KR20030053119A - A method for manufacturng spring steel without ferrite decarburization - Google Patents

Abstract

PURPOSE: A method for manufacturing suspension spring steel without peeling operation of ferrite decarburized layer is provided which is capable of skipping peeling process during processing of suspension spring steel wire rod by properly controlling hot rolling conditions, thereby suppressing formation of ferrite decarburized layer. CONSTITUTION: The method comprises the steps of preparing a billet comprising 0.4 to 0.6 wt.% of C, 1.3 to 1.6 wt.% of Si, 0.5 to 0.7 wt.% of Mn, 0.4 to 0.7 wt.% of Cr and a balance of Fe and other inevitable impurities; rough rolling the reheated billet after reheating the billet to a temperature of 1,000 to 1,100 deg.C; intermediate finish rolling the rough rolled billet at a temperature of 750 to 850 deg.C, and subsequently finish rolling the intermediate finish rolled billet to a final surface reduction ratio of 30% or more in the same temperature range; and cooling the coiled wire rod to a temperature of 600 to 650 deg.C at a cooling rate of 1 deg.C/sec or less after coiling the rolled wire rod in the temperature range of 750 to 800 deg.C, and subsequently air-cooling the resultant wire rod to an ordinary temperature in the air.

Description

A method for manufacturng spring steel without ferrite decarburization}

The present invention relates to a method for manufacturing spring steel for automobile suspension, and more particularly, to a method for manufacturing spring steel without ferrite decarburization while having similar fatigue strength and deformation resistance to existing spring steel.

In the reheating and hot rolling processes of the spring steel manufacturing process, surface decarburization, or ferrite decarburization layer, is formed on the material, which reduces the fatigue life of the material, and is therefore usually a surface processing, i.e., a process for shaving the surface. Peeling process is performed. However, since the peeling process is very expensive, if omitted, the yield can be increased by 14% or more.

On the other hand, SAE9254 has been developed in which silicon (Si), which promotes surface decarburization, is added and chromium (Cr), an element effective in preventing decarburization, has been developed in a component range that does not significantly reduce deformation resistance. In this case, however, the ferrite decarburized layer could not be completely controlled, and a step of shaving the surface of the spring wire rod had to be carried out normally.

Further, conventional techniques for suppressing the decarburized layer generated during spring production include Japanese Patent Laid-Open Nos. 2-301541, No. 1-31960, No. 63-216591, No. 63-153240 And (S) 58-67847 and (S) 58-27956. In the above-described techniques, copper, molybdenum, tin, etc. may be added while increasing the chromium content or lowering the carbon content. The method is presented. However, in this case, it was effective in reducing the decarburized layer but was not completely controlled. In addition, there was an uneconomical disadvantage due to the addition of expensive alloying elements.

Accordingly, the inventors of the present invention have repeatedly conducted research and experiments to solve the above problems, and based on the results, the present invention proposes a hot rolling condition such as steel reduction and material reduction rate in finishing rolling. It is an object of the present invention to provide a method for producing a spring steel which can omit the filling process during the processing of the spring steel wire by appropriately controlling the formation of the ferrite decarburized layer.

1 is a schematic configuration diagram of a wire rod rolling equipment used in the present invention

Figure 2 is a photograph (magnification: X100) showing the ferrite decarburization structure of the inventive material (1, 2, 3) of the present invention

Figure 3 is a photograph (magnification: X100) showing the ferrite decarburization structure of the present comparative material (primary material) (1, 2, 3)

* Description of Signs of Main Parts of Drawings *

1. Heating Furnace 2. Roughing Mill

3. First intermediate water chiller 4. Intermediate sand mill

5. Second intermediate water cooler 6. Filament mill

7. Water chiller 8. Winder

9. Air Cooler 10. Integrator

The present invention for achieving the above object,

By weight%, carbon (C): 0.4-0.6%, silicon (Si): 1.3-1.6%, manganese (Mn): 0.5-0.7%, chromium (Cr): 0.4-0.7%, balance Fe and other unavoidable impurities Providing a billet (Billet) is composed of; Reheating the billet to a temperature of 1000 ~ 1100 ℃ and then rough rolling; Intermediate rough rolling of the roughly rolled billet at 750 ° C. to 850 ° C., followed by finishing rolling at a final reduction rate of 30% or more in the same temperature range; After winding the rolled wire at 750 ~ 800 ℃ to the temperature of 600 ~ 650 ℃ 1 ℃ / s or less cooling rate, and then air-cooled to room temperature; comprising a method for producing ferrite decarburization-free spring steel It is about.

Hereinafter, the steel component of the present invention and the reason for limitation thereof will be described.

