ES2565857B1 - High strength spring steel that has excellent wire rod rolling properties - Google Patents

Abstract

The present invention relates to a high strength spring steel having excellent wire rod rolling properties, consisting essentially of, in terms of mass%: C: from 0.40% to 0.65%; Yes: from 1.20% to 2.80%; Mn: from 0.30% to 1.20%; P: 0.020% or less; S: 0.020% or less; Cu: 0.40% or less; Ni: 0.80% or less; Cr: 0.70% or less; Ti: from 0.060% to 0.134%; At: 0.10% or less; N: 0.010% or less; and O: 0.0015% or less, and optionally: B from 0.0005% to 0.0050%, the remainder being Fe and unavoidable impurities, in which the content in terms of mass% of the specified chemical components meets the following expressions (1) to (3): # X1 = 0.14 x [Si] - 0.11 x [Mn] - 0.05 x [Cu] - 0.11 x [Ni] - 0.03 x [ Cr] + 0.02 <= 0.2… Expression (1) # X2 = ({al} - 500) / {be}> = 3.0… Expression (2) # {al} = 912 - 231 x [C] + 32 x [Si] - 20 x [Mn] - 40 x [Cu] - 18 x [Ni] - 15 x [Cr] # {be} = 10 ^ (0.322 - 0.538 x [C] + 0.018 x [Yes ] + 1,294 x [Mn] + 0.693 x [Cu] + 0.609 x [Ni] + 0.847 x [Cr]) # X3 = 31 x [C] + 2.3 x [Si] + 2.3 x [Mn] + 1.25 x [Cu] + 2.68 x [Ni] + 3.57 x [Cr] - 6 x [Ti]> = 24.0… Expression (3)

Description

STEEL FOR HIGH RESISTANCE SPRINGS THAT HAS A PROPERTY OF EXCELLENT WIRE ROLLING

FIELD OF THE INVENTION The present invention relates to a high strength spring steel having excellent wire rod rolling properties.

BACKGROUND OF THE INVENTION

On the occasion of the search for weight reduction of suspension springs in response to the demands of weight reduction of vehicles (automobiles), the development of springs that allow a stress of allo design has been pursued. In order to raise the design tension of the springs, it is necessary to engineer improvements in various spring characteristics, and more specifically, it is essential to add alloy elements. For example, you can think of adding Si when you want to improve the sedimentation property, while you can think of adding an element such as Cu, Ni or Cr when you want to improve corrosion resistance

Incidentally, the increase in amounts of alloy elements added in order to improve the spring characteristics tends to produce detriments such as the appearance of decarburization of ferrite and the formation of bainite during cooling after rod rolling. The detriment formed is fatal for the springs to which a grenade must be given, while the latter detriment can become a detrimental factor at the time of secondary work, and therefore it becomes important to avoid both detriments. As techniques to avoid both detriments, there have been known techniques described, for example, in the following patent documents 1 and 2

The following patent document 1 has disclosed the technique of heating a steel material at a temperature of 1,170 oC or more for at least 2 minutes according to hot rolling, cooling the material at an average cooling rate of 5 to 300 oC / minute in a temperature range of 750 oC to 600 oC after rolling, and also adopt a husking process. The following patent document 2 has disclosed the technique of subjecting a hot rolled steel material in a condition that, after heating the furnace extraction, the temperature is set before finishing at less than 1,000 ° C, maintaining the material of steel in a temperature range of 1,000 oC at 1,150 OC for 5 seconds or less after the finishing laminate and then rolling it, thereafter cooling the rolled steel material to a temperature of 750 OC or less at a cooling rate of 2 to 8 oC / s, and additionally cooling gradually to 600 OC after 150 seconds or more after rolling

Patent Document 1. Japanese Patent No. 4031267

Patent Document 2: Japanese Patent No. 5330181

SUMMARY OF THE INVENTION

However, each of the techniques disclosed in patent documents 1 and 2 requires the execution of a lamination process specified individually. Therefore, it has been desired to avoid the occurrence of decarburization of ferrite and foonation of bainite by adjusting the chemical components of a steel material instead of adopting a technique of providing a specific rolling process, thereby developing a spring steel of high strength that has excellent wire rod rolling properties

The present invention has been made in a context of the above circumstances, and an object of the present invention is to provide a high strength spring steel having excellent wire rod rolling properties by adjusting the chemical components of a steel material to avoid the appearance of decarburization of ferrite and the formation of bainite

