KR100797327B1 - Steel wire rod for high strength and high toughness spring having excellent cold workability, method for producing the same and method for producing spring by using the same - Google Patents

Abstract

A steel wire rod having excellent cold formability in the postprocess as a wire rod for producing a spring generally having high strength and high toughness is provided, a method for producing the steel wire rod is provided, and a method for producing a spring generally having high strength and high toughness by using the steel wire rod is provided. A steel wire rod for high strength and high toughness spring having excellent cold formability has a composition comprising, by weight percent, 0.4 to 0.7% of C, 1.5 to 3.5% of Si, 0.3 to 1.0% of Mn, 0.01 to 1.5% of Cr, 0.01 to 1.0% or less of Ni, 0.01 to 1.0% or less of Cu, 0.005 to 0.02% of B, 0.1% or less of Al, 0.0015% or less of O, 0.02% or less of P, 0.2% or less of S, 0.02% or less of N, and the balance of Fe and other inevitable impurities, and has an internal structure comprising ferrite and pearlite, wherein the internal structure has a spherical austenite grain size of 8 mum or less. In the internal structure of the steel wire rod, the total area fraction of bainite and martensite structures is less than 1%. The composition for the steel wire rod further comprises, by weight percent, 0.5% or less of V and 0.5% or less of Ti.

Description

Steel wire for high strength, high toughness spring with excellent cold workability, method for manufacturing the steel wire and method for manufacturing spring from the steel wire {STEEL WIRE ROD FOR HIGH STRENGTH AND HIGH TOUGHNESS SPRING HAVING EXCELLENT COLD WORKABILITY, METHOD FOR PRODUCING THE SAME AND METHOD FOR PRODUCING SPRING BY USING THE SAME}

1 is a graph showing a CCT diagram when cooling for a conventional wire rod,

2 is a graph showing a CCT diagram when cooling after rolling with a wire rod having fine grains, and

3 is a graph comparing the grain size when the rolling temperature is lowered in the second rolling mill from the final rolling mill and the grain size when it is not.

The present invention relates to a high-strength, high toughness spring steel wire material having excellent cold workability, a method for manufacturing the steel wire material and a method for manufacturing a spring from the steel wire material, and more specifically, coil springs, leaf springs, torsion bars for automobiles And a steel wire for producing a spring having high strength and toughness at the same time as a spring used in a stabilizer, etc., the steel wire having excellent cold workability without the need for softening heat treatment for peeling processing or shaving processing in a later process, the steel wire material It relates to a method of manufacturing a high strength, high toughness spring using the steel wire.

Recently, the seriousness of air pollution caused by the pollution sources caused by the combustion of the petroleum fuels has increased worldwide as the use of fossil fuels, especially petroleum fuels, and in addition to the oil spill accident of large tankers, oil prices In order to avoid the damage caused by the petroleum fuel, the research on the technology to reduce the consumption of the petroleum fuel, if possible, has been conducted at various angles.

As a demand source for using a large amount of petroleum fuel, an automobile may be cited, and the automobile manufacturer is continuously conducting various attempts and studies to reduce the amount of petroleum fuel used. One of the most traditional ways to reduce the use of petroleum fuel is to improve the fuel efficiency of automobiles, which is the mainstream of the currently developed and applied technologies. One of the methods for improving is mentioned. Another method is to reduce the amount of energy required to move the unit distance by reducing the weight of the vehicle.

In order to reduce the weight of the vehicle body, there may be a method of replacing the parts in the car with a light weight material having a low specific gravity, but there are not many fields that can replace the superiority of steel parts. Therefore, steel parts are still often used as automobile parts, and it is common to attempt to improve automobile fuel efficiency by lightening the steel parts.

The simple weight reduction of steel parts can cause a fatal problem for the safety of automobiles because the load that can be supported per unit weight is determined. Therefore, the weight reduction of parts becomes a feasible task after solving the task of increasing the strength of parts.

In particular, automotive spring is a concept similar to high strength is a component that strongly demands excellent permanent deformation resistance. Permanent deformation resistance means resistance to the phenomenon of permanent deformation in which the spring height does not change completely after the spring is used for a long time. In order to increase the permanent deformation resistance of the spring, steel wire rods in which Si is added a large amount It has been used as a spring material. The Si serves to prevent permanent deformation by increasing the yield strength of the steel.

