JPH1143721A - High strength spring steel and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique of manufacturing a spring having high strength equal to or higher than that of the conventional spring requiring a quench-and- temper process, while obviating te necessity of the quench-and-temper process. SOLUTION: An ingot or billet of a steel, having a composition containing, by mass, 0.30-0.60% C and 0.60-3.00% Si or further containing 1.20-3.50% Mn and 0.20-1.50% Cr, is hot-rolled at a rolling finishing temp. between Ac1 +30 deg.C and Ac3 -20 deg.C. Subsequently, cooling rate is controlled to (10 to 50) deg.C/sec and further the structure is regulated to a microstructure consisting of 20 to 50% ferrite by area ratio and the balance martensite. In this case, cooling rate is regulated to (5 to 50) deg.C/sec, and the structure is controlled so that it is composed of 20 to 70% ferrite by area ratio and the balance martensite, and then, the steel is work-hardened by cold working at 5 to <20% draft. By this method, the spring manufacturing process can be abbreviated and also an energy cost can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for a high-strength spring and a method for producing the same. Hot rolling is performed so that the termination temperature is in the two-phase region of ferrite and austenite, then the cooling rate after hot rolling is appropriately adjusted, and further, the martensite structure in which the area ratio of ferrite is adjusted Or a cold-worked steel material having high strength imparted thereto, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, suspension springs for automobiles are manufactured from a wire by two methods, hot forming and cold forming. Among these, in the production of a spring by hot forming, a steel material for a spring is hot-rolled, and then quenched and tempered to hot-form the wire material that has been tempered to a predetermined strength. On the other hand, in the production of a spring by cold forming, after a spring steel material is formed into a wire by hot rolling, a spheroidizing annealing or patenting process is performed, followed by a cold drawing process, and an oil tempering process. And cold coiled (formed).

[0003] For example, Japanese Patent Application Laid-Open No. Sho 59-170241 discloses that the chemical composition of steel is appropriately selected and controlled rolling is performed to reduce the crystal grain size of the steel to a fine grain of 9 or more. High strength with increased fatigue strength as well as ruggedness
Technology for obtaining high toughness spring steel (hereinafter referred to as prior art)
Is disclosed.

In recent years, in the manufacture of automobiles and various industrial machines, there has been a strong demand for cost reduction by omitting manufacturing steps and reducing energy consumption.

[0005]

According to the above-mentioned prior art, there is an advantage that a high-strength and high-toughness spring steel material excellent in fatigue strength can be obtained. However, in the method of manufacturing a spring steel material according to the prior art and the conventional method, the quenching / tempering step is performed in any of the spring manufacturing processes, regardless of whether the spring forming step is hot forming or cold forming. Is required, so that predetermined equipment is required and energy costs are high.

In order to solve the above problems, the present inventors have focused on imparting high strength without quenching and tempering by optimizing the control of the microstructure of steel.
The relationship between steel composition design, hot rolling conditions and cooling rate after rolling, controlled microstructure, and strength and ductility of steel materials were studied. Then, in order to omit the conventional spring manufacturing process and reduce the energy cost, a spring having a strength equal to or higher than that of the conventional spring in which the quenching / tempering process is performed is manufactured without performing quenching / tempering. We decided to develop technology to do that. Thus, an object of the present invention is to provide a high-strength steel material for a spring and a method for manufacturing the same, which does not require quenching and tempering in a spring manufacturing process by solving the above-mentioned problems.

[0007]

SUMMARY OF THE INVENTION The present inventors have conducted intensive studies from the above viewpoints in order to develop a high-strength steel material for a spring and a method for producing the same. As a result, the composition of the steel, the rolling conditions and the cooling rate after the rolling are variously changed, and the microstructure of the steel material is changed to a two-phase structure composed of ferrite and martensite with an appropriate area ratio, whereby the steel is conventionally used. It has been found that it is possible to obtain a high-strength product equivalent to or higher than a quenched / tempered product of a spring steel material. Furthermore, by performing cold working at an appropriate working rate, a steel material for springs with higher strength can be obtained,
In addition, they have found that it is possible to expand the appropriate range of the area ratio of ferrite having a two-phase structure.

