KR102208165B1 - Steel Material for Springs with Tempering Process Omitting - Google Patents

Abstract

The present invention is nickel (Ni) 0.003 to 0.2 wt%, copper (Cu) 0.005 to 0.2 wt%, molybdenum (Mo) 0.01 to 0.5 wt%, titanium (Ti) 0.01 to 0.04 wt%, vanadium (V) 0.01 to 0.04 It contains at least one selected from 0.001 to 0.2% by weight of niobium (Nb), 0.001 to 0.01% by weight of aluminum (Al), 0.1 to 0.4% by weight of carbon (C), and 01. to 1.0 of silicon (Si). Wt%, manganese (Mn) 0.1 to 1.5 wt%, chromium (Cr) 0.1 to 0.7 wt%, boron (B) 0.001 to 0.004 wt%, nitrogen (N) 0.004 to 0.015 wt%, oxygen (O) 0.003 wt% Hereinafter, phosphorus (P) 0.01% by weight or less, sulfur (S) 0.01% by weight or less, the balance is composed of iron and other inevitable impurities, and the bainite structure fraction is formed by more than 90%, a heat treatment process consisting of quenching and tempering It relates to a steel material for a spring for omitting the tempering process in which the middle tempering process can be omitted.

Description

Steel Material for Springs with Tempering Process Omitting}

The present invention relates to a spring steel for omitting the tempering process.

More specifically, a steel for spring manufactured by providing an appropriate carbon equivalent to the spring steel to increase the bainite structure fraction, thereby reducing or minimizing the amount of martensite and pearlite produced, and properly securing the effective amount of boron. Improves strength and toughness.

This relates to a spring steel material for omitting the tempering process to improve the production efficiency of the spring by allowing the tempering process to be omitted in the spring heat treatment process consisting of quenching and tempering.

In general, since the spring is elastically deformed by an external impact and returns to its original state, heat treatment is performed such