In addition, Si is an element belonging to Group 4 on the periodic table and is thermodynamically similar to C. As described above, the increase in strength (high tensile strength) of the component is not an exception to the spring, but an element added essentially for the high strength is C. C is easy to add and improves the strength of the steel through the action of strengthening the solid solution or causing precipitation strength together with other alloying elements added together. However, when C is added to the alloy together with a large amount of Si, the two elements compete with each other by the similar thermodynamic behavior of C and Si, and as a result, decarburization occurs in which C is removed from the alloy.

Conventional Si-added spring steel is SAE9250 and the like, since the Si content of 1.8 to 2.0% by weight in the spring steel, the surface decarburization of C in the steel species becomes more severe, as a result Steel grades have a problem such as fatigue life degradation due to surface decarburization layer, which makes it difficult to use them in springs.

In order to solve this problem, by lowering the overall carbon content and adding Ni to prevent the presence of decarburized portion in the surface layer, further adjust the Si content to compensate for the decrease in strength as the carbon content decreases, Mo High-strength spring steels (Japanese Patent Application Nos. JP1998-110247 and JP1996-176737, Korean Patent Application No. 1997-0073576 and Korean Patent Application No. 1999-0048929) have been developed that further increase the maximum design stress to 1200 MPa.

However, this development steel has increased the Si content for the improvement of yield strength and deformation resistance in terms of alloy design. Since the Si segregation zone is mainly formed in the center of the wire rod, the formation of such segregation zone is a major cause of ferrite formation and encourages nonuniformity of the central microstructure, and it is a major cause of large change in physical properties and deterioration of spring toughness. Cause.

In addition, the developed high stress spring steel has a low temperature structure (Bainite) even when the wire rod is cooled at a relatively low speed in the manufacture of the wire rod, due to the addition of a large amount of alloying elements, thereby increasing the manufacturing cost. + Complexes of martensite) occur. If low temperature structure occurs during wire rod manufacturing, it may cause problems during processing in the post process. In other words, low-temperature tissues such as bainite and martensite have very high hardness due to internal stress generated during transformation, and these low-temperature tissues are peeled to adjust the wire diameter or improve the surface quality before spring-forming the wire rod. (peeling) or shaving (shaving) processing is a cause that makes the machining difficult. Therefore, in order to smooth the processing, heat treatment such as softening heat treatment is performed on the wire rod, which is another factor of cost increase and deterioration of operability.

In addition, strength and toughness in general, it is a concept that is arranged to each other, it is also difficult to secure them at one time is another problem to be solved for the high-strength spring steel. In other words, in order to improve the strength of the spring, it is essential to form a hard tissue such as martensite or bainite in the steel, and such a hard tissue such as martensite or bainite is weak in general. This is because the impact toughness is poor due to the (brittle) property.

As discussed above, the spring requires both high strength and high toughness apart from those required to secure high permanent strain resistance and fatigue strength, but until now, steel for springs having these properties has not been developed yet. to be. In addition, since the low-temperature structure is partially generated inside the steel wire for the spring, in most cases, the softening heat treatment should be carried out by the spring customer.

Accordingly, it is an object of the present invention to provide a steel wire having excellent cold workability in a post process and a method of manufacturing the same as a wire rod for producing a spring having both high strength and high toughness.

In addition, another object of the present invention is to provide a method for producing a spring having high strength and high toughness using the wire.

As one aspect of the present invention for achieving the above object, the steel wire of the present invention is a weight%, C: 0.4 ~ 0.7%, Si: 1.5 ~ 3.5%, Mn: 0.3 ~ 1.0%, Cr: 0.01 ~ 1.5% , Ni: 0.01 to 1.0% or less, Cu: 0.01 to 1.0% or less, B: 0.005 to 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: It has a composition containing 0.02% or less, remainder Fe and other unavoidable impurities, has an internal structure composed of ferrite and pearlite, and has a spherical austenite grain size of 8 µm or less.

At this time, it is preferable that the sum of the area fractions of the bainite and martensite structures in the internal structure of the steel wire is less than 1%.

In addition, the composition of the steel wire is effective to further include V: 0.5% or less and Ti: 0.5% or less by weight.