[0008] The present invention has been made based on the above findings, and has the following features. The steel material for a high-strength spring according to claim 1 has a chemical composition of C: 0.30 to 0.6.
0 mass% and a steel ingot or a slab containing Si: 0.60 to 3.00 mass%, and the rolling end temperature is Ac
₁ + 30 ° C. to Ac ₃ -20 ° C. hot rolling is performed, and the cooling rate of the steel material after hot rolling is 10 to 50 ° C.
Control within the range of / sec. Thus, the microstructure of the steel material after cooling is characterized in that the ferrite is adjusted to a microstructure of martensite with an area ratio of ferrite of 20 to 50% and a balance of martensite. In this case, among the chemical components of the steel material, the component composition other than the C and Si contents may be the same as that of the conventional high-strength spring steel. For example, SU specified in JIS G4801 as a material for JIS B2702 hot-formed coil springs
P7 and SUP9 correspond thereto.

[0009] The steel material for a high-strength spring according to the second aspect is the invention according to the first aspect, wherein the chemical composition of the steel ingot or the billet is C: 0.30 to 0.60 mass%, Si: 0. .6
0 to 3.00 mass%, Mn: 1.20 to 3.50 mass
% And Cr: 0.20 to 1.50 mass%.

According to a third aspect of the present invention, there is provided a method for producing a high-strength spring steel material, wherein C: 0.30 to 0.60 mas as a chemical component composition.
A steel ingot or a slab containing s% and Si: 0.60 to 3.00 mass% was manufactured, and the rolling end temperature was set to Ac ₁ +3.
Hot rolling in a temperature range of 0 ° C. to Ac ₃ -20 ° C. is performed, and then the steel material after hot rolling is cooled at a cooling rate of 10 to 50 ° C. /
It is characterized in that it is cooled within the range of sec, and controlled so that the ferrite area ratio of the thus obtained steel material after cooling is 20 to 50% and the remainder is a microstructure composed of martensite. is there.

According to a fourth aspect of the present invention, there is provided a method for producing a steel material for a high-strength spring according to the third aspect of the present invention, wherein the steel ingot or slab is C: 0.30 to 0.60 mass%, Si: 0. 6
0 to 3.00 mass%, Mn: 1.20 to 3.50 mass
% And Cr: 0.20 to 1.50 mass%.

According to a fifth aspect of the present invention, there is provided a method of manufacturing a high-strength spring steel material, wherein the chemical composition is C: 0.30 to 0.60 m
A steel ingot or a slab containing ass% and Si: 0.60 to 3.00 mass% is manufactured, and the rolling end temperature is Ac ₁ +
Hot rolling in the temperature range of 30 ° C. to Ac ₃ -20 ° C. is performed, and then the steel material after hot rolling is cooled at a cooling rate of 5 to 50 ° C. /
The temperature is controlled so that the ferrite area ratio of the thus obtained steel material after cooling is within the range of 20 to 70%, and the remainder is a microstructure composed of martensite. It is characterized in that the steel material after cooling having a microstructure is work-hardened by cold working within a working ratio of less than 5 to 20%.

According to a sixth aspect of the present invention, there is provided a method for producing a steel material for a high-strength spring according to the fifth aspect of the present invention, wherein the steel ingot or slab is C: 0.30 to 0.60 mass%, Si: 0. 6
0 to 3.00 mass%, Mn: 1.20 to 3.50 mass
% And Cr: 0.20 to 1.50 mass%.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention include:
It is desirable that the features of the invention described in each of the first to sixth aspects described above be adopted in the prior art. That is, the production of ingots or billets is performed by refining in a steelmaking furnace such as a converter and an electric furnace, tapping into a ladle, and then appropriately processing in a secondary refining furnace such as an RH degassing device. A molten steel having a chemical composition is cast by a continuous casting method or an ingot casting method. The cast steel ingot or slab is charged into a predetermined heating furnace, heated to a predetermined temperature, and rolled into a wire at a rolling end temperature within the range of the present invention. Here, the rolling end temperature is determined according to the Ac ₁ transformation point and the Ac ₃ transformation point determined depending on the chemical composition of the wire. That is, based on Ac ₁ and Ac ₃ ,
After rolling into a wire having a predetermined cross-sectional dimension using a stable two-phase region temperature of ferrite and austenite as a rolling end temperature, it is directly cooled. This cooling is performed by adjusting the cooling rate within the range of the present invention so that the microstructure of the wire is a two-phase structure of ferrite + martensite, and the area ratio of both is adjusted within an appropriate range. Alternatively, cold working at a predetermined working rate is performed on the area ratio exceeding the appropriate range within a certain range. Further, in order to obtain a steel material for a spring having higher strength, a steel having a ferrite area ratio within the above-mentioned appropriate range is subjected to cold working at a predetermined working ratio.