Specifically, the present invention relates to the following points 1 to 4. 1 A high strength spring steel having excellent wire rod rolling properties, consisting essentially of, in terms of mass%:

C: from 0.40% to 0.65%; Yes: from 1.20% to 2.80%; Mn: from 0.30% to 1.20%;

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P: 0.020% or less;

S: 0.020% or less; Cu: 0.40% or less; Ni: 0.80% or less; Cr: 0.70% or less; Ti: from 0.060% to 0.134%; At: 0.10% or less;

N: 0.010% or less; Y

Or: 0.0015% or less, and optionally:

B: from 0.0005% to 0.0050%, being the Fe and inevitable impurities, in which the contents in terms of mass% of the specified chemical components meet the following expressions (1) to (3): X1 = 0.14 x [Si] -0.11 x [Mn] -0.05 x [Cu] -0.11 x [Ni] -0.03 x [Cr] - + 0.02 s 0.2 Expression (1) X2: or (at -500) 1 13 2: 3.0 Expression (2)

0: = 912231 x [CJ - + 32x [Sil -20 x [Mn]. 40x [Cu]. 18 x [Ni]. 15 x [Cr] 13 o: 1011 (0.322. 0.538 x [CJ - + 0.018 x [Si] - + 1.294 x [Mn] - + 0.693 x [Cu] - + 0.609 x [Ni] - + 0.847 x [Cr]) X3 = 31 x [CJ - + 2.3 x [Si] - + 2.3 x [Mn] - + 1.25 x [Cu] + 2.68 x [Ni] - + 3.57 x [Cr] -6 x [Ti] 2: 24 Expression (3)

2. High strength spring steel having excellent wire rod rolling properties in accordance with point 1, which has a 400 ° C tempering hardness of 53.0 HRC

or more ·

3. High strength spring steel that has excellent wire rod rolling properties according to points 1 or 2, which has a grain size number of 9 or more.

The present inventors have discovered that it is possible to formulate (as the expression (1 ») the relationship between a ferrite decarburization depth and a parameter (X1) determined by converting the degrees of depth contributions from the respective chemical components of a Steel material in numerical values, formulate (as the expression (2 ») the relationship between bainite formation in the case of cooling at a normal cooling rate after wire rod lamination and a parameter (X2) determined by converting the degrees of the contributions to the formation of bainite from the respective chemical components of a steel material in numerical values and formulate (as the expression (3) the relationship between the hardness in the case of subjecting to a tempering treatment at 400 oC and a parameter (X3) determined by converting the degrees of hardness contributions from the respective chemical components s of a steel material in numerical values Specifically, it is possible to obtain a high strength spring steel having excellent wire rod rolling properties by adjusting the contents of the chemical components in a steel material in order to meet the above expressions (1) to (3).

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph to illustrate the conditions of the expression (1).

Figure 2 is a graph to illustrate the conditions of the expression (2)

Figure 3 is a graph to illustrate the conditions of the expression (3).

Figure 4 is a graph to show the reason for setting the lower limit of Ti content at 0.060% by mass.

Detailed description of the invention

The following are descriptions of reasons and conditions for limiting the individual chemical components (elements) in the composition of the present steel for high strength springs. For the rest, the content of each component is shown in terms of% by mass, and "% by mass" is the same as "% by weight".

(1) C: 0.40% to 0.65%

The C is an essential element of spring steel to ensure resistance. When the content of C is less than 0.40%, it is impossible to achieve the expected spring resistance. On the other hand, when C is added in an amount greater than 0.65%, degradation of the toughness and fatigue characteristics is caused, and therefore the upper limit of the C content is set at 0.65%. The content of C is preferably 0.45% to 0.60%.

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(2) Yes: 1.20% to 2.80%

Si is an effective element in improving the sedimentation resistance of spring steel. Si is added in an amount therefore of 1.20% or more. However, the addition of Si in excess of 2.80% tends to cause not only the degradation of the sedimentation properties, but also the appearance of decarburization of ferrite, and therefore the upper limit of the Si content is set to 2.80%. The Si content is preferably greater than 1.50% and 2.50% or less, more preferably greater than 2.00% and 2.50% or less.