As another aspect of the present invention, the method for producing a high strength, high toughness spring steel wire having excellent cold workability is weight%, C: 0.4 to 0.7%, Si: 1.5 to 3.5%, Mn: 0.3 to 1.0%, Cr. : 0.01 to 1.5%, Ni: 0.01 to 1.0% or less, Cu: 0.01 to 1.0% or less, B: 0.005 to 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02 When rolling billets having a composition containing% or less, N: 0.02% or less, balance Fe and other unavoidable impurities by hot rolling, the rolling temperature in the second or more rolling mill from the final rolling mill of the finishing mill is 850 ° C or less. It features.

At this time, it is preferable that the composition of the steel wire further includes V: 0.5% or less and Ti: 0.5% or less by weight.

And the said rolling temperature is more than Ar3, and it is effective.

In addition, it is preferable to start cooling at a temperature of 700 ~ 850 ° C to the rolled wire rod to perform cooling at a rate of 5 ° C / sec or less to room temperature.

As another aspect of the present invention, a method for producing a spring having high strength and high toughness is% by weight, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01 to 1.0% or less, Cu: 0.01 to 1.0% or less, B: 0.005 to 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02 High strength and high cold workability, having a composition containing less than%, balance Fe and other unavoidable impurities, having an internal structure composed of ferrite and pearlite, and having a spherical austenite grain size of 8 μm or less. A method of manufacturing a high strength, high toughness spring for producing a spring using a steel wire for toughness spring, comprising: peeling and shaving the steel wire without softening heat treatment; Austenitizing the steel wire; Oil-cooling the austenitized steel wire; Treating the oil-cooled steel wires; And cold working the thinned steel wire into a spring shape.

Another spring manufacturing method of the present invention for producing a spring by hot working separately from the spring manufacturing method by the cold working, by weight, C: 0.4 ~ 0.7%, Si: 1.5 ~ 3.5%, Mn: 0.3 to 1.0%, Cr: 0.01 to 1.5%, Ni: 0.01 to 1.0% or less, Cu: 0.01 to 1.0% or less, B: 0.005 to 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02 % Or less, S: 0.02% or less, N: 0.02% or less, has a composition containing the balance Fe and other unavoidable impurities, has an internal structure composed of ferrite and pearlite, and the old austenite grain size of the internal structure is 8 µm A method of manufacturing a high strength, high toughness spring for producing a spring using a high strength, high toughness spring steel wire having excellent cold workability, characterized in that the peeling and shaving without softening heat treatment for the steel wire ); Hot working the spring wire in a spring shape; Austenitizing the hot worked spring; Cooling the austenitized spring; And considering the oil-cooled spring.

At this time, the austenitization temperature is preferably 900 ~ 1000 ℃.

In addition, it is preferable that the said soaking treatment temperature is 350-450 degreeC.

Hereinafter, the present invention will be described in detail.

In general, tensile strength and impact toughness, which are the most important properties in the development of spring steel, exhibit opposite properties. Therefore, it is the most important development goal to maximize the opposite impact toughness while keeping the tensile strength value at a minimum. Therefore, the composition of the spring steel material of the present invention described below is a composition capable of maximizing the impact toughness while maintaining a high tensile strength.

The inventors of the present invention control the composition of the steel wire as described below, in order to realize the technical idea, the acid / carbon / of Al, B, V, and Ti inside when the steel wire having the following composition is manufactured with a spring By forming nitride-based precipitates to improve the strength and toughness, at the same time to improve the hardenability and grain boundary strengthening during the heat treatment using the hardenability enhancing element B to improve the high strength and toughness at the same time.

Hereinafter, the component system of the steel wire of this invention is demonstrated.

C: 0.4-0.7 wt%

C is an essential element added to secure the strength of the spring. When the content of C is less than 0.4% by weight, hardenability may not be secured and thus strength required for spring steel may not be secured. In addition, when the C content is more than 0.7% by weight, the fatigue strength is significantly reduced because twin martensite structures are formed during the hardening and annealing treatment to cause material cracks. In addition, it is preferable to limit the C content to 0.4 to 0.7% by weight because it is difficult to secure sufficient toughness due to the high strength and suppress the material decarburization caused by high Si addition.

Si: 1.5-3.5 wt%

Si is dissolved in ferrite and has the effect of strengthening the base material strength and improving the deformation resistance. By the way, when the Si content is less than 1.5% by weight, the lower limit of Si needs to be limited to 1.5% by weight because Si is not sufficiently effective in solid solution in the ferrite to strengthen the base material strength and improve the deformation resistance. In addition, when the Si content exceeds 3.5% by weight, the effect of improving the deformation resistance is saturated, so that the effect of addition may not be obtained, and the content of Si is limited to 1.5 to 3.5% by weight because it promotes surface decarburization during heat treatment. It is desirable to.