Next, the reasons for limiting the chemical composition of the steel, the rolling end temperature, the cooling rate after the rolling, the microstructure of the steel after the cooling, and the cold working ratio in the present invention as described above will be described.

[Chemical Component Composition] (1) C C is contained in order to give necessary strength as a spring. However, if the content is less than 0.30 mass%, it becomes difficult to secure the necessary strength as a spring. on the other hand,
If the C content exceeds 0.60 mass%, as estimated from the Fe-C phase diagram, the C content approaches the eutectoid point as the C content increases, and the two-phase temperature region of ferrite + austenite narrows. Therefore, it is difficult to perform two-phase rolling. Therefore, the C content is limited to the range of 0.30 to 0.60 mass%.

(2) Si Si forms a solid solution in ferrite to improve the strength of steel and effectively acts to improve the sag resistance of the spring. Further, Si acts as a ferrite forming element and is an element necessary for improving the ductility of ferrite. To satisfy both of these, Si must be 0.60 m
It is necessary to contain ass% or more. Further, since the Ac ₃ wire shifts to a higher temperature side by the addition of Si, the temperature of the two-phase region is expanded, and the narrowing of the two-phase temperature region due to an increase in the C content is alleviated. Therefore, the two-phase rolling of ferrite + austenite is facilitated. On the other hand, adding a large amount of Si exceeding 3.00 mass% is not only difficult in steelmaking work, but also increases nonmetallic inclusions such as SiO ₂ , impairs cleanliness of steel, and causes fatigue fracture. It becomes easy to be a starting point. Therefore,
The Si content is limited to the range of 0.60 to 3.00 mass%.

(3) Mn Mn is an element that is effective in deoxidizing steel and also effective in improving the hardenability of steel. In addition, it is important as a martensite forming element, and is effective for providing high strength by forming a solid solution in ferrite to reduce the amount of C solid solution in ferrite and increasing the amount of C solid solution in martensite. .
In order to satisfy these simultaneously, Mn must be 1.20 mass%
It is desirable to contain the above. On the other hand, Mn was 3.50.
If the content exceeds mass%, the hardenability will increase,
The toughness tends to decrease, and the workability also tends to decrease. Therefore, it is desirable that the Mn content be limited to the range of 1.20 to 3.50 mass%.

(4) Cr Cr is an element effective in suppressing decarburization and graphitization of high carbon steel. However, its content is 0.20 mas
If it is less than s%, these effects cannot be sufficiently exerted. on the other hand,
If the Cr content exceeds 1.50 mass%, the toughness tends to decrease. Therefore, the Cr content is 0.20 to 1.50 m
It is desirable to limit it within the range of ass%.

[Rolling Temperature and Cooling Rate After Rolling] (1) Preliminary Test The inventors of the present invention grasped the ferrite + austenite two-phase region temperature of steels having various component compositions and formed by two-phase region rolling. In order to grasp the microstructure quantitatively, and to grasp the relationship between the microstructure and the strength, using a steel ingot of various chemical composition within and outside the range of the present invention, a two-time quenching test was performed as follows. Done.

