Spring steel and its spheroidizing annealing method

Abstract

A step of putting a spring steel in a hot-rolled state into a heat treatment furnace and rapidly heating it to a two-phase region temperature, a step of holding the spring steel at the two-phase region temperature, a step of rapidly cooling the spring steel to a first temperature, a step of isothermally spheroidizing the spring steel at the first temperature, and a step of slowly cooling the spring steel after the isothermal spheroidizing treatment together with the heat treatment furnace to a second temperature and then discharging it from the furnace and air-cooling it, and a spring steel and a spheroidizing annealing method thereof including these steps.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal heat treatment, and specifically relates to spring steel and its spheroidizing annealing method.

Background Art

[0002] Spring steel is a raw material for manufacturing various parts such as coil springs, leaf springs, torsion springs, etc., and is widely applied in fields such as national defense, industry and agriculture, and daily life, including automobiles, instruments, aviation, spaceflight, electrical products, etc. All kinds of machinery cannot do without the use of spring steel.

[0003] Spring steel must have good performance and quality, for example, to meet the needs of spring manufacturing such as high elastic limit, tensile strength, hardness, plasticity, etc., and be able to have a high fatigue limit and anti-elastic relaxation ability when operating under alternating loads.

Summary of the Invention

Problems to be Solved by the Invention

[0004] Spring steel generally adopts rolling production, but the materials after rolling have problems such as non-uniform structure, large residual stress, high hardness, and poor plasticity, which are disadvantageous for subsequent processing such as cold forming. Cold forming technology is used to manufacture springs with high elastic limit, and there are strict requirements for the original structure, spheroidization rate of carbides and hardness of spring steel. If the existing general spheroidizing annealing technology is adopted, the spheroidization rate is low and the requirements of cold forming cannot be met.

Means for Solving the Problems

[0005] In view of the above problems, the object of the present application is to provide spring steel and its spheroidizing annealing method.

[0006] The spheroidizing annealing method of spring steel provided by the present application is characterized by a fast heating rate and a short spheroidizing time, and has a good spheroidizing effect and a high spheroidization rate of the spring steel after spheroidizing annealing.

[0007] To achieve the above object, the present application adopts the following technical solutions. The first aspect of the present application is putting the spring steel in the hot-rolled state into a heat treatment furnace and rapidly heating it to the two-phase region temperature, i.e., the rapid heating stage, holding the spring steel at the two-phase region temperature, i.e., the two-phase region holding stage rapidly cooling the spring steel to the first temperature, i.e., the first cooling stage, isothermally spheroidizing the spring steel at the first temperature, i.e., the isothermal spheroidization, and providing a spheroidizing annealing method for spring steel including the step of slowly cooling the spring steel after the isothermal spheroidizing treatment together with the heat treatment furnace to the second temperature, and then discharging the furnace and air-cooling it, i.e., the second cooling stage.

[0008] In the heating-up stage, a rapid heating method of heating the spring steel to the two-phase region is adopted. When the temperature reaches the two-phase region (between the Ac1 and Ac3 temperatures), the ferrite in the pearlite is first austenitized, and the carbon element in the groove has a high energy density, so it is easy to dissolve into the austenite matrix. During the holding process in the two-phase region, the sheet-like carbide gradually breaks and dissolves, leaving undissolved particulate carbides distributed dispersedly. At this time, the material is rapidly cooled below the Ar1 temperature, and the dissolved carbide performs non-spontaneous nucleation with the undissolved granular carbide as the core to form a spherical carbide structure.

[0009] In some embodiments, the components of the spring steel, by mass%, contain C: 0.56 - 0.64%, Si: 0.17 - 0.37%, Mn: 0.7 - 1.0%, P: ≤0.025%, S: ≤0.02%, Cr: 0.7 - 1.0%, Ni: ≤0.35%, Cu ≤0.25%, and the balance is the matrix Fe and inevitable impurities.

[0010] In some embodiments, the heating rate of the rapid heating is 120 - 160 °C / h. By adopting rapid heating, the heating efficiency is increased, and at the same time, the surface burning loss and the depth of the decarburized layer are reduced.

[0011] In some embodiments, the two-phase region temperature is 740 - 760 °C, and the holding time at the two-phase region temperature is 96 min - 160 min.