Mn: 0.3 ~ 1.0 wt%

Mn is an element that is beneficial to secure strength by improving the hardenability of steel when present in steel. Therefore, when the Mn content is less than 0.3% by weight, it is difficult to obtain sufficient strength and hardenability required as a material for high strength springs. On the contrary, when the Mn content exceeds 1.0% by weight, the toughness decreases, so the Mn content is 0.3 to 1.0% by weight. It is desirable to limit to%.

Cr: 0.01 ~ 1.5 wt%

Cr is a useful element for securing oxidation resistance, temper softening, surface decarburization prevention and quenching. However, when the Cr content is less than 0.01% by weight, it is difficult to secure sufficient oxidation resistance, temper softening property, surface decarburization and quenching effect. In addition, when the content exceeds 1.5% by weight, the deformation resistance may be lowered, which may lead to a decrease in strength. Therefore, the addition amount of preferable Cr is 0.01 to 1.5 weight%.

Ni: 0.01-1.0 wt%

Ni is an element added to improve the hardenability and toughness. If the content of Ni is less than 0.01% by weight, the effect of improving hardenability and toughness is not sufficient. If the content of Ni is more than 1% by weight, the amount of retained austenite is increased to reduce the fatigue life. Since the production cost rises, the amount of addition needs to be limited to 0.01 to 1% by weight.

Cu: 0.01 ~ 1.0 wt%

The addition of Cu is effective for preventing decarburization and improving corrosion resistance. The decarburized layer significantly reduces fatigue life after spring processing. This effect is weak at less than 0.01% by weight, and more than 1.0% by weight is likely to cause rolling defects due to embrittlement.

B: 0.005 to 0.02 wt%

The addition of B has an effect of densifying rust generated on the surface, increasing corrosion resistance and enhancing grain boundary strength by improving hardenability. If less than 0.005% by weight, the hardenability is not secured, and thus the strength required for the spring steel cannot be secured. If it exceeds 0.02% by weight, the carbonitride-based precipitates are coarsened, which adversely affects the fatigue characteristics.

O: 0.0015 wt% or less

The content of O is limited to 0.0015% by weight or less. When it exceeds 0.0015% by weight, oxide-based nonmetallic inclusions are coarsened, and fatigue life is drastically reduced.

Al: 0.1 wt% or less

The addition of Al refines the grain size and improves toughness. When the Al content exceeds 0.1% by weight, the amount of oxide precipitates is increased and the size thereof is coarsened, which adversely affects the fatigue characteristics.

P and S: 0.02% by weight or less

The content of P and S is limited to 0.02% by weight or less. Since P segregates at grain boundaries to reduce toughness, the upper limit thereof is limited to 0.01% by weight. It is preferable to limit the upper limit to 0.02% by weight since it has a detrimental effect on the spring characteristics.

A sufficient effect can be obtained only by the above composition, but in addition to the advantageous steel composition, the strength and toughness of the steel can be further improved by adding V and Ti as necessary as described below.

V: 0.005 to 0.5% by weight or less, Ti: 0.005 to 0.5% by weight or less

The V or Ti is one of the more preferable elements of the spring steel composition of the present invention, an element that improves the spring characteristics by forming a carbon / nitride by a single or compound addition to cause precipitation hardening action, the content of each 0.005 It is limited to the range of 0.5 wt% or less and 0.005 to 0.5 wt%. When the content is low, the precipitation of V and Ti-based carbon / nitride is reduced, and the effect of improving grain size control and spring characteristics (fatigue characteristics and permanent strain resistance) is insufficient. If the content is high, the manufacturing cost rises sharply, the spring property improvement effect by the precipitate is saturated, and the coarse alloy carbide which does not dissolve in the base material increases during the austenite heat treatment. And precipitation strengthening effect is lowered.

When the spring is manufactured using the wire rod having the composition described above, a spring having excellent strength and toughness can be obtained.

However, as described above, when the composition is controlled to improve the strength of the spring, a low temperature structure is easily generated when the wire is cooled, thereby increasing the hardness of the wire. Therefore, since the cold workability is poor, even a wire rod having the above-described composition cannot secure excellent cold workability by a general manufacturing method.