(Twice Quenching Test) Table 1 shows the chemical composition of the steel ingot used in this test, which is within the scope of the present invention (steel 1 to 6 of the present invention) and out of the scope of the present invention. 2 shows the chemical composition of Comparative Examples (Comparative Steels 1 to 7). A 150 kg steel ingot having a cross section of 180 × 180 mm having the 13 kinds of chemical component compositions shown in Table 1 was prepared, and each was hot forged to 20 mmφ, and the length was 3 mm.
It was cut into 00 mm units. Then, it was subjected to the following tests (a) and (b). (A) The test material of 20 mmφ × 300 mm was hardened twice with the heat treatment pattern shown in FIG. In this double quenching, a uniform martensite structure is obtained by primary quenching from the austenite region, and then the quenching temperature is set to five levels (750, 775, 800, 825 and 8
(50 ° C.) and secondary quenching was performed. The test material thus obtained was subjected to hardness measurement and microstructure observation. (B) A heat treatment shown in FIG. 2 was applied to another test material of 20 mmφ × 300 mm. A test piece was cut out from the obtained test material, and the heating transformation points Ac ₁ and Ac ₃ of each steel were measured by a fully automatic transformation point measuring device using a high-frequency heating method. Among the results obtained in (a) and (b) above, the microstructure including the ferrite area ratio when the secondary quenching temperature is 800 ° C., and Ac ₁ and Ac ₃ are also shown in Table 1, and Graphs showing the effect of the secondary quenching temperature on the hardness for each steel are shown in FIGS. In these figures, the hardness corresponding to the tensile strength of 1500 N / mm ² or more to be provided as the high-strength spring of the present invention is as follows.
The Vickers hardness (HV10) of 470 or more is shown.

[0022]

[Table 1]

From the results shown in Table 1 and FIGS. 3 and 4,
The following matters are understood. Comparative steel 1 where the area ratio of ferrite exceeds 50%
No. 3 is 1500 N / mm ² (470 Vickers hardness HV10, which is a minimum value of tensile strength practical for spring steel).
Is difficult to obtain by heat treatment alone. In such a low C content region, it is difficult to secure a tensile strength of 1500 N / mm ² or more even when cold working is combined. Therefore, it is understood that the C content is preferably limited to 0.30 mass% or more.

In Comparative Steels 6 and 7 having a ferrite area ratio of less than 20%, the two-phase temperature range shown in Table 1 is extremely narrow (the value of Ac ₃ -Ac ₁ is small). It is difficult. Therefore, it is understood that the C content is preferably set to 0.60 mass% or less.

The steels 1 and 2 of the present invention and the comparative steel 4 are:
The hardenability was changed by changing the Mn content while keeping the C content constant at about 0.35 mass%. From FIG.
In Comparative Steel 4 (Mn: 0.55 mass%), a sufficient martensite structure was not obtained due to a low Mn content, and the secondary quench hardness was also low. On the other hand, Steel 2 of the present invention (Mn: 1.40 mass%) and Steel 1 of the present invention (Mn:
At 3.35 mass%), a sufficient martensite structure is obtained, and the secondary quenching hardness is also ensured, which is a value without any problem.

Inventive steel 1 (Si: 2.00 mass%)
Comparative steel 5 (Si: 0.30 mass%), inventive steel 3 (S
i: 0.63 mass%), Steel 4 of the present invention (Si: 2.87 m)
(ass%) tests the effect of the Si content on the secondary quench hardness. FIG. 4 shows that the Si content is 0.60
In the case of mass% or more, stable high hardness is obtained in the region where the secondary quenching temperature is high, and the Si content is 0.60 mas
It turns out that s% or more is necessary.

(2) Rolling Finish Temperature As described above, C, Si and M suitable for the steel material of the present invention are used.
The approximate values of the range of the n content and the appropriate range of the quenching temperature after hot rolling were grasped in association with the microstructure. In consideration of these results, the rolling end temperature and the cooling rate of the steel material after rolling in the present invention were limited. That is, the reason why the rolling end temperature is limited to the temperature range of Ac ₁ + 30 ° C. to Ac ₃ −20 ° C. is as follows.

In order to adjust the microstructure of the wire after hot rolling and before cooling to a two-phase structure of ferrite + austenite, it is essential that at least the rolling end temperature is in the two-phase region of ferrite + austenite. In order to adjust the ferrite area ratio within the range of the present invention by adjusting the cooling rate after rolling, most of the rolling temperature range, which is the temperature from the start to the end of rolling, is maintained in the two-phase range. Is desirable.