[0012] The spring steel is close to eutectoid steel, the two-phase region is narrow. By controlling the temperature at 740 - 760 °C and the holding time at 96 - 160 min, a good spheroidization effect can be obtained. If the temperature or holding time is lower than this, the dissolution amount of sheet-like carbides is too small, and some carbides still exist in sheet form, reducing the spheroidization rate. If the temperature or holding time exceeds this, the dissolution amount of carbides is too large, the nucleation sites decrease, and the spheroidization rate decreases.

[0013] In some embodiments, the heat treatment furnace is a continuous annealing furnace, which consists of five parts: a heating zone, a holding zone, a rapid cooling zone, an isothermal zone, and a slow cooling zone. The heating zone, the holding zone, the rapid cooling zone, the isothermal zone, and the slow cooling zone sequentially perform the rapid heating stage, the two-phase region holding stage, the first cooling stage, the isothermal spheroidization stage, and the second cooling stage on the spring steel.

[0014] In some embodiments, the cooling rate in the first cooling stage is 20 - 30 °C / h.

[0015] In the first cooling stage, cooling at this cooling rate can retain undissolved granular carbides and promote spheroidization. If the cooling rate is too fast, the degree of supercooling increases, which is disadvantageous for spheroidization. If the cooling rate is too slow, the undissolved granular carbides will dissolve further, which is also disadvantageous for spheroidization.

[0016] The first cooling stage is carried out in the rapid cooling zone of the continuous annealing furnace, and the cooling method is to turn on the fan for cooling.

[0017] In some embodiments, the first temperature is 700 - 720 °C, and the isothermal time for isothermal spheroidization is 216 min - 360 min.

[0018] When the first temperature of isothermal spheroidization is controlled within this range, the spheroidization effect is the best. If the temperature is too high or too low, the spheroidization rate of the spring steel will decrease. At the same time, it is necessary to reasonably control the isothermal time of isothermal spheroidization so that if the spheroidization time is too short, the spheroidization effect is poor, and if the spheroidization time is too long, the accumulation of carbides will not increase significantly.

[0019] In some embodiments, the second temperature is 525 - 595 °C.

[0020] If the second temperature is too high, when air-cooled after discharging from the furnace, the temperature difference between the inside and outside of the spring steel is large, the stress is large, and deformation cracking is likely to occur. If the second temperature is too low, the production effect is too low, which affects the roll speed and problems occur in continuous production.

[0021] In some embodiments, the slow cooling is to cool together with the furnace in the slow cooling zone.

[0022] In some embodiments, the inside of the heat treatment furnace adopts nitrogen gas atmosphere protection, the height of the material arrangement entering the furnace is 120 mm - 180 mm, and the roller speed is 1.5 - 2.5 m / h.

[0023] Compared with the hydrogen gas atmosphere, the present application adopts nitrogen gas atmosphere protection to improve the production safety.

[0024] The present application adopts rapid heating and isothermal spheroidization annealing by a continuous annealing furnace. The total heating time is about 15 h. For continuous production, the annealing time is significantly shortened and the annealing efficiency is improved. After protection by introducing nitrogen gas, there is no obvious burning loss on the surface, there is no total decarburized layer on the material, and the depth of the total decarburized layer can be controlled within 0.3 mm, reducing the depth of the decarburized layer that requires polishing. At the same time, the present application controls the cooling process after isothermal spheroidization so that no large cooling stress is generated, and the cold working performance is improved.

[0025] The second aspect of the present application provides a spring steel manufactured by the above-mentioned spheroidization annealing method of spring steel.

[0026] In some embodiments, the spheroidization rate of the spring steel produced by the above-mentioned spheroidizing annealing method of spring steel is ≥80%, the hardness is ≤190 HBW, the crystal grain size is ≥Grade 6, there is no obvious burning loss on the surface, there is no total decarburized layer in the material, and the depth of the total decarburized layer can be controlled within 0.3 mm.

[0027] Compared with the prior art, the beneficial effects of the present application are as follows.

[0028] 1) The present application performs spheroidizing annealing of spring steel using a continuous annealing furnace. By rapidly heating the spring steel to the temperature between Ac1 and Ac3, non-uniform austenite and a large amount of undissolved granular carbides are obtained. Then, it is rapidly cooled below the Ar1 temperature. The carbides non-spontaneously nucleate with undissolved particles and hetero-eutectoid to form spheroidized carbides. This technology is characterized by a fast heating rate and a short spheroidization time.