The inventors of the present invention have investigated to determine the cause of such a problem, and as a result, even if relatively slow cooling in the composition range of the conventional spring steel material, the cooling curve does not pass through the ferrite or pearlite region on the CCT diagram shown in FIG. Since it immediately enters the bainite or martensite region, it can be seen that a large amount of low-temperature tissue such as bainite or martensite is generated.

Therefore, in order not to generate a low temperature structure, a method of slowing the cooling rate and passing the pearlite or ferrite region may be considered. However, the inventors have found that in order to allow the cooling curve to pass through the ferrite or pearlite region on the TTT diagram in the conventional spring steel composition including the spring steel composition of the present invention, the cooling rate should be at most 3 ° C / sec. . However, the cooling capacity of most of the wire rod cooling equipment currently adopted is 5 ℃ / sec or less, it is very difficult to accurately control the cooling rate at such a low cooling rate (less than 3 ℃ / sec). Therefore, it is not desirable at this stage to manufacture a wire rod having excellent cold workability by slowing the cooling rate.

In the other method, when the pearlite nose described in FIG. 1 is moved to the left side, the cooling curve is sufficiently sufficient even at a relatively high cooling rate (that is, when the horizontal axis of the CCT diagram is time-consuming). One way is to allow it to pass through an area. In this case, the CCT diagram may be in the form shown in FIG.

Mainly, the shape of the CCT diagram is affected by the composition, but the inventors of the present invention confirmed that the shape of the CCT diagram can be controlled by adjusting the grain size even if the composition of the steel wire is fixed.

That is, in a general wire rod manufacturing process, the size of the austenite grains before cooling in the wire rod internal structure is about 12 μm. In this case, the CCT diagram has the shape shown in FIG. 1. However, as one of the main conditions of the present invention, when managing the austenite grain size before cooling to 8 µm or less, the CCT diagram shows that the pearlite and ferrite regions are shifted to the far left (i.e., the short time side) as shown in FIG. It has a form. The reason is that the ferrite or pearlite grains are transformed at the austenite grain boundary. If the austenite grain size (AGS) is small before the transformation, the grain boundary required for the ferrite or pearlite transformation is rapidly increased, so that ferrite or Perlite transformation will increase.

Therefore, it is important to control the size of the austenite grains before cooling to 8 μm in order to produce wire rods excellent in cold workability even at a relatively high cooling rate without changing the composition. Accordingly, the steel wire of the present invention is characterized in that the wire structure having the above-mentioned advantageous composition is composed of ferrite and pearlite, and the grain size of the sphere austenite in the internal structure is 8 µm or less.

In addition, low-temperature tissues such as bainite and martensite are preferably not formed, but since they may be inevitably formed to some extent, the amount is preferably less than 1% as a fraction of the total tissue area.

There may be several ways to control the austenite grain size. In other words, the austenite grain size is greatly influenced by the amount of deformation during deformation, the rate of deformation, the hot rolling temperature and the like. These hot rolling conditions result in static recrystallization, dynamic recrystallization, quasi-dynamic recrystallization and grain growth.If the cross-sectional shape of the material being processed like hot wire rolling is circular and the rolling speed is high, the deformation amount and deformation rate are changed. It's hard to do Therefore, it is desirable to control the recrystallization behavior and the grain growth behavior by adjusting the hot rolling temperature.

In the related art, in order to refine the grains by controlling the hot rolling temperature, a method of pancakeizing and miniaturizing the austenite grain shape while suppressing recrystallization by rolling while keeping the temperature of the total rolling section low has been widely used. However, in this case, the load acting on the roll during the whole rolling process is increased, which causes a burden on the equipment, and thus adversely affects power consumption and equipment life.

However, according to the inventors of the present invention, even if rolling in the entire rolling section as shown in FIG. 3, the rolling section that actually affects the austenite grain size is the rolling section within the last two of the final rolling mill. When the rolling temperature of the rolling mill is maintained at 750 ~ 850 ℃ it can be adjusted to austenite grain size of 8㎛ or less. In FIG. 3, the square mark indicates a temperature behavior as a case of manufacturing a wire rod with a normal manufacturing pattern, and a mark indicates a change in austenite grain size. Similarly, the circle mark indicates a case where the wire rod is manufactured by the manufacturing pattern according to the present invention, where o indicates a temperature behavior and o indicates a change in austenite grain size. As shown in FIG. 3, in the case of the manufacturing pattern according to the present invention, the austenite grain size was found to be less than 5 μm as a result of maintaining the rolling temperature of the final two rolling mills at 850 ° C. or less, while In this case, the rolling temperature in the final two rolling mills was 950 ° C. or more, and the grain size inside the steel wire thus produced was 12 μm or more. This is due to the static loading crystallization occurring in the first half of the rolling and the grain size of the wire rod did not fluctuate significantly, while the static recrystallization of the wire occurred in the second half (especially the last two mills), resulting in slow recrystallization behavior and delayed grain growth. This is because the crystal grain refinement effect can be obtained.