If the rolling end temperature is less than Ac ₁ + 30 ° C., the steel material temperature may become Ac ₁ or lower due to a temperature drop from the end of rolling to the start of cooling rate control. On the other hand, when the rolling end temperature is equal to or higher than Ac ₃ -20 ° C., the rolling time at a temperature exceeding Ac ₃ tends to be too long.
c ₃ exceeded becomes an austenite single-phase structure can not be obtained ferrite + austenite dual phase structure when the, can not be adjusted ferrite area ratio within a proper range.

By the way, when the rolling end temperature is Ac ₁ +30
℃-Ac ₃ -20 ℃ If the temperature range is
Under normal operating conditions, the quenching temperature, which is the cooling start temperature, is always maintained at the two-phase region temperature, even if the temperature drop after rolling is considered. Therefore, the necessary conditions for changing the microstructure of the steel material after quenching to a martensite structure in which the ferrite area ratio is adjusted are satisfied.

(3) Cooling rate after rolling The reason for limiting the cooling rate after rolling will be described. (A) The reason for setting the temperature in the range of 10 to 50 ° C./sec in claims 1 to 4 In order to generate martensite, it is necessary to increase the cooling rate. The subsequent ferrite area ratio is less than 20%, and the elongation and the drawing are reduced, so that the spring cannot withstand cold coiling. On the other hand, if the cooling rate after rolling is less than 10 ° C./sec,
This is because the ferrite area ratio of the wire exceeds 50%, and if cold working is not performed after rolling, 1500 N / mm ^2, which is the minimum value of practical tensile strength as a spring, cannot be secured.

(B) In Claims 5 and 6, 5 to 50
Reason for setting within the range of ° C / sec Of the above cooling rates, when the cooling rate is within the range of 10 to 50 ° C / sec, the strength of the wire is further improved by performing cold working at a predetermined working rate. It is because there is an advantage that can be. On the other hand, when the cooling rate is 10 ° C./se
If it is less than c, as described in (a) above, the area ratio of ferrite becomes large. To ensure practical tensile strength as a spring, perform predetermined cold working. Is required. In this case, if the ferrite area ratio of the microstructure of the wire after cooling exceeds 70% even if it exceeds 50%, the minimum value of the practical tensile strength (1500 N / mm) is obtained by a predetermined amount of cold working. ² ) can be secured, and the level of ductility can be secured. Thus, the ferrite area ratio is 70%.
In order to keep the cooling rate below, the cooling rate may be adjusted to 5 ° C./sec or more.

As described above, the cooling rate is 10 to 50 ° C./se.
When the temperature is within the range of c, cold working is performed to further improve the strength. When the temperature is within the range of 5 to 10 ° C./sec, cold working is performed to secure the required minimum strength. Apply.
However, in any case, in claims 5 and 6, the cooling rate after rolling is limited to the range of 5 to 50 ° C / sec.

[Microstructure of Steel after Cooling] The reason for changing the microstructure of the wire into a two-phase structure of ferrite + martensite is that both the strength and ductility, which are the mechanical properties required for a spring, are excellent. It is necessary. And in Claims 1-4, the area ratio of ferrite is
The reason why the area ratio of ferrite is limited to the range of 20 to 70% will be described. (A) In claim 1, the area ratio of ferrite is 2
Reason for being in the range of 0 to 50%: As described above, when the ferrite area ratio of the wire is set in the range of 20 to 50% and the remainder is adjusted to a martensite two-phase structure, the desired ratio can be obtained without cold working. This is because mechanical properties are satisfied. (B) In Claims 5 and 6, the reason why the area ratio of ferrite is within the range of 20 to 70%: the area ratio of ferrite of the wire is within the range of 20 to 50%, and the remainder is a two-phase structure of martensite. When adjusted, the strength can be further improved by performing a predetermined amount of cold working, and when the ferrite area ratio is in the range of more than 50 to 70%, a predetermined amount of cold working is performed. This is because a desired strength can be ensured by applying.

[Cold work ratio] According to claims 5 and 6, a wire is manufactured by limiting the chemical composition of steel and the hot rolling and the cooling rate after rolling to the above-described conditions. Cold working and hardening to improve strength. The cold working rate at this time is limited to the cold working within a range of 5 to less than 20%. The reason for limiting the cold working ratio in this way is 5
%, The effect of work hardening is not sufficiently exhibited. On the other hand, if cold working of 20% or more is performed, ductility such as elongation and drawing is greatly reduced, and this is not practical.