[0029] 2) By controlling the holding temperature in the two-phase region, the holding time, and the isothermal temperature and isothermal time of the spheroidizing annealing, the spheroidization rate of the obtained spring steel is ≥80%, the hardness is ≤190 HBW, the crystal grain size is ≥Grade 6, there is no obvious burning loss on the surface, there is no complete decarburized layer in the material, and the depth of the total decarburized layer can be controlled within 0.3 mm, reducing the depth of the decarburized layer that requires grinding. The spring steel produced by the present application meets the raw material requirements of high elastic limit spring steel for cold forming.

Brief Description of the Drawings

[0030]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0031] The following examples further elaborate on the content of this application. The protection scope of this application includes, but is not limited to, the following examples. For those where specific conditions are not specified in the examples, operations are carried out according to normal conditions or the conditions proposed by the manufacturer. For those where the manufacturing manufacturer of the chemicals and equipment used is not specified, all are ordinary products that can be obtained commercially.

[0032] The spring steel in the examples of this application is in the state of hot-rolled bar before spheroidizing annealing, and the diameter range is Φ20~Φ80mm. The spring steel structure in the hot-rolled state mainly consists of sheet-like pearlite and ferrite.

[0033] The following elaborates on the present invention in detail through specific examples. The number of the hot-rolled spring steel bars used in the examples is 60CrMn, which complies with the standard GB / T1222-2016, and the mass percentage of each element is as shown in Table 1. Example 1

[0034] The components of the 60CrMn steel bars in this example are shown in Table 1. Table 1 Mass % of each element in 60CrMn (the balance is Fe and inevitable impurities)

[0035]

Table 1

[0036] The hot-rolled 60CrMn steel bars are put into a continuous annealing furnace, and heating, heat preservation, rapid cooling, isothermal treatment, slow cooling, and furnace discharging are carried out sequentially to obtain the bars after spheroidizing annealing. Specifically, (1) Place the hot-rolled spring steel bars on the transmission rollers of the continuous annealing furnace, arrange the materials at a height of 150mm. After the material arrangement is completed, transmit at the set roller speed of 2m / h and enter the continuous annealing furnace for spheroidizing annealing material arrangement, and (2) Rapidly heat the bars to the two-phase region temperature of 750°C at a heating rate of 150°C / h, and keep the spring steel at this temperature for 120 minutes for rapid heating, and (3) Send the spring steel that has completed heat preservation in the two-phase region to the rapid cooling zone of the continuous annealing furnace, turn on the fan, and rapidly cool the bar from 750 °C to 710 °C at a cooling rate of 25 °C / h, (4) Isothermal spheroidizing annealing of the spring steel at 710 °C, with an isothermal time of 300 min, (5) Slowly cool the spring steel that has completed isothermal spheroidizing in the furnace to 560 °C, (6) Discharge the spring steel bar that has completed slow cooling from the furnace. The steps include

[0037] Produce 9 batches of 60CrMn based on the designed continuous furnace spheroidizing annealing process, with specifications of Φ20~Φ80mm, length of 4~7m. The spheroidization rate measurement is carried out based on SEP-1520 1998-09, the hardness measurement is carried out based on GBT231.1-2018, and the crystal grain size measurement is carried out based on ISO 643.

[0038] Table 2 shows the performance of 60CrMn produced by the continuous furnace spheroidizing annealing process designed according to this example. As can be seen from Table 2, the carbide spheroidization rate of 60CrMn spring steel is 85%~92%, the hardness value is 172HBW~181HBW, the crystal grain size is 7~8 grades, there is no obvious burning loss on the surface, there is no total decarburized layer in the material, and the depth of the total decarburized layer can be controlled within 0.3 mm, meeting the requirements. Table 2 Performance of 60CrMn manufactured by the continuous furnace spheroidizing annealing process designed according to Example 1

[0039]

Table 2

[0040] Figure 1 is the spheroidized structure diagram of 60CrMn spring steel manufactured using the spheroidizing annealing process of this application, and the spheroidizing effect is good. Example 2

