No.1. Each of 26-B to 29-B had a Cr segregation degree of 0.1% or less, a vE- ₅₀ of 70 J / cm ² or more, and exhibited excellent impact characteristics.
In addition, rolled material No. 1 manufactured to satisfy the conditions for reducing the degree of segregation of Cr and the conditions for refining TiN. 1-C-5-C and no. Each of 26-C to 29-C is a quadrilateral defined by Cr segregation degree of 0.1% or less and a radial length from the outermost surface of the rolled material: 2 mm × length in the rolling direction: 0.10 mm. In the region, the number of TiN having a radial size of 8 μm or more was 10 or less, and vE- ₅₀ was 85 J / cm ² or more, which showed more excellent impact characteristics.

Claims (7)

% By mass
C: 0.55-0.63%,
Si: 1.90-2.30%,
Mn: 0.15 to 0.55%,
P: more than 0%, 0.015% or less,
S: more than 0%, 0.015% or less,
Al: 0.001 to 0.1%,
Cu: 0.15-0.45%,
Ni: 0.35 to 0.75%,
Cr: 0.36-0.65%, and Ti: 0.04-0.11%
And the balance is iron and inevitable impurities, the ideal critical diameter DCI represented by the following formula (1) is 120 or less, and the decarburization index CA represented by the following formula (2) is 70 or less. A high-strength rolled material for springs, characterized in that it exists.
DCI = 25.4 × (0.171 + 0.001 × [C] + 0.265 × [C] ² )
× (3.3333 × [Mn] +1) × (1 + 0.7 × [Si])
× (1 + 0.363 × [Ni]) × (1 + 2.16 × [Cr])
× (1 + 0.365 × [Cu]) × (1 + 1.73 × [V]) × (1 + 3 × [Mo]) (1)
CA = 98 × [Si] −171 × [Mn] −40 × [Cr] −142 × [Ni] −60 × [Cu] (2)
In said formula (1) and formula (2), [element name] means content in steel in the mass% of each element.   In the cross section perpendicular to the rolling direction of the rolled material, the C concentration in the region from the outermost surface of the rolled material to the depth of 20 μm in the radial direction is 50% or more of the C concentration of the parent phase, and martensite and bainite in the entire structure When a region from the outermost surface of the rolled material to a depth of 0.5 mm in the radial direction is observed, there is no ferrite single phase having a depth of 20 μm or more and a width of 500 μm or more. The rolled material for high-strength springs according to claim 1, further having a tensile strength of 1170 MPa or less.   The rolled material for high-strength springs according to claim 1 or 2, wherein the ideal critical diameter DCI is 70 or more.   The rolled material for high-strength springs according to any one of claims 1 to 3, further comprising, in mass%, B: more than 0% and 0.01% or less.   Furthermore, at least 1 selected from the group consisting of V: more than 0%, not more than 0.3%, Nb: more than 0%, not more than 0.3%, and Mo: more than 0%, not more than 0.5% by mass%. The rolled material for high-strength springs according to any one of claims 1 to 4, comprising seeds. In a horizontal cross section in the rolling direction including the center of the rolled material, the Cr segregation degree at a position of 2.0 mm in the radial direction from the outermost surface of the rolled material satisfies the following formula (3): Item 6. The rolled material for high-strength springs according to any one of Items 1 to 5.
Cr segregation degree ≦ 0.1% (3)
Here, the degree of Cr segregation is a value of + 2σ of the distribution of Cr content in the rolling direction measured by line analysis using EPMA.   In a cross section horizontal to the rolling direction including the center of the rolled material, the radial size in the quadrilateral region defined by the radial length from the outermost surface of the rolled material: 2 mm × the length in the rolling direction: 0.10 mm The rolled material for high-strength springs according to claim 6, wherein the number of TiN having a thickness of 8 μm or more is 10 or less.