[0036]

Next, the present invention will be described in more detail with reference to examples. (Test 1) A 150 kg steel ingot having a cross section of 180 mm × 180 mm containing the 19 chemical composition shown in Tables 2 and 4 was prepared. The steel ingot is hot forged into a billet having a cross section of 116 mm x 116 mm, then heated to a predetermined temperature, and hot rolled into a 12 mmφ wire, and a part of the wire is further cold-rolled. The wire was drawn to produce a steel wire for a spring. In the wire rod rolling, the rolling temperature and the cooling rate after the rolling were variously changed, and in the cold drawing, the wire drawing reduction rate was variously changed.

Thus, the hot-rolled wire and the cold-drawn wire obtained by the manufacturing method within the scope of the present invention (Example),
In addition, for all of the hot-rolled wire and the cold-drawn wire (comparative example) obtained by the manufacturing method outside the scope of the present invention,
Measurement tests of the Ac ₁ and Ac ₃ transformation points, microstructure tests, tensile strength, elongation and mechanical properties tests of drawing, and investigation of cold coiling properties of springs were performed. Some of the comparative examples include a wire rod SUP7 (JIS) as a material for a hot-formed coil spring (JISB2702) conventionally used in practice.
G4801) (Comparative Examples 6 to 6).
7).

The test conditions and test results are shown in Tables 2 to 5.

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

Tables 2 to 5 show the following. Effect of Rolling Temperature on Various Properties As can be seen from Comparative Examples 1 and 2 and Examples 1 and 2, in Comparative Examples 1 and 2, the rolling temperature was out of the range of the present invention. A phase structure cannot be obtained, elongation and reduction of drawing are remarkable, and cold coiling cannot be performed.

Influence of Cooling Rate after Rolling on Various Properties As can be seen from Examples 3 and 4 and Comparative Examples 3 and 4, Comparative Example 3 has a cooling rate of less than 10 ° C./sec and is within the scope of the present invention. , The minimum value of 1500 N / mm ² of practical tensile strength as spring steel cannot be secured. On the other hand, in Comparative Example 4, since the cooling rate was larger than 50 ° C./sec and was out of the range of the present invention, the ferrite content was 20%.
, The elongation and drawing are low, and cold coiling is not possible.

Influence of Cold Wire Drawing Reduction Ratio on Strength and Ductility As can be seen from Examples 5 to 7, the chemical composition of the steel and the rolling temperature are within the range of the present invention, and When the cooling rate is in the range of 10 to 50 ° C./sec, the ferrite area ratio is 20 to 50%, and the remainder is adjusted to a martensite microstructure, cold drawing with a reduction in area of less than 20% is performed. Thereby, the strength of the wire rod of the embodiment is further improved, and the elongation and the drawing are secured. Then, referring to Comparative Example 5, the wire-drawing limit is 20% of the area reduction rate.
It can be seen that the elongation and the reduction in drawing were remarkable, and cold coiling was not possible. However, among the conditions of the above Examples 5 to 7,
Since the cooling rate after rolling is less than 10 ° C./sec, when the ferrite area ratio exceeds 50% and the microstructure of the remaining martensite is obtained, wire drawing of 10 to 15% is possible, The required strength as a spring is ensured, the ductility is also ensured, and the cold coiling property is also sufficient (Example 8).
To 11).

When the results of Examples 5 to 11 and Comparative Example 5 are combined, the range of the cooling rate after rolling and the area ratio of ferrite of the present invention are 5 to 50 ° C./se, respectively.
c, and within the range of 20 to 70%, which indicates that the present invention is widely applicable.

Note that Comparative Examples 6 and 7 are JIS B270
2 A SUP7 heat-treated material conventionally used as a material for a hot-formed coil spring. (Test 2) Cold-formed springs having the spring specifications shown in Table 6 were produced using the wires produced in Examples 1 to 11 and Comparative Examples 6 to 8 shown in Tables 2 to 5.

[0048]

[Table 6]

For each of the above springs, a tightening stress 110
0 N / mm ² , ambient temperature 20 ° C., tightening time 72
A time crimping test was performed. Table 7 shows the results. From Table 7, it can be seen that all of the examples have the same sag resistance as Comparative Examples 6 and 7, which are current SUP27 heat-treated materials.