(3) Mn "0.30% to 1.20%

The Mn functions as an ingredient to fix the S, which is a degrading element of toughness, in the form of the MnS. The Mn also functions as a quencher property improver. In order to make good use of these functions, the Mn is added in an amount of 0.30% or more. However, the addition of Mn in an amount greater than 1.20% results in the degradation of the toughness, and therefore the upper limit of the content of Mn is set at 1.20%. The content of Mn is preferably greater than 0.50% and 1.10% or less, more preferably less than 1.00%

(4) P: 0.020% or less

Since P makes the grain boundaries of the crystal brittle, its contents need to be minimized. As long as the content of P is 0.020% or less, the impact of the reduction in the resistance of the grain boundaries is slight, while the extreme reduction in the content of P is undesirable from an industrial point of view, since causes elongation of the fusion process resulting in a higher cost

(5) S: 0.020% or less

The S is inevitably present in the steel and, as mentioned above, it combines with the Mn to form inclusions of MnS that become the starting points of concentration of efforts. An unduly high S content not only increases the amount of MnS inclusions, but also causes the reduction of fatigue resistance. However, as long as the content of S is 0.020% or less, the reduction in fatigue resistance is extremely slight.

(6) Cu: 0.40% or less

Cu is an effective element in improvising corrosion resistance. In addition, it is also effective in preventing the decarburization of ferrite. The Cu content is preferably from 0.20% to 0.37%.

(7) Ni: 0.80% or less

Ni is an effective element in improving corrosion resistance. In addition, it is also effective in preventing decarburization of ferrite. The incorporation of Ni, however, causes an increase in cost, and therefore the upper limit of the Ni content is set at 0.80% The Ni content is preferably from 0.50% to 0.75% .

(8) Cr: 0.70% or less

Cr is an effective element in improving corrosion resistance In addition, it is also effective for adjusting quenching properties. An excessive addition of Cr causes the formation of sharp corrosion pitting, and therefore the upper limit of the Cr content is set at 0.70%. The Cr content is preferably from 0.20% to 0.50%

(9) Ti: 0.060% to 0.134%

Ti is an element that is apt to form carbide. Ti-based carbides contribute to fine-tuning the crystal grains and improving a fatigue characteristic, a delayed fracture characteristic and a sedimentation resistance. For these reasons, Ti is added in an amount of 0.060% or more. However, when the Ti content is greater than 0.140%, the effects of the addition of Ti become saturated; on the contrary, it causes the deterioration of the rolling properties. The upper limit of Ti content is therefore set at 0.134%. The Ti content is preferably 0.080% to 0.120%. The reasons why the lower limit of Ti content is set at 0.060% will be described later.

(10) At: 0.10% or less

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Al is an element that acts as a deoxidizer during liquid steel cleaning. However, when Al is added in an amount greater than 0.10%, inclusions are increased, rather the decrease in resistance to faliga is caused. Therefore, the upper limit of Al content is set at 0.10%

5 (11) N: 0.010% or less

The N combines with Ti to form nitride, which results in the decrease of the resistance to the phallus. Therefore, the upper limit of the N content is set at 0.010%

(12) OR: 0.0015% or less

10 Since O forms oxide-based inclusions, its content is set at 0.0015% or less

(13) The reslo: Faith and inevitable impurities

15 By the way, descriptions of Faith and the inevitable impurities in Table 1 are omitted

(14) Satisfying the following expression (1) X1 = 0.14 x [Si] -0.11 x [Mn] -0.05 x [Cu] -0.11 x [Ni] -0.03 x [Cr ] + 0.02 '"0.2 ... Expression (1)

In order to examine the adequacy of Expression (1), simulations of

20 decarburization of ferrite. In the simulations, the steel samples that have chemical compositions are shown in Table 1, respectively, each formed independently by fusion and hot rolled on bars having $ 22 mm. From then on, these samples were machined in bars that had dimensions of 14 mm41 x 20 mm, subjected to a heat treatment with the condition that they were kept at 900 oC for 100 minutes, and then

25 cooled in oil. Subsequently, deep measurements of ferrite decarburization were made on the samples after the technical treatment. The measurement results obtained are shown in Table 1 and in Figure 1

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Eg comp. one

0.48 2.01 0.60 0.37 0.65 0.15 0.091 ° 0.141 7.17 825 45.3 23.1 0.057 absent absent absent 52.1 9.7

Eg comp. 2

0.52 2.13 0.76 0.30 0.50 0.25 0.004 ° 0, 157 5.21 820 61.4 25.4 0.066 absent absent absent 53.5 7.9