The content of C in the steel component of the present invention is limited to 0.4 to 0.6% by weight (hereinafter referred to as only%), because the carbon content is less than 0.4%, sufficient strength as a high stress spring steel by hardening and consideration This is because it is difficult to secure, and when it exceeds 0.80%, securing of toughness is insufficient and it is difficult to suppress material decarburization derived from the content of high silicon.

The content of Si is limited to 1.3 to 1.6%, because if the content of Si is less than 1.3%, silicon is dissolved in the ferrite, and the effect of strengthening the strength of the base material and improving the deformation resistance is insufficient, whereas 1.6% If it exceeds, the effect of improving the deformation resistance is saturated and there is a high possibility of decarburization during heat treatment.

The content of Mn is limited to 0.5 to 0.7%, because if the content of manganese is less than 0.5%, the strength and quenchability as spring steel is insufficient, and if the content exceeds 0.7%, the toughness decreases.

Cr is an element for reducing the ferrite decarburized layer, and the content of the present invention is limited to 0.4 to 0.7%. If the content of Cr is less than 0.4%, it is difficult to obtain an effect of sufficient hardenability and decarburization, and if it exceeds 0.7%, the decarburization inhibitory effect is saturated and there is no improvement effect.

In the present invention, after the billet is prepared as described above, and then subjected to a process such as reheating, hot rolling and winding, and then used for the spring, the manufacturing process will be described in detail below.

First, the reheating temperature is preferably set to 1000 ~ 1100 ℃, the reason is that when the reheating temperature exceeds 1100 ℃ the carbon activity is very increased, carbon is escaped from the surface to the outside to facilitate decarburization, If it is less than 1000 ℃ due to the effect of the alloying elements added for the characteristics of the spring steel is not possible due to the rolling load in the rough rolling.

Next, hot rolling is performed. Hereinafter, the hot rolling conditions of the present invention will be described in detail with reference to FIG. 1.

As shown in Figure 1, hot rolling generally consists of rough rolling (2) -water cooling (3) -medium sand rolling (4) -water cooling (5) -ideal rolling (6) -water cooling. It is characterized by controlling the temperature conditions of intermediate sand rolling and finishing rolling during the hot rolling process, and controlling the reduction rate of the raw material during rolling.

That is, in the present invention, the steel billet roughly rolled at a temperature range of 950 to 1050 ° C. is intermediately rolled at 750 to 850 ° C., and the ferrite layer of the ferrite decarburized layer, which is generated and grown in a heating furnace, is formed in a columnar shape. It is refined to polygonal form.

Then, the reheated material is cooled to a temperature range of 750 ° C. to 850 ° C., and then subjected to filament rolling so that the reduction rate of the material is 30% or more. The ferrite decarburized part of the surface coarsened by grain growth is further refined to prevent ferrite from growing, and the material is given a certain reduction rate to improve the activation energy, thereby quickly passing the section where the formation of ferrite is made.

As a result, the ferrite decarburized layer, in which ferrite grows to form columnar bands on the surface of the material, is removed, and through the interface inside the material rather than the amount of carbon that escapes from the surface through the microstructured structure during rolling or cooling. It is possible to increase the amount of carbon to be diffused and to reduce the amount of the formation of the cornerstone ferrite, and to maintain the same amount of carbon on the surface and the amount of carbon in the internal structure, thereby effectively removing the ferrite decarburized layer.

Meanwhile, in the present invention, the reason for limiting the intermediate sand rolling temperature and the finishing rolling temperature to 750 to 850 ° C is as follows.

In the present invention, unlike the conventional rolling method, the roughly rolled material is cooled to the unrecrystallized region (750 ~ 850 ℃) and then subjected to intermediate sand rolling, and the process heat generated at this time is removed again by water cooling to remove the temperature. After cooling to the unrecrystallized region (750 ~ 850 ℃) of the material is characterized in that the finishing rolling, the dislocation density is rapidly increased inside the material, the grain is also very fine.

If intermediate sand rolling temperature and finishing rolling temperature are higher than 850 ℃, there is an effect of microstructure in comparison with the conventional rolling method, but there is insufficient microstructure in rolling, so that ferrite grain grows locally during cooling, so that some circumference in the material section A columnar ferrite layer is generated, and thus, a ferrite decarburization layer is generated, which makes it impossible to control the ferrite decarburization over the entire material. And if it is less than 750 degreeC, rolling will become impossible due to the lack of rolling capability of an intermediate sand mill and a finishing mill.