[0050]

[Table 7]

Further, the above Examples 1 to 11 and Comparative Example 6
From ~ 8 to the cold-formed spring manufactured according to the specifications in Table 6,
Stress 600 ± 500N / mm ² , ambient temperature 20 ℃,
A fatigue test was performed at a cycle of 60 Hz and a cutoff of 3.00 × 10 ⁷ times. Table 8 shows the results. Table 8
Thus, it can be seen that all the examples have the same durability as Comparative Examples 6 and 7, which are the current SUP27 heat-treated materials.

[0052]

[Table 8]

[0053]

As described above, according to the present invention,
It has an appropriate chemical composition as a spring steel, and has a microstructure of a steel material with a two-phase structure of ferrite and martensite with an appropriate ferrite area ratio, and further increases the strength by cold work hardening as necessary. The applied spring steel material can be manufactured. Thus, the present invention can provide a steel material for a high-strength spring and a method for producing the same, and bring about an industrially useful effect.

[Brief description of the drawings]

FIG. 1 is a heat treatment pattern showing a twice quenching test method in a test for solving a problem.

2 is a heat treatment pattern of the test piece for measuring heat transformation point Ac ₁ and Ac _3.

FIG. 3 is a graph showing the effect of a secondary quenching temperature on the hardness of steel having various component compositions.

FIG. 4 is a graph showing the effect of the secondary quenching temperature on the hardness of steels having other various component compositions.

Claims (6)

[Claims] 1. A steel ingot or a slab having a chemical composition containing C: 0.30 to 0.60 mass% and Si: 0.60 to 3.00 mass% has a rolling end temperature of Ac _1. Hot rolling in a temperature range of + 30 ° C. to Ac ₃ −20 ° C., and then controlling a cooling rate of the steel material after the hot rolling to a range of 10 to 50 ° C./sec.
A steel material for a high-strength spring, wherein the steel material after cooling has a ferrite area ratio of 20 to 50% and a balance adjusted to a martensite microstructure. 2. The steel ingot or slab comprises: C: 0.30 to 0.60 mass%, Si: 0.60 to 3.00 mass%, Mn: 1.20 to 3.50 mass%, and The steel material for a high-strength spring according to claim 1, wherein the steel material has a chemical composition containing 0.20 to 1.50 mass% of Cr. 3. A steel ingot or a slab having a chemical composition containing 0.30 to 0.60 mass% of C and 0.60 to 3.00 mass% of Si is rolled at a rolling end temperature of Ac _1. Hot rolling in a temperature range of + 30 ° C. to Ac ₃ -20 ° C., and then cooling the steel material after the hot rolling at a cooling rate of 10 to 50 ° C./sec. The ferrite area ratio of the steel material after
A method for producing a high-strength steel for springs, characterized in that the balance is controlled to a martensite microstructure at 50%. 4. The steel ingot or slab as follows: C: 0.30 to 0.60 mass%, Si: 0.60 to 3.00 mass%, Mn: 1.20 to 3.50 mass%, and 4. The method for producing a high-strength spring steel material according to claim 3, wherein a material having a chemical composition containing 0.20 to 1.50 mass% of Cr is used. 5. A steel ingot or a steel slab having a chemical composition containing C: 0.30 to 0.60 mass% and Si: 0.60 to 3.00 mass% is rolled at an end temperature of Ac _1. Hot rolling in a temperature range of + 30 ° C. to Ac ₃ -20 ° C., and then cooling the hot-rolled steel at a cooling rate of 5 to 50 ° C./sec. 20% to 70% ferrite area ratio of steel
And the remainder is controlled to a martensitic microstructure, and then the steel material after cooling having the microstructure is work hardened by cold working within a working ratio of less than 5 to 20%. A method for producing a high-strength spring steel material. 6. The steel ingot or steel slab includes: C: 0.30 to 0.60 mass%, Si: 0.60 to 3.00 mass%, Mn: 1.20 to 3.50 mass%, and 6. The method for producing a high-strength steel material for a spring according to claim 5, wherein a material having a chemical composition containing 0.20 to 1.50 mass% of Cr is used.