Therefore, it is important to make the rolling temperature in the last two or more rolling machines into 850 degreeC or less.

However, when the filament rolling temperature is less than Ar3, the austenite / ferrite transformation occurs before the austenite is refined by rolling to form coarse ferrite, so the filament rolling temperature needs to exceed Ar3.

Ar3 varies depending on the composition of the steel wire, but in the steel wire composition of the present invention, it is generally determined at about 740 ° C.

The process other than controlling the temperature of the last two rolling mills in the said wire rod manufacturing process is the same as a general wire rod process. That is, those skilled in the art can easily manufacture the steel wire for the spring by reheating the billet using various techniques already known, rough rolling and finishing rolling (temperature control in the final two or more rolling mills are required), and then cooling.

The wire rod cooling conditions at the time of cooling start cooling at a temperature of 700 ~ 850 ℃ and preferably cooled at a rate of 5 ℃ / sec or less to room temperature.

Subsequently, the steel wire manufactured by the above process undergoes peeling and shaving without softening heat treatment in a later process, is austenitized, oil-cooled and then soaked, and then cold-processed into a spring shape to be manufactured as a spring or Alternatively, after processing into a spring shape in the hot (850 ~ 1000 ^o C) and then austenitized and oil-cooled to be processed by the spring can be produced as a spring.

The general temperature range of the spring production method is the same as the general spring production conditions, but the feature of the spring production method of the present invention is that the softening heat treatment is not performed.

Therefore, the above peeling condition, shaving condition, austenitization temperature, oil cooling temperature, and soaking temperature are in accordance with ordinary spring production conditions.

However, the austenitization temperature is preferably carried out at a temperature of 900 ~ 1000 ℃ in order to prevent the crystal grains from becoming too coarse by recrystallization. That is, when the austenitization treatment temperature is less than 900 ° C., it is not preferable that the superfine ferrite is generated during cooling because of its low temperature, and when it is above 1000 ° C., it is not preferable because it promotes decarburization and grain growth. After the austenitization treatment, the quenching process is completed by quenching.

Since the quenched spring is high in strength but martensite structure is not desirable for improving toughness, it is preferable that the soaking process is followed. The internal structure is changed from martensite to sour martensite by the soaking process.

Preferred consideration temperature is 350-450 degreeC. If the soaking temperature is less than 350 ° C., the martensite effect may not be sufficient, and the toughness of the spring may deteriorate. If the soaking temperature exceeds 450 ° C., the martensite may transform into a higher temperature structure. Therefore, it is preferable that the soaking temperature is 350-450 degreeC.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

After casting steels having a composition as shown in Table 1 and fabricating steel strips, the wire was manufactured by performing hot rolling under the conditions of Table 2 below. Thus, the hot-rolled steel wire was processed into a spring shape, heat-treated at 950 ° C., and then cooled in oil to prepare a specimen by heat-treating at a temperature of 390 ° C. and 420 ° C. as shown in Table 3.

In the case of Example 1 to Example 6 during spring processing has excellent cold workability as can be seen from the results of Table 2 below, but after the peeling and shaving process without the additional softening heat treatment, the spring processing, but in the case of Comparative Example cold workability When peeling and shaving as it is lacking there is a possibility that the material is damaged, it is maintained by 120 ~ 180 minutes at 500 ~ 700 ℃ after softening heat treatment and peeling and shaving process and then manufactured by spring.