Eg comp. 3

0.52 2.09 0.76 0.30 0.50 0.25 0.032 0.0013 0.152 5.20 819 61.3 25.1 0.063 absent absent absent 53.8 8.2

Eg comp. 4

0.52 2.37 0.35 0, 29 0.37 0.23 0.081 0.0020 0.251 23.51 839 14.4 24.1 0.163 Present absent absent 53.4 9.3

Eg comp. 5

0.56 2.39 0.39 0.20 0.43 0.25 0.090 0.0020 0,247 21, 89 832 15.2 25.5 0.116 Present absent absent 54.1 9.4

Eg comp. 6

0.48 2.39 0.40 0.20 0.40 0.25 0.089 0.0019 0.249 21.21 851 16.5 23.0 0.158 Present absent absent 52.0 9.5

Eg comp. 7

0.61 2.39 0.40 0, 20 0.40 0.25 0.092 0.0017 0.249 22.78 821 14.1 27.0 0.126 Present absent absent 55.6 9.7

Eg comp. 8

0.57 2.20 0.40 0.20 0.40 0.25 0.091 0.0015 0.223 22.07 824 14.7 25.3 0.109 Present absent absent 54.2 9.8

Eg comp. 9

0.56 2.78 0.40 0.20 0.40 0.25 0.090 0.0017 0.304 22.65 845 15.2 26.3 0.156 Present absent absent 55.2 9.6

Eg comp. 10

0.42 2.54 1.11 0.31 0.21 0.25 0.097 0.0020 0.221 2.62 857 136.3 22.9 0.088 Present absent Present 52.2 10.6

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Eg comp. eleven

0.41 2.56 0.96 0.29 0.21 0.14 0.091 0.0020 0.231 5.27 863 68.7 21.7 0.114 Present absent absent 51.5 9.6

Eg comp. 12

0.44 2.50 0.98 0.28 0.78 0.58 0.087 0.0016 0.145 0.93 837 362.0 25.6 0.048 absent Present Present 54.4 9.4

Eg comp. 13

0.44 2.52 0.73 0.30 0.80 0.59 0.079 0.0015 0.172 1.83 841 186.2 25.3 0.063 absent Present Present 53.2 9.3

Eg comp. one.

0.41 2.10 1.01 0.24 0.24 0.36 0088 OR 0.154 2.98 845 115.8 21.6 0.087 absent absent Present 51.5 9.5

Eg comp. fifteen

0.40 2.18 0.84 0.26 0.17 0.12 0.001 OR 0.198 8.60 857 41.5 20.6 0.064 absent absent absent 50.1 7.6

Eg comp. 16

0.60 1.97 0.83 0.10 0.11 0.12 0.003 OR 0.184 14.05 812 22.2 25.9 0.072 absent absent absent 53.2 7.4

Eg comp. 17

0.54 1.29 0.74 0.09 0.06 0.63 0.002 OR 0.089 6.79 800 44, 1 23.9 0.031 absent absent absent 52.4 7.9

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Figure 1 is a graph made by plotting the mud coordinate data of the steel species with the depth of decarburization of ferrite as a vertical axis, and X1 in the expression (1) as a horizontal axis. X1 includes a polynomial formed by adding or subtracting component terms, each of which is obtained by multiplying each of the contents of the specified chemical components (Si, Mn, Cu, Ni and Cr) by the specified individual coefficient and , as clearly seen in Figure 1, makes an almost linear correspondence relationship with the depth of decarburization of ferrite

On the other hand, separately from the above, each kind of steel was formed by melting and subjected to roughing and also a rolling of wire rod in relation to the rolled wire rod ($ 13.5 mm) using a real machine at a temperature of lamination of 900 oC. The cooling rate in this case was set at 0.5 oC / s. And an assessment of a real result of the decarburization of ferrite in each wire rod rolling material, that is, a decision was made as to whether the decarburization of ferrite was performed (the decarburization of ferrite was present) or not (decarburization of ferrite was absent). The results of the assessment are shown in Figure 1 in the form of coordinate data of all steel species with no decarburization of ferrite as a white circle and the presence of decarburization of ferrite as a black circle. In addition, in Table 1, the absence of decarburization of ferrite is described as "absent," while the presence of decarburization of ferrite is described as "present."