In the present invention, it is also preferable that the rolling is performed so that the material reduction ratio is 30% or more. Because intermediate sanding and finishing rolling are carried out at the temperature range of 750 ~ 850 ℃, if the reduction rate of the material is less than 30%, the effect of grain refinement is insufficient, and the ferrite layer of ferrite decarburization layer produced and grown in the furnace is circumferential. It cannot be refined from columnar to polygonal form, and also, it is impossible to obtain some micronized grains, the effect of secondary tissue micronization during finishing rolling, and the grains grow to form a ferrite layer to form a ferrite decarburized layer. Because it forms.

As described above, the hot rolled steel wire is then wound, and at this time, the temperature is preferably limited to 750 to 800 ° C. If the coiling temperature is less than 750 ° C, in order to reach the target temperature instantaneously in the water cooling system, the material must be cooled with high pressure water, so the friction between the material and the water cooling system and the temperature drop of the material in the water cooling system are reduced. Due to the increased rigidity caused by the friction with the winder causes surface scratches.

In addition, when the coiling temperature exceeds 800 ℃, the deformation energy is contained even after hot rolling, and the transformation is accelerated, and the transformation time is accelerated on the CCT curve, thereby causing the low temperature structure of brittle tissue such as bainart and martensite. There is a problem.

The wire wound in this way can be manufactured in a spring steel without ferrite decarburization by slow cooling at a rate of 1 ° C./s or less to a temperature range of 600 to 650 ° C. in a conventional manner and then air cooling to room temperature.

Hereinafter, the present invention will be described in detail through examples.

(Example)

By weight%, billets composed of C: 0.53%, Si: 1.41%, Cr: 0.58%, Mn: 0.61%, P: 0.001%, S: 0.008%, balance Fe and other unavoidable impurities were prepared. The billet thus prepared is reheated by varying its reheating temperature as shown in Table 1, and then roughly rolled at 980 to 1020 ° C. Then, as shown in Table 1, while varying the intermediate sand rolling, filamentous rolling, reduction rate and hot rolling temperature, Hot rolled.

The ferrite decarburization depth and the occupancy rate of each wire rod thus manufactured were measured, and the results are shown in Table 2. Meanwhile, the ferrite decarburization depth means the maximum depth generated on the wire rod surface after the wire rod rolling, and the share of ferrite decarburization means the percentage of the circumference of the wire rod surface after the wire rod rolling. In addition, the total decarburization depth means the distance having the same structure as the carbon concentration of the matrix in the surface decarburization layer.

division Reheating Temperature (℃) Hot Rolling Temperature (℃) Reduction rate of filament rolling material Winding temperature (℃) Middle ideological rolling Sasang Rolling Invention 1 1000 805 830 40% 780 Invention 2 1100 780 810 35% 772 Invention 3 1020 820 830 30% 790 Comparative Material 1 1010 820 810 20% 800 Comparative Material 2 1020 798 820 10% 792 Comparative Material 3 1015 1020 996 35% 780

division Ferrite Decarburization Depth (mm) Ferrite decarburization share (%) Decarburization depth (mm) Invention 1 0 0 0.13 Invention 2 0 0 0.11 Invention 3 0.004 2 0.13 Comparative Material 1 0.02 40 0.12 Comparative Material 2 0.04 60 0.10 Comparative Material 3 0.06 100 0.17

As shown in Table 2, the ferrite decarburization depth was less than 0.004 mm for the invention materials 1 to 3 whose reheating temperature and rolling conditions were properly controlled, whereas the comparative materials 1 to 3 had a decarburization depth. It can be seen that more than 0.02mm and the share of the circumference is also more than 40%.

Meanwhile, FIGS. 2 and 3 are ferrite decarburized tissue photographs of the present invention and the comparative material described above, and confirms that the inventive materials 1 to 3 have a reduced depth of ferrite decarburization compared to the comparative materials 1 to 3. Can be.

As described above, the present invention can effectively produce the spring steel without performing the conventional peeling process by suppressing the production of the ferrite decarburized layer by optimally controlling the steel component and the hot rolling conditions, etc. In addition to improving by more than 14%, there is an effect that can improve the fatigue life of steel.

Claims (1)

By weight%, carbon (C): 0.4-0.6%, silicon (Si): 1.3-1.6%, manganese (Mn): 0.5-0.7%, chromium (Cr): 0.4-0.7%, balance Fe and other unavoidable impurities Providing a billet (Billet) is composed of; Reheating the billet to a temperature of 1000 ~ 1100 ℃ and then rough rolling; Intermediate rough rolling of the roughly rolled billet at 750 ° C. to 850 ° C., followed by finishing rolling at a final reduction rate of 30% or more in the same temperature range; Winding the rolled wire at 750 ~ 800 ℃ and then cooled to a temperature of less than 1 ℃ / s to a temperature of 600 ~ 650 ℃, and then air-cooled to room temperature; manufacturing method of the ferrite decarburization-free spring steel