Tensile tests were conducted to examine the cold workability of the wire rods produced under the conditions of Table 2. Tensile test pieces were taken in the rolling direction and processed to ASTM-Sub size. Tensile tests were conducted at a cross head speed of 2 mm / min, and the detailed values are shown in Table 2.

division C Si Mn Ni Cr V Ti Cu B P S Al Comparative Example 1 0.55 3.0 0.5 0.25 0.7 0.05 - 0.1 0.001 0.01 0.03 0.001 Comparative Example 2 0.55 2.2 0.5 0.25 0.7 0.20 - 0.1 - 0.008 0.008 0.01 Comparative Example 3 0.50 2.2 0.7 0.30 1.0 0.20 0.07 0.3 0.03 0.009 0.007 0.06 Comparative Example 4 0.6 1.4 0.6 - 0.5 - - - - 0.03 0.01 0.07 Example 1 0.45 2.9 0.7 0.5 1.2 0.4 0.3 0.3 0.006 0.008 0.009 0.03 Example 2 0.49 3.1 0.6 0.3 0.4 0.2 0.4 0.5 0.001 0.012 0.008 0.02 Example 3 0.55 2.6 0.7 0.1 0.6 0.4 0.2 0.8 0.008 0.009 0.015 0.05 Example 4 0.59 2.6 0.4 0.7 1.2 0.2 0.4 0.5 0.014 0.015 0.009 0.06 Example 5 0.64 1.9 0.8 0.5 1.3 0.3 0.4 0.1 0.017 0.018 0.015 0.04 6 0.69 1.6 0.9 0.8 0.9 0.2 0.09 0.4 0.007 0.005 0.016 0.07

However, the content of each component in Table 1 means weight%

division 4th rolling mill temperature (℃) from the final mill Old austenite grain size (㎛) Cooling Speed (3 ℃ / sec) Cooling rate (5 ℃ / sec) Cooling Speed (7 ℃ / sec) Cold tissue fraction (%) Wire Strength (MPa) Cold tissue fraction (%) Wire Strength (MPa) Cold tissue fraction (%) Wire Strength (MPa) Comparative Example 1 960 12 2 1100 3 1140 10 1200 Comparative Example 2 980 14 3 1098 4 1120 13 1198 Comparative Example 3 970 13 2.2 1060 4 1100 12 1150 Comparative Example 4 975 15 3.3 1110 5 1143 14 1200 Example 1 850 6 0.5 980 0.5 983 1.1 1030 Example 2 830 4 0.2 950 0.3 965 0.9 1000 Example 3 790 5 0.9 990 0.9 995 1.3 1040 Example 4 800 6 0.8 984 0.8 993 1.5 1060 Example 5 830 5 0.7 960 0.7 964 1.0 1020 Example 6 780 5 0.6 950 0.7 958 0.9 1040

However, in Table 2, the low temperature tissue fraction refers to the area fraction, and the wire strength refers to the tensile strength. In addition, the temperature from the final rolling mill to the fourth rolling mill to the final rolling mill was rolled while keeping substantially the same.

division Consideration temperature: 390 ℃ Consideration temperature: 420 ℃ Tensile Strength (MPa) Elongation (%) Impact value (J) Tensile Strength (MPa) Elongation (%) Impact value (J) Comparative Example 1 1987 6 3.2 1890 7 3.7 Comparative Example 2 1923 6 4.1 1884 6 4.7 Comparative Example 3 1930 5 3.7 1872 6 4.5 Comparative Example 4 2001 6 2.8 1930 7 3.5 Example 1 2097 15 6.5 2035 16 7.4 Example 2 2100 13 5.9 2060 15 6.6 Example 3 2198 10 6.1 2120 12 6.9 Example 4 2200 9 5.4 2145 10 6.3 Example 5 2235 9 5.6 2197 10 6.2 Example 6 2309 8 5.3 2265 10 6.0

As can be seen from the results of Table 2, when the cooling rate is 3 ℃ / second and 5 ℃ / second not only out of the component range of the present invention but also the rolling temperature conditions of the rolling mill Comparative Example 1 outside the range prescribed by the present invention In Comparative Example 4, the low temperature tissue fraction was very high as 2% or more, and as a result, the wire strength was much higher than that of Examples 1 to 6. On the other hand, in the case of Examples 1 to 6, the fraction of the low-temperature structure was 1% or less, belonging to the range suitable for cold working, and as a result, the wire strength was also good value of 1000 MPa or less. However, in the case of the cooling rate of 7 ℃ / sec, even in the Example it was able to observe the low-temperature fraction of more than 1%, it was confirmed that the tensile strength of the wire rod is relatively high as 1000MPa or more. The difference between the comparative example and the example is due to the austenite grain size before cooling, and the austenite grain size, which is a value capable of confirming the austenite grain size at room temperature, is 12 µm or more in the comparative example. In the case of the example, 6 micrometers or less showed the big difference.