As can be seen in Figure 1, it is convenient to organize the depths of decarburization of ferrite in X1 in the expression (1). And considering the actual results of ferrite decarburization in accordance with the practical wire rod lamination, it was determined that the threshold value of Xl for the decision as to whether or not ferrite decarburization occurred is 0.2. In other words, by adjusting Xl to 0.2 or less, it becomes possible to obtain a structure free of decarburization of ferrite

(15) Satisfying the following expression (2)

X2 = (ex -500) / 1} ~ 3.0 ... Expression (2) a = 912-231 x [C] + 32 x [Si] -20 x [Mn] -40x [Cu] -18 x [Ni] -15 x [Cr] 1} = 10 "(0.322 -0.538 x [C] + 0.018 x [Si] + 1.294 x [Mn] + 0.693 x [Cu] + 0.609 x [Ni] + 0.847 x [ Cr])

In order to examine the adequacy of the Expression (2), each species of steel was subjected, in a similar way to the previous one, to a devastated and in addition to a rolling of wire rod (13.5 mm (jl) using a real machine at a lamination temperature of 900 ° C. In this case, the cooling was carried out at two different speeds of 1.5 ° C / s and 0.5 ° C / s and an assessment of a real result of the formation of bainite in each of the rolled wire rod materials, that is, a decision was made as to whether bainite was formed (presence of bainite formation) or not (absence of bainite formation) In addition, in Table 1 and in Figure 2, the unit of a cooling rate is expressed in oC / s

The results obtained are shown in Table 1 and in Figure 2. Figure 2 is a graph made by plotting the coordinate data of all steel species with the cooling speed as a vertical axis and X2 in Expression (2) as horizontal axis. Although X2 includes ct and 13 as variables, the concept of equality itself is known (for example, see Matter, vol. 36, No. 6, 1997, pages 603 to 608). ct includes a polynomial formed by adding or subtracting the terms of components from each of which is obtained by multiplying each of the contents of the specified chemical components (C, Si, Mn, Cu, Ni and Cr) by the coefficient specified individually, and 13 is 10 to the power of such a polynomial. As shown in Figure 2, considering the actual results of bainite formation according to practical wire rod lamination, it was determined that the threshold value of X2 for the decision as to whether or not bainite formation occurred is 3.0. In other words, by adjusting X2 to 3.0 or more, it becomes possible to obtain bainite-free structures, as long as the cooling is performed at usual execution speeds.

(16) Satisfying the following expression (3)

X3 = 31 x [C] + 2.3 x [Si) + 2.3 x [Mn] + 1.25 x [Cu] + 2.68 x [Ni] + 3.57 x [Cr] 6 x [ Ti) ~ 24.0 Expression (3)

In order to examine the adequacy of the Expression (3), some steel samples prepared for the individual steel species that are formed by fusion, hot forging them in their respective bars that have $ 22 mm and then machining them in their respective bars having dimensions of 20 mm x 10 mm, they were maintained at 950 ° C for 60 minutes, subjected to oil quenching, then kept at 400 ° C for 30 minutes, and further tempered according to air cooling. hardness (HRC) in steel samples treated in this way

The measurement results obtained are shown in Table 1 and in Figure 3. Figure 3 is a graph made by plotting the coordinate data of all steel species with a hardness as a vertical axis and X3 in Expression (3) as horizontal axis. X3 includes a polynomial formed by adding or subtracting component terms, each of which is obtained by multiplying each of the contents of the specified chemical components (C, Si, Mn, Cu, Ni, Cr and Ti) by the individually specified coefficient.

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As can be seen in Figure 3, it is appropriate to organize the hardness in X3 in Expression (3). And in order that the high-strength spring steel according to the invention guarantees a hardness (HRC) of at least 53.0 in the case of setting the tempering temperature at 400 oC, it was determined that the threshold value of X3 is 24.0. In other words, by adjusting X3 to 24.0 or more, it becomes possible to obtain a high strength structure with a hardness (HRC) of 53.0 or more in the case of setting the tempering temperature at 400 oC

(17) B: from 0.0005% to 0.0050%

B is an effective element in improving the toughness of spring steel by preventing P and S from segregating glass grain boundaries. Therefore, the content of B is preferably 0.0005% or more. On the other hand, the excessive addition of B causes the formation of B nitride, which results in the degradation of toughness. Therefore, the content of B is preferably 0.0050% or less

