阅读说明：本技术 弹簧钢用轧制线材 (Rolled wire for spring steel ) 是由 铃木崇久 根石丰 小泽修司 于 2018-06-15 设计创作，主要内容包括：该弹簧钢用轧制线材的化学成分以质量％计含有C：0.42～0.60％、Si：0.90～3.00％、Mn：0.10～1.50％、Cr：0.10～1.50％、B：0.0010～0.0060％、N：0.0010～0.0070％、Mo：0～1.00％、V：0～1.00％、Ni：0～1.00％、Cu：0～0.50％、Al：0～0.100％、Ti：0～0.100％、Nb：0～0.100％,P限制为低于0.020％、S限制为低于0.020％,剩余部分包含Fe及杂质,其中,碳当量(Ceq)为0.75～1.00％,金属组织包含以面积分率计为90％以上的回火马氏体及贝氏体,抗拉强度为1350MPa以下,并且断面收缩率值为40％以上。(The rolled wire rod for spring steel contains, in mass%, C: 0.42 to 0.60%, Si: 0.90-3.00%, Mn: 0.10 to 1.50%, Cr: 0.10-1.50%, B: 0.0010-0.0060%, N: 0.0010-0.0070%, Mo: 0-1.00%, V: 0-1.00%, Ni: 0-1.00%, Cu: 0-0.50%, Al: 0-0.100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, P is limited to less than 0.020%, S is limited to less than 0.020%, and the balance contains Fe and impurities, wherein the carbon equivalent (Ceq) is 0.75 to 1.00%, the microstructure contains tempered martensite and bainite in an area fraction of 90% or more, the tensile strength is 1350MPa or less, and the reduction of area value is 40% or more.)

1. A rolled wire rod for spring steel, which comprises the following chemical components in mass%:

C：0.42～0.60％、

Si：0.90～3.00％、

Mn：0.10～1.50％、

Cr：0.10～1.50％、

B：0.0010～0.0060％、

N：0.0010～0.0070％、

Mo：0～1.00％、

V：0～1.00％、

Ni：0～1.00％、

Cu：0～0.50％、

Al：0～0.100％、

Ti：0～0.100％、

Nb：0～0.100％，

p is limited to less than 0.020%,

S is limited to less than 0.020%,

the remainder comprising Fe and impurities,

wherein the carbon equivalent (Ceq) defined by the following formula (1) is 0.75 to 1.00%,

the microstructure includes tempered martensite and bainite in an area fraction of 90% or more,

a tensile strength of 1350MPa or less and a reduction of area of 40% or more,

Ceq＝[C％]+「Si％」/24+[Mn％]/6+[Cr％]/5+[Mo％]/4+[V％]/14+「Ni％」/40(1)。

2. the rolled wire rod for spring steel according to claim 1, further containing Mo: 0.10-1.00%, V: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Cu: 0.05 to 0.50%, Al: 0.005-0.100%, Ti: 0.005-0.100%, Nb: 0.005-0.100% of 1 or more than 2.

Technical Field

The present invention relates to a rolled wire rod for spring steel.

The present application claims priority based on Japanese application No. 2017-118110, applied in 2017, 06, 15, the contents of which are incorporated herein by reference.

Background

As automobiles have become higher in performance and lighter in weight, springs used for automobile parts have also become higher in strength. In order to increase the strength of springs, high-strength steels having tensile strengths exceeding 1800MPa have been used for the manufacture of springs after heat treatment. In recent years, steels having a tensile strength of more than 2000MPa have also come to be used as spring materials.

On the other hand, suspension springs for automobiles are required to have not only high strength but also high toughness so as not to be broken even under an impact load due to irregularities on road surfaces.

In recent years, with the demand for further higher strength of springs, a method has been proposed in which both strength and toughness are achieved.