In addition, as can be seen from Table 3, in the case of satisfying the composition of the present invention, the tensile strength thereof was satisfied to be 2000 MPa or more, but in Comparative Examples 1 to 4, the strength value was remarkably higher than that of the Examples. It was confirmed that the lack. This advantageous effect of the invention is due to the steel composition of the invention. That is, the steel composition defined in the present invention is to reduce the addition amount of Si to reduce the effect of surface decarburization, and is a composition in which B, V and Ti are added to replace the loss of strength to be generated due to this reduction, The additive effect is to minimize the degradation of strength and toughness by minimizing the crystal grains occurring during quenching by precipitates such as V (C, N) and Ti (C, N), enhancing the hardenability of boron and strengthening grain boundaries, This is due to the enhanced strength due to the enhanced precipitation strengthening.

As described above, the present invention not only can provide a spring with high strength and toughness, but also has excellent cold workability of the wire rod manufactured to provide the spring, and thus, without peeling and shaving without special heat treatment. It has the advantageous effect that it can be carried out.

Claims (11)

By weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0% or less, Cu: 0.01-1.0% or less, B: 0.005 ~ 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less and have a composition containing residual Fe and other unavoidable impurities. A high-strength, high toughness spring steel wire having excellent cold workability, characterized by having an internal structure composed of spherical austenite grains of 8 µm or less. The high-strength, high toughness spring steel wire having excellent cold workability according to claim 1, wherein the sum of the area fractions of the bainite and martensite structures in the internal structure of the steel wire is less than 1%. According to claim 1 or 2, wherein the composition of the steel wire is a high-strength, high toughness spring steel wire having excellent cold workability, characterized in that it further comprises V: 0.5% or less and Ti: 0.5% or less by weight. . By weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0% or less, Cu: 0.01-1.0% or less, B: Hot rolled billet having a composition containing 0.005 to 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, residual Fe and other unavoidable impurities To produce a wire rod, the method of producing a high strength, high toughness spring steel wire having excellent cold workability, characterized in that the rolling temperature in the second or more rolling mill of the finishing mill is less than or equal to 850 ° C. The method of claim 4, wherein the composition of the steel wire further comprises V: 0.5% or less and Ti: 0.5% or less by weight. The method according to claim 4 or 5, wherein the rolling temperature is higher than or equal to Ar3. 6. The excellent cold workability according to claim 4 or 5, wherein cooling of the rolled wire is started at a temperature of 700 to 850 ° C and cooled to a room temperature at a rate of 5 ° C / sec or less. Method for manufacturing high strength, high toughness spring steel wire. By weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0% or less, Cu: 0.01-1.0% or less, B: 0.005 ~ 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less and have a composition containing residual Fe and other unavoidable impurities. It has an internal structure consisting of a high strength, high toughness spring for producing a spring using a high strength, high toughness spring steel wire having excellent cold workability, characterized in that the old austenite grain size of the internal structure is 8㎛ or less As a manufacturing method, Peeling and shaving the steel wire without softening heat treatment; Austenitizing the steel wire; Oil-cooling the austenitized steel wire; Treating the oil-cooled steel wires; And Cold working the thinned steel wire in a spring shape; manufacturing method of high strength, high toughness spring comprising a. By weight%, C: 0.4-0.7%, Si: 1.5-3.5%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0% or less, Cu: 0.01-1.0% or less, B: 0.005 ~ 0.02%, Al: 0.1% or less, O: 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less and have a composition containing residual Fe and other unavoidable impurities. It has an internal structure consisting of a high strength, high toughness spring for producing a spring using a high strength, high toughness spring steel wire having excellent cold workability, characterized in that the old austenite grain size of the internal structure is 8㎛ or less As a manufacturing method, Peeling and shaving the steel wire without softening heat treatment; Hot working the spring wire in a spring shape; Austenitizing the hot worked spring; Cooling the austenitized spring; And The method of manufacturing a high-strength, high-toughness spring comprising the; 10. The method according to claim 8 or 9, wherein the austenitization temperature is 900 to 1000 ° C. The method for producing a high strength, high toughness spring according to claim 8 or 9, wherein the light treatment temperature is 350 to 450 ° C.

Images