(Others) Reasons for setting the lower limit of Ti content at 0.060%

In samples that have experienced a hot forge, a temper after 950 oC and an additional tempering at 400 OC, measurements of crystal grain size (grain size of austenitic crystal) were performed according to the test method of the grain size of the austenitic crystal (JIS G 0551: 2005). The measurement results (crystal grain size numbers) obtained are shown in Table 1 and in Figure 4. Figure 4 is a graph made by plotting the coordinate data of each steel species with a grain size number of the clistal as vertical axis and Ti content as horizontal axis

The grain size of the austenitic crystal influences various characteristics (a characteristic of fatigue, a characteristic of delayed fracture, a property of sedimentation), and generally, it is possible to improve these characteristics through the refining of the grains of the clistal. In the high strength steel of the present invention, the lower limit of Ti content is set at 0.060 based on Figure 4 so that the grain size of the crystal after quenching and tempering becomes No. 9 or more. . In other words, by adjusting the Ti content to 0.060% or more, it becomes possible to obtain a fine structure that is No. 9 or more in crystal grain size number.

The calculation results, the measurement results and the valuation results of the Expressions

(1) to (3) corresponding to each steel species (in each of examples 1 to 12 and comparative examples 1 to 17) are shown in Table 1 As shown in examples 1 to 12, steels for high strength springs that have excellent wire rod rolling properties, and more specifically, steels that do not cause ferrite decarburization or bainite formation during wire rod rolling and have a tempering hardness at 400 oC of 53, 0 HRC or more and a grain size number of 9 or more, can be obtained by adjusting each of the chemical components so that they fall within the range of individually specified content and meet the expressions (1) to (3)

On the other hand, in each of the comparative examples 1, 6, 10, 11, 14, 15 and 17, the expression (3) was not fulfilled; As a result, the tempering hardness at 400 oC was below 53.0 HRC. In addition, in each of the comparative examples 4 to 11, the expression (1) was not fulfilled; As a result, a decarburization of ferrite occurred during wire rod lamination.

In addition, in each of comparative examples 2, 3 and 15 to 17, the Ti content was below 0.060% by mass; as a result, the grain size number of the crystal thereof was below No. 9. In addition, in each of comparative examples 10 and 12 to 14, the expression (2) was not met; As a result, bainite formation occurred during wire rod lamination

As can be clearly seen from the above descriptions, in accordance with the present invention, it is possible to obtain a high strength spring steel having excellent wire rod rolling properties. By the way, the present invention should not be construed as being limited to the previous examples, but can be carried out so that they undergo various changes and modifications as long as they do not depart from the essence of the invention.

This application is based on Japanese patent application No. 2014-206311 filed on October 7, 2014, and the content is incorporated herein by reference.

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Claims (3)

A high strength spring steel having excellent wire rod rolling properties, consisting essentially of, in terms of mass%: 5 C: from 0.40% to 0.65%; Yes: from 1.20% to 2.80%; Mn: from 0.30% to 1.20%; P: 0.020% or less; s: 0.020% or less; 10 Cu: 0.40% or less; Ni: 0.80% or less; Cr: 0.70% or less; Ti: from 0.060% to 0.134%; At: 0.10% or less; 15 N: 0.010% or less; Y Or: 0.0015% or less, and optionally " B: from 0.0005% to 0.0050% the faith being the inevitable impurities, 20 in which the content in terms of mass% of the specified chemical components meets the following expressions (1) to (3) "X1 = 0.14 x [Si] -0.11 x [Mn] -0.05 x [Cu] -0.11 x [Ni] -0.03 x [Cr] + 0.02: S 0.2 .. Expression (1) X2 = (0.-500) I ~ <'! 3, 0 ... Expression (2) a = 912-231 x [C] + 32 x [Si] -20 x [Mn] -40x [eu] -18 x [Ni] -15 x [er] 25 ~ = 10 "(0.322 -0.538 x [C] + 0.018 x [Si] + 1.294 x [Mn] + 0.693 x [eu] + 0.609 x [Ni] + 0.847 x [er]] X3 = 31 x [C ] + 2.3 x [Si] + 2.3 x [Mn] + 1.25 x [eu] + 2.68 x [Ni] + 3.57 x [Cr] -6 x [Ti] <'! 24.0 .. Expression (3) 2. The high strength spring steel having excellent wire rod rolling properties 30 according to claim 1, having a 400 ° C tempering hardness of 53.0 HRe or more 3. The high strength spring steel having excellent wire rod rolling properties according to claim 1 or 2, which has a grain size number of 9 or more.