[0041] The components of the 60CrMn steel bar in this example are the same as those of the 60CrMn steel bar in Example 1. The spheroidizing annealing method of spring steel provided by this example is basically the same as the spheroidizing annealing method of spring steel in Example 1, except that the heating rate of rapid heating in step (2) is 120 °C / h. Specifically, (1) Place the spring steel bar after hot rolling on the transmission roller of the continuous annealing furnace, arrange the materials at a height of 150 mm. After the material arrangement is completed, it is transmitted at the set roller speed of 2 m / h and enters the continuous annealing furnace to perform the material arrangement for spheroidizing annealing, (2) Rapidly heat the bar at a heating rate of 120 °C / h to the two-phase region temperature of 750 °C, and keep the spring steel at this temperature for 120 min for rapid heating, (3) Send the spring steel that has been kept warm in the two-phase region to the rapid cooling zone of the continuous annealing furnace, turn on the fan, and rapidly cool the bar from 750 °C to 710 °C at a cooling rate of 25 °C / h for rapid cooling, (4) Isothermal spheroidizing annealing of the spring steel at 710 °C, and the isothermal time is 300 min for isothermal spheroidizing, (5) Slowly cool the spring steel that has completed isothermal spheroidizing in the furnace to 560 °C for slow cooling in the furnace, (6) Take out the spring steel bar after slow cooling is completed. It includes the steps of taking out of the furnace.

[0042] 60CrMn is produced by the continuous furnace spheroidizing annealing process designed according to this example, with a specification of Φ20 mm. Measure the performance of 60CrMn produced in this example, and the results are shown in Table 3. Example 3

[0043] The components of the 60CrMn steel bar in this example are the same as those of the 60CrMn steel bar in Example 1. The spheroidizing annealing method of spring steel provided by this example is basically the same as the spheroidizing annealing method of spring steel in Example 1, except that the isothermal time in step (4) is 216 min. Specifically, (1) Place the spring steel bar after hot rolling on the transmission roller of the continuous annealing furnace, arrange the materials at a height of 150 mm. After the material arrangement is completed, it is transmitted at the set roller speed of 2 m / h and enters the continuous annealing furnace to perform the material arrangement for spheroidizing annealing, (2) Rapidly heat the bar to the two-phase region temperature of 750°C at a heating rate of 150°C / h, and hold the spring steel at this temperature for 120 min for rapid heating, and (3) Send the spring steel that has completed heat preservation in the two-phase region to the rapid cooling zone of the continuous annealing furnace, turn on the fan, and rapidly cool the bar from 750°C to 710°C at a cooling rate of 25°C / h for rapid cooling, and (4) Isothermal spheroidizing annealing of the spring steel at 710°C, and the isothermal time is 216 min for isothermal spheroidizing, and (5) Slowly cool the spring steel that has completed isothermal spheroidizing to 560°C in the furnace for in-furnace slow cooling, and (6) Taking out the spring steel bar after slow cooling is completed. The steps include taking out of the furnace.

[0044] 60CrMn is produced by the continuous furnace spheroidizing annealing process designed according to this example, with a specification of Φ60mm. Measure the performance of 60CrMn produced in this example, and the results are shown in Table 3. Table 3 Performance of 60CrMn produced by the continuous furnace spheroidizing annealing process designed according to Examples 2-3

[0045]

Table 3

[0046] Comparative Example 1 The composition, state before annealing, and dimensional specifications of the 60CrMn steel bar are the same as those in Example 1, and the normal annealing process was adopted. The general annealing process is specifically (1) Place the spring steel bar after hot rolling on the transmission roller of the continuous annealing furnace, arrange the materials at a height of 150 mm, and after the material arrangement is completed, transmit at the set roller speed of 2 m / h and enter the continuous annealing furnace to perform spheroidizing annealing for material arrangement, and (2) Rapidly heat the bar to the two-phase region temperature of 750°C at a heating rate of 150°C / h for rapid heating, and (3) After reaching the temperature, hold the spring steel at this temperature for 650 min for heat preservation, and (4) Slowly cool the spring steel that has completed isothermal spheroidizing to 560°C in the furnace for in-furnace slow cooling, and (5) The step and process parameters of discharging the spring steel bar after the slow cooling is completed.

[0047] The normal annealing process was adopted, and the spheroidization rate was only about 50%. As shown in Figure 2, some carbides showed a layered or short rod-shaped distribution, and the hardness was 210 - 230 HBW, which could not meet the usage requirements.