For example, patent document 1 discloses a method of controlling carbide precipitation after quenching and tempering by optimizing the amount of addition of alloying elements, thereby achieving both high strength and high toughness. However, the chemical composition of the steel material and the contents other than the quenching and tempering step are not particularly mentioned, and the influence of the microstructure of the wire rod rolling step and the rolled wire rod, which are the steps prior to the quenching and tempering, on the material after the quenching and tempering is not mentioned.

Patent document 2 mentions a structure before rolling, and shows that the drawability of a rolled wire rod is improved and the hydrogen embrittlement resistance after quenching and tempering is improved by mainly using ferrite and pearlite and reducing martensite and bainite. However, there is no mention of the relationship between mechanical properties such as strength and toughness and the microstructure of the rolled wire rod.

Disclosure of Invention

Problems to be solved by the invention

The invention provides a rolled wire rod for spring steel, which is suitable for spring steel having a tensile strength of 2000MPa or more and high toughness after heat treatment such as quenching and tempering.

Means for solving the problems

The present inventors have conducted studies and found that: by controlling not only the chemical composition but also the structure of the rolled wire rod, a high-strength and high-toughness spring steel can be obtained by the subsequent quenching and tempering heat treatment. The present invention is mainly made of steel as shown below.

(1) According to one aspect of the present invention, a rolled wire rod for spring steel contains, in terms of mass%, C: 0.42 to 0.60%, Si: 0.90-3.00%, Mn: 0.10 to 1.50%, Cr: 0.10-1.50%, B: 0.0010-0.0060%, N: 0.0010-0.0070%, Mo: 0-1.00%, V: 0-1.00%, Ni: 0-1.00%, Cu: 0-0.50%, Al: 0-0.100%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, P is limited to less than 0.020%, S is limited to less than 0.020%, and the balance contains Fe and impurities, wherein the carbon equivalent (Ceq) defined by the following formula (1) is 0.75 to 1.00%, the metal structure contains tempered martensite and bainite in an area fraction of 90% or more, the tensile strength is 1350MPa or less, and the reduction of area value is 40% or more.

Ceq＝[C％]+「Si％」/24+[Mn％]/6+[Cr％]/5+[Mo％]/4+[V％]/14+「Ni％」/40 (1)

(2) The rolled wire rod for spring steel according to the above (1), wherein the chemical component may further contain, in mass%: 0.10-1.00%, V: 0.05 to 1.00%, Ni: 0.05 to 1.00%, Cu: 0.05 to 0.50%, Al: 0.005-0.100%, Ti: 0.005-0.100%, Nb: 0.005-0.100% of 1 or more than 2.

Effects of the invention

According to the rolled wire rod for spring steel of the present invention, a spring steel exhibiting a tensile strength of 2000MPa or more and high toughness can be obtained by performing heat treatment such as quenching and tempering. That is, the rolled wire rod for spring steel according to the above aspect of the present invention can be suitably used as a material for high-strength and high-toughness spring steel. In particular, it can be suitably used as a material for spring steel such as a suspension spring.

Drawings

Fig. 1A is a photograph (magnification 400 times) of a structure showing an example of the structure of a rolled wire rod for spring steel according to the present embodiment.

Fig. 1B is a photograph (magnification: 1000 times) of a structure showing an example of the structure of the rolled wire rod for spring steel according to the present embodiment.

Fig. 2A is a photograph (magnification 400 times) of a structure showing an example of the structure of a conventional rolled wire rod for spring steel.

Fig. 2B is a photograph (magnification: 1000 times) of a structure showing an example of the structure of a conventional rolled wire rod for spring steel.

Detailed Description

The inventors of the present invention have studied a rolled wire rod for spring steel, which is a material for obtaining spring steel having sufficient toughness even when the tensile strength after quenching and tempering is high at 2000MPa or more.

As a result, the present inventors have found that: in order to obtain a spring steel having both high strength and high toughness after quenching and tempering, it is effective to control the microstructure of a rolled wire rod for a spring steel before quenching and tempering.

Generally, spring steel (suspension spring steel) is adjusted to a desired strength by wire drawing a rolled wire rod, adjusting the wire rod to a desired wire diameter while increasing the roundness, and then performing quenching and tempering. Therefore, the rolled wire rod is generally adjusted to have a soft pearlite or a complex phase structure including ferrite and pearlite which is excellent in wire drawability. In the case where soft ferrite and pearlite are mixed with hard bainite and martensite in a rolled wire rod, since the soft phase and the hard phase have different deformation behaviors and there is a possibility of wire breakage during wire drawing, conventionally, control is performed so that bainite and martensite are not mixed in the structure of the rolled wire rod.

On the other hand, in recent years, there has been a demand for improving the tensile strength of a suspension spring steel obtained by quenching and tempering a rolled wire rod. In order to increase the tensile strength after quenching and tempering, it is conceivable to add an alloy element such as Cr, Mo, or V, which increases hardenability. However, if the hardenability is high, bainite and martensite are easily generated in cooling after rolling, and soft ferrite and pearlite, and hard bainite and martensite are easily mixed in the rolled wire rod. Therefore, the following methods have been conventionally adopted: by reducing the cooling rate after rolling and adjusting the alloy composition, the mixed presence of bainite and martensite in the structure of the rolled wire rod is suppressed.

In contrast, the present invention is characterized in that hot-rolled wire rods are directly fed into a cooling water tank and subjected to in-line quenching to form a structure including bainite and martensite as main phases, and then subjected to softening annealing to ensure wire drawability. The martensite produced by the in-line quenching is changed into tempered martensite by the softening annealing. Therefore, the rolled wire rod for spring steel according to the present invention has a structure including 90% or more of bainite and tempered martensite.

As described above, conventionally: it is not preferable that bainite and martensite are mixed in the structure of the rolled wire rod. However, the inventors of the present invention newly found that: even if the structure after rolling is a structure including bainite and martensite as main phases, the tensile strength is not more than a certain level and the reduction in area is not less than a certain level by soft annealing, and thus wire drawability equivalent to that of the structure of pearlite can be ensured. In addition, it has been found that: if the cooling rate after rolling is insufficient, hardenability is insufficient due to the influence of chemical components of the steel material, and thus ferrite and pearlite are present in a certain amount or more in a mixture with bainite and martensite, the wire drawability is lowered.

Further, the present inventors have studied and found that: by making the rolled structure into bainite and martensite main phases and annealing the bainite and tempered martensite main phases, carbides in the steel material can be uniformly and finely dispersed as compared with conventional pearlite. By forming the structure of the rolled wire rod into such a structure, the solid solution of carbide is facilitated in the quenching and tempering treatment of the rolled wire rod for spring steel. As a result, the prior austenite grain size can be made finer by lowering the quenching temperature, and the remaining of undissolved carbide after quenching can be suppressed. Namely, the present inventors have found that: in the rolled wire rod, the rolled structure is made into a bainite and martensite main phase, and the bainite and tempered martensite are used as the main phase by annealing, whereby the toughness after quenching and tempering is also improved.

Thus, the present inventors have found that: by making the structure after rolling mainly into bainite and martensite and then performing softening annealing, it is possible to improve the mechanical properties (high strength and high toughness) after quenching and tempering while ensuring wire drawability in the subsequent step (wire drawing treatment) performed for producing spring steel.

A rolled wire rod for spring steel according to an embodiment of the present invention based on this finding (a rolled wire rod for spring steel according to the present embodiment) will be described below.

The reason for limiting the chemical composition of the rolled wire rod for spring steel according to the present embodiment will be described.

[C：0.42～0.60％]

C is an element that greatly affects the strength of steel. The C content is set to 0.42% or more in order to impart sufficient strength to the steel after quenching and tempering. The C content is preferably 0.43% or more, more preferably 0.45% or more.

On the other hand, if the C content is excessive, the non-transformed austenite (retained austenite) increases in the steel after the quenching and tempering, and the strength-increasing effect by the C content decreases. In addition, toughness is significantly reduced. Therefore, the C content is set to 0.60% or less. The C content is preferably 0.58% or less.

[Si：0.90～3.00％]

Si is an element that increases the strength of spring steel produced from a rolled wire rod for spring steel, and particularly suppresses softening at the time of tempering performed after quenching. Further, Si is an element that improves resistance to a shape change in use of the spring, that is, a decrease in spring force (spring force decrease resistance characteristic). In order to obtain such an effect, the Si content in the rolled wire rod for spring steel of the present embodiment is set to 0.90% or more. The Si content is preferably 1.20% or more, more preferably 1.40% or more.

On the other hand, if the Si content is excessive, the steel is significantly embrittled. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less.

[Mn：0.10～1.50％]

Mn is an element that improves hardenability of steel, and is an element necessary for obtaining bainite and martensite in direct quenching after hot rolling. In order to obtain such an effect, the Mn content in the rolled wire rod for spring steel of the present embodiment is set to 0.10% or more. The Mn content is preferably 0.30% or more.

On the other hand, if the Mn content is excessive, the soft retained austenite increases after quenching and tempering, and the tensile strength decreases. In the rolled wire rod for spring steel of the present embodiment, the Mn content is set to 1.50% or less in order to suppress the generation of retained austenite. The Mn content is preferably 1.00% or less, and more preferably 0.70% or less.

[Cr：0.10～1.50％]

Cr is an element necessary for improving hardenability of steel and for obtaining bainite and martensite in direct quenching after hot rolling. Cr is an element necessary for controlling the precipitation state of carbide and ensuring the strength of the steel after quenching and tempering. In order to obtain such effects, the content of Cr in the rolled wire rod for spring steel of the present embodiment is set to 0.10% or more. The Cr content is preferably 0.30% or more, and more preferably 0.50% or more.

On the other hand, if the Cr content is excessive, the soft retained austenite increases after quenching and tempering, the tensile strength decreases, and the steel material becomes brittle. Therefore, in the rolled wire rod for spring steel of the present embodiment, the Cr content is set to 1.50% or less. The Cr content is preferably 1.00% or less.

[B：0.0010～0.0060％]

B is an element necessary for improving hardenability of steel and for obtaining bainite and martensite in direct quenching after hot rolling. Further, B is an element that preferentially segregates at prior austenite grain boundaries that are likely to become starting points of fracture, thereby suppressing segregation of P, S, and the like into the grain boundaries, and as a result, contributes to an increase in grain boundary strength and an improvement in toughness. In order to obtain these effects, the B content in the rolled wire rod for spring steel of the present embodiment is set to 0.0010% or more. The content of B is preferably 0.0020% or more.

On the other hand, even if B is contained excessively, not only these effects are saturated, but also Fe may be precipitated at grain boundaries₂₃(CB)₆And the toughness of the steel is lowered. Therefore, the B content is set to 0.0060% or less. The content of B is preferably 0.0050% or less.

[N：0.0010～0.0070％]

N is an element that generates various nitrides in steel. The nitride particles that are stable even at high temperatures contribute to the refinement of prior austenite grains due to the pinning effect of the austenite grain growth. In the rolled wire rod for spring steel of the present embodiment, the N content is set to 0.0010% or more. The N content is preferably 0.0020% or more.

On the other hand, if the N content is excessive, coarse nitrides which become starting points of fracture are formed, and toughness and fatigue characteristics are degraded. When the N content is excessive, N is linked with B to generate BN, and the amount of solid solution B is reduced. If the amount of solid-solution B is reduced, the above-mentioned effect of improving hardenability and the effect of improving grain boundary strength by B may be impaired. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0060% or less.

[ P: less than 0.020% ]

P is an element which is present in steel as an impurity element and embrittles the steel. In particular, P segregated in the prior austenite grain boundary causes a reduction in the grain boundary strength and causes embrittlement of the steel. Therefore, the P content is preferably small. In order to prevent embrittlement of the steel, the P content in the rolled wire rod for spring steel of the present embodiment is limited to less than 0.020%. The P content is preferably 0.015% or less.

[ S: less than 0.020% ]

S is an element which is present as an impurity element in steel and embrittles the steel, as in P. S can be fixed as MnS by containing Mn, but MnS, if coarsened, acts as a fracture origin and deteriorates the fracture characteristics of the steel. In order to suppress these adverse effects, the S content is preferably small, and the S content is limited to less than 0.020% in the rolled wire rod for spring steel of the present embodiment. The S content is preferably 0.015% or less, and more preferably 0.010% or less.

The rolled wire rod for spring steel according to the present embodiment basically contains the above elements, and the remainder contains Fe and impurities. However, instead of part of Fe, 1 or 2 or more of Mo, V, Ni, Cu, Al, Ti, and Nb may be further contained. In particular, Mo, V, Ni, Cu, Al, Ti, and Nb are optional elements, and the chemical composition of the steel of the present embodiment may not contain these elements. Therefore, the lower limit of the content of each of Mo, V, Ni, Cu, Al, Ti and Nb is 0%.

The impurities are components mixed from raw materials such as ores and scraps or from various environments in a manufacturing process in the industrial production of steel materials, and are components that are allowed within a range that does not adversely affect steel.

[Mo：0～1.00％]

Mo is an element effective for improving hardenability of steel and for obtaining bainite and martensite in direct quenching after hot rolling. Further, the element is effective for controlling the precipitation state of carbide and ensuring the strength of the steel after quenching and tempering. In order to obtain such an effect, the Mo content may be set to 0.10% or more. On the other hand, in the case where the Mo content exceeds 1.00%, these effects are saturated. Since Mo is a high-priced element, it is not preferable to contain Mo more than necessary, and therefore even when Mo is contained, the Mo content is set to 1.00% or less. The Mo content is preferably 0.60% or less.

[V：0～1.00％]

V is an element effective for improving hardenability of steel and for obtaining bainite and martensite in direct quenching after hot rolling. Further, the element is effective for controlling the precipitation state of carbide and ensuring the strength of the steel after quenching and tempering. In order to obtain such an effect, the V content may be set to 0.05% or more. On the other hand, when the V content exceeds 1.00%, coarse undissolved precipitates are formed and the steel becomes brittle. Therefore, even when V is contained, the upper limit of the V content is set to 1.00% or less. The preferable upper limit of the V content is 0.50% or less.

[Ni：0～1.00％]

Ni is an element that improves hardenability of steel, and also has an effect of improving corrosion resistance of steel. In order to obtain these effects, the Ni content may be set to 0.05% or more, and more preferably 0.10% or more, in the rolled wire rod for spring steel of the present embodiment. On the other hand, if the Ni content is excessive, the soft retained austenite increases after quenching and tempering, and the tensile strength decreases. Therefore, even when Ni is contained, the Ni content is set to 1.00% or less. The preferable upper limit of the Ni content is 0.50% or less.

[Cu：0～0.50％]

Cu is an element that improves the hardenability of steel, and also has an effect of improving the corrosion resistance of steel. In order to obtain these effects, the Cu content in the rolled wire rod for spring steel according to the present embodiment may be set to 0.05% or more, and more preferably 0.10% or more. On the other hand, if the Cu content is excessive, the hot ductility of the steel decreases, which may cause cracking during hot rolling. Therefore, even when Cu is contained, the Cu content is set to 0.50% or less. The preferable upper limit of the Cu content is 0.30% or less.

[Al：0～0.100％]

Al is an element used as a deoxidizing element, and reacts with N in steel to form AlN. Since AlN suppresses coarsening by pinning the growth of austenite grains during heat treatment, Al is an element effective for grain refinement. In addition, Al also has an effect of suppressing the formation of BN by fixing N, thereby improving the effect of B. In order to obtain these effects, the Al content may be set to 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Al content is excessive, coarse AlN is generated and toughness is reduced. The Al content in the rolled wire rod for spring steel of the present embodiment is set to 0.100% or less. The Al content is preferably 0.050% or less, and more preferably 0.035% or less.

[Ti：0～0.100％]

Ti reacts with N, C in the steel to form TiN and TiC, and suppresses the growth of austenite grains during heat treatment to inhibit coarsening. Therefore, Ti is an element effective for grain refinement. Ti also has an effect of suppressing the formation of BN by fixing N, thereby improving the effect of B. In order to obtain these effects, the Ti content may be set to 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ti content is excessive, coarse TiN is formed and the toughness is lowered. Therefore, even when Ti is contained in the rolled wire rod for spring steel according to the present embodiment, the Ti content is set to 0.100% or less. The Ti content is preferably 0.070% or less.

[Nb：0～0.100％]

Nb is an element that reacts with N, C in steel to form Nb (cn), and suppresses coarsening by pinning the growth of austenite grains during heat treatment, and is effective for grain refinement. Nb also has an effect of suppressing the formation of BN by fixing N, thereby improving the effect of B. In order to obtain these effects, the Nb content may be set to 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Nb content is excessive, coarse Nb (cn) is generated and the toughness is lowered. In the rolled wire rod for spring steel of the present embodiment, the Nb content is set to 0.100% or less even when Nb is contained. The Nb content is preferably 0.050% or less.

The rolled wire rod for spring steel according to the present embodiment is characterized in that bainite and martensite are obtained in direct quenching after hot rolling. Therefore, Ceq (carbon equivalent) calculated by the following formula (1) is set to 0.75% or more in order to secure hardenability. The lower limit of Ceq is preferably 0.80% or more. Further, if Ceq is too high, there is a problem of seizure and increase in retained austenite during quenching. Further, if Ceq is too high, there is a possibility that undissolved carbides remain when the rolled wire rod for spring steel is quenched and tempered. Therefore, the upper limit of Ceq is set to 1.00% or less. The upper limit of Ceq is preferably 0.90% or less. The symbol of the element in the formula (1) is substituted for the mass% of each element. That is, for example, if [ C% ], the content of C in mass% is substituted. In the case of steel not actively containing Mo, V, or Ni, [ Mo% ], [ V% ] or "Ni%" is substituted for 0%.

Ceq＝[C％]+「Si％」/24+[Mn％]/6+[Cr％]/5+[Mo％]/4+[V％]/14+「Ni％」/40 (1)

The metal structure of the rolled wire rod for spring steel according to the present embodiment is a structure in which the total of bainite and tempered martensite is 90% or more, and more preferably 95% or more in terms of an area fraction. The sum of bainite and tempered martensite may be 100%. The area ratio of each of bainite and tempered martensite is not necessarily limited. The remaining portion of the structure is 0% or more and less than 10%, more preferably 0% or more and less than 5%. The rest part of the structure comprises 1 or more than 2 of ferrite, pearlite and retained austenite. If the total area ratio of bainite and tempered martensite is less than 90% (the remaining structure is 10% or more), ductility decreases, the reduction in area value in the tensile test decreases, and wire drawability decreases.

The metal structure is formed by quenching after hot rolling and then softening annealing for adjusting strength.

The rolled wire rod for spring steel of the present embodiment has a tensile strength of 1350MPa or less and a reduction in area value of 40% or more. If the tensile strength exceeds 1350MPa or the reduction of area value becomes lower than 40%, the steel tends to break during drawing at the time of production of spring steel. Since the tensile strength of the rapidly cooled rolled wire rod is high, softening annealing is performed so that the tensile strength becomes 1350MPa or less in order to set the strength suitable for wire drawing. The tensile strength is 1350MPa or less and the reduction of area is 40% or more by the softening annealing.

The metal structure of the rolled wire rod for spring steel was observed by collecting a structure observation test piece from the rolled wire rod for spring steel. Specifically, the rolled wire rod for spring steel was cut at the center L-section, etched with 3% nitroethanol (3% nitric acid-ethanol solution) after forming and polishing, and observed with a metallographic microscope at a magnification of 400 times at an observation position of only a position inside 1/4 having a diameter equal to the surface of the rolled wire rod in the L-section, and the obtained area ratios were averaged.

The observed structure separation was judged as "bainite and tempered martensite", "ferrite" and "pearlite", and the area fraction of "bainite and tempered martensite" was determined. Since it is difficult to identify bainite and tempered martensite, it is sufficient to treat them together.

Fig. 1A and 1B show an example of the structure of the rolled wire rod for spring steel according to the present embodiment, and the structure includes bainite and tempered martensite. On the other hand, fig. 2A and 2B are examples of the structure of a conventional rolled wire rod for spring steel, and are structures including ferrite and pearlite.

The tensile strength was measured by a tensile test method according to JIS Z2241 using a No. 2 round bar test piece, and the maximum tensile strength before breaking was measured. The reduction of area value is measured from the diameter of the maximum reduction of area after fracture.

Next, an example of the method for producing a rolled wire rod for spring steel according to the present embodiment will be described. The rolled wire rod for spring steel of the present embodiment can obtain this effect as long as it has the above-described configuration regardless of the production method, but is preferably obtained stably by the following production method, for example.

The steel ingot having the above chemical composition is heated at a temperature of, for example, 950 to 1200 ℃ for a time not exceeding 120 minutes, and hot rolled to produce a rolled wire rod having a wire diameter of about 12 to 18mm (hot rolling step). The rolled wire rod in the red hot state is processed so as to be in a loop shape suitable for winding, and then is put into a water tank (cooling step).

The temperature for finishing rolling in the hot rolling step is set to 900 to 1000 ℃, and the time from finishing rolling to putting into the water tank is set to 30 seconds or less.

In the cooling step, the rolled wire rod put into the water tank is cooled to 200 ℃ or lower. The rolled wire rod is cooled to 200 ℃ or lower and then taken out from a water tank, thereby being cooled at an average cooling rate of 5 to 30 ℃/sec. The heating temperature of the steel material, the rolling completion temperature of the steel material, and the temperature of the steel material during cooling are set to the surface temperature of the steel material. The average cooling rate is an average cooling rate having a numerator of a temperature difference between the temperature of the steel material at the start of cooling and the cooling completion temperature and a denominator of a time difference between the start time of cooling and the completion time of cooling. When the cooling start is set to be put into the water tank, the cooling end is set to be pulled out from the water tank.

The hot rolling step and the subsequent cooling step form a microstructure having bainite and martensite as main phases. When the rolling completion temperature is less than 900 ℃ or more than 1000 ℃ and the average cooling rate during cooling is less than 5 ℃/sec, ferrite and pearlite are likely to be precipitated, and the area fraction of bainite and martensite is lowered. The average cooling rate is preferably 10 ℃/sec or more. The higher the average cooling rate, the better, but since the effect is saturated when it exceeds 30 ℃/sec, the upper limit is set to 30 ℃/sec or less.

The cooled rolled wire rod is subjected to softening annealing under conditions of 300 to 500 ℃ for 2 to 24 hours so that the rolled wire rod has a drawable strength, that is, a tensile strength of 1350MPa or less. By the softening annealing, the martensite becomes tempered martensite. Under the annealing conditions, the tensile strength can be set to 1350MPa or less, and the reduction of area can be set to 40% or less.

By the above-described manufacturing method, the rolled wire rod for spring steel according to the present embodiment can be manufactured.

In order to obtain spring steel from the above-described rolled wire rod for spring steel, the rolled wire rod for spring steel is quenched and tempered after wire drawing. The quenching is preferably performed by high-frequency quenching. The condition for quenching and tempering is preferably set such that the tensile strength of the spring steel becomes 2000MPa or more. According to the rolled wire rod for spring steel of the present embodiment, even if the tensile strength is set to 2000MPa or more by quenching and tempering, it is possible to obtain a rolled wire rod having high toughness at 23 ± 5 ℃, for example, 60.0J/cm²Spring steel having a charpy impact value of above.