[0048] Comparative Example 2 The components of the 60CrMn steel bar in this comparative example are as shown in Table 1. The spheroidizing annealing method of the spring steel is basically the same as that of Example 1, except that the rapid heating rate in step (2) is 60 °C / h. Specifically, (1) Place the spring steel bar after hot rolling on the transmission roller of the continuous annealing furnace, arrange the material at a height of 150 mm. After the material arrangement is completed, it is transmitted at the set roller speed of 2 m / h and enters the continuous annealing furnace for spheroidizing annealing. (2) Rapidly heat the bar to the two-phase region temperature of 750 °C at a heating rate of 60 °C / h, and hold the spring steel at this temperature for 120 min. (3) Send the spring steel after holding in the two-phase region to the rapid cooling zone of the continuous annealing furnace, turn on the fan, and rapidly cool the bar from 750 °C to 710 °C at a cooling rate of 25 °C / h. (4) Isothermal spheroidizing annealing of the spring steel at 710 °C, and the isothermal time is 300 min. (5) Slowly cool the spring steel after isothermal spheroidizing to 560 °C in the furnace. (6) The steps include discharging the spring steel bar after the slow cooling is completed.

[0049] 60CrMn was produced by the continuous furnace spheroidizing annealing process designed according to this comparative example, and the specification is Φ30 mm. The performance of 60CrMn produced in this comparative example was measured, and the results are shown in Table 4. As is obvious from Table 4, the depth of the decarburized layer is 0.38 mm, which is significantly larger than that of 60CrMn with a specification of Φ30 mm in Example 1, and it could not meet the usage requirements.

[0050] Comparative Example 3 As shown in Table 1, the components of the 60CrMn steel bar in this comparative example are as follows. The spheroidizing annealing method for the spring steel is basically the same as that of Example 1, except that in step (3), it is rapidly cooled to 680°C. Specifically, (1) Place the spring steel bar after hot rolling on the transmission roller of the continuous annealing furnace, arrange the materials at a height of 150 mm. After the material arrangement is completed, it is transmitted at the set roller speed of 2 m / h and enters the continuous annealing furnace for spheroidizing annealing material arrangement, and (2) Rapidly heat the bar at a heating rate of 150°C / h to the two-phase region temperature of 750°C, and rapidly heat the spring steel and hold it at this temperature for 120 min, (3) Send the spring steel that has been held in the two-phase region to the rapid cooling zone of the continuous annealing furnace, turn on the fan, and rapidly cool the bar from 750°C to 680°C at a cooling rate of 25°C / h, (4) Isothermal spheroidizing annealing of the spring steel at 710°C, and the isothermal time is 300 min, (5) Slowly cool the spring steel that has completed isothermal spheroidizing to 560°C in the furnace, (6) Take out the spring steel bar after slow cooling, including the steps of taking out of the furnace.

[0051] 60CrMn was produced by the continuous furnace spheroidizing annealing process designed according to this comparative example, and the specification is Φ30 mm. The performance of the 60CrMn produced in this comparative example was measured, and the results are shown in Table 4. As is clear from Table 4, the spheroidization rates are 68% and 62%, which are significantly lower than those of the 60CrMn with a specification of Φ30 mm in Example 1. The hardnesses are 195 and 197 HBW, which are significantly higher than those of the 60CrMn with a specification of Φ30 mm in Example 1, and could not meet the usage requirements.

[0052] Comparative Example 4 As shown in Table 1, the components of the 60CrMn steel bar in this comparative example are as follows. The spheroidizing annealing method for the spring steel is basically the same as that of Example 1, except that in step (3), the cooling rate is 110°C / h. Specifically, (1) Place the hot-rolled spring steel bar on the transmission roller of the continuous annealing furnace, arrange the materials at a height of 150 mm. After the material arrangement is completed, it is transmitted at the set roller speed of 2 m / h and enters the continuous annealing furnace for globular annealing material arrangement, and (2) Rapidly heat the bar at a heating rate of 150 °C / h to the two-phase region temperature of 750 °C, and keep the spring steel at this temperature for 120 min for rapid heating, and (3) Send the spring steel that has been kept warm in the two-phase region to the rapid cooling zone of the continuous annealing furnace, turn on the fan, and rapidly cool the bar from 750 °C to 710 °C at a cooling rate of 110 °C / h for rapid cooling, and (4) Isothermal globular annealing of the spring steel at 710 °C, and the isothermal time is 300 min for isothermal globularization, and (5) Slowly cool the spring steel that has completed isothermal globular annealing to 560 °C in the furnace for slow cooling in the furnace, and (6) The step of discharging the spring steel bar after slow cooling is completed. It includes the steps of discharging.

[0053] 60CrMn was produced by the continuous furnace globular annealing process designed according to this comparative example, and the specification is Φ30 mm. The performance of 60CrMn produced in this comparative example was measured, and the results are shown in Table 4. As is clear from Table 4, the globularization rates are 74% and 72%, which are significantly lower than those of 60CrMn with a specification of Φ30 mm in Example 1. The hardnesses are 192 and 194 HBW, which are significantly higher than those of 60CrMn with a specification of Φ30 mm in Example 1, and the usage requirements could not be met. Table 4 Performance of 60CrMn manufactured by the continuous furnace globular annealing process designed according to Comparative Examples 2-4

[0054]

Table 4

[0055] The above are only preferred embodiments of the present invention and do not limit the present invention. All modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A spheroidizing annealing method for spring steel, comprising: a rapid heating step of putting the spring steel in the hot-rolled state into a heat treatment furnace and rapidly heating it to the two-phase region temperature; a two-phase region holding step of holding the spring steel at the two-phase region temperature; a first cooling step of rapidly cooling the spring steel to a first temperature; isothermal spheroidizing of spheroidizing the spring steel isothermally at the first temperature; a second cooling step of slowly cooling the spring steel together with the heat treatment furnace to a second temperature after the isothermal spheroidizing treatment, and then discharging the furnace and air-cooling it, wherein the components of the spring steel are, by mass%, C: 0.56 to 0.64%, Si: 0.17 to 0.37%, Mn: 0.7 to 1.0%, P: ≤0.025%, S: ≤0.02%, Cr: 0.7 to 1.0%, Ni: ≤0.35%, Cu ≤0.25%, the balance being matrix Fe and unavoidable impurities; the heating rate of the rapid heating is 120 to 160 °C / h; the two-phase region temperature is 740 to 760 °C, and the holding time at the two-phase region temperature is 96 min to 160 min; the cooling rate of the first cooling step is 20 to 30 °C / h; the first temperature is 700 to 720 °C, and the isothermal time of the isothermal spheroidizing is 216 min to 360 min; the second temperature is 525 to 595 °C, and a spheroidizing annealing method for spring steel is characterized in that.

2. The heat treatment furnace is a continuous annealing furnace, and the continuous annealing furnace consists of five parts: a heating zone, a holding zone, a rapid cooling zone, an isothermal zone, and a slow cooling zone. The first cooling step is performed in the rapid cooling zone of the continuous annealing furnace, and the cooling method is to turn on the fan for cooling. The spheroidizing annealing method for spring steel according to claim 1 is characterized in that.

3. The slow cooling is characterized in that it is cooled together with the furnace in the slow cooling zone. The spheroidizing annealing method for spring steel according to claim 1 is characterized in that.

4. Nitrogen gas atmosphere protection is adopted inside the heat treatment furnace, the height of the material arrangement entering the furnace is 120 mm to 180 mm, and the roller speed is 1.5 to 2.5 m / h. The spheroidizing annealing method for spring steel according to claim 1 is characterized in that.

5. Nitrogen gas atmosphere protection is adopted inside the heat treatment furnace, the height of the material arrangement entering the furnace is 120 mm to 180 mm, and the roller speed is 1.5 to 2.5 m / h. The spheroidizing annealing method for spring steel according to claim 2 is characterized in that.

6. The inside of the heat treatment furnace adopts nitrogen gas atmosphere protection, the height of the material arrangement entering the furnace is 120 mm to 180 mm, and the roller speed is 1.5 to 2.5 m / h. The spheroidizing annealing method for spring steel according to claim 3 is characterized in that.

7. Spring steel manufactured by the spheroidizing annealing method for spring steel according to claim 1.

8. Spring steel manufactured by the spheroidizing annealing method for spring steel according to claim 2.

9. Spring steel manufactured by the spheroidizing annealing method for spring steel according to claim 3.

10. Spring steel manufactured by the spheroidizing annealing method for spring steel according to claim 4.

11. The spring steel according to claim 7, characterized in that the spheroidization rate of the spring steel is ≧ 80%, the hardness is ≦ 190 HBW, and the crystal grain size is ≧ 6 grades.

12. The spring steel according to claim 8, characterized in that the spheroidization rate of the spring steel is ≧ 80%, the hardness is ≦ 190 HBW, and the crystal grain size is ≧ 6 grades.

13. The spring steel according to claim 9, characterized in that the spheroidization rate of the spring steel is ≧ 80%, the hardness is ≦ 190 HBW, and the crystal grain size is ≧ 6 grades.

14. The spring steel according to claim 10, characterized in that the spheroidization rate of the spring steel is ≧ 80%, the hardness is ≦ 190 HBW, and the crystal grain size is ≧ 6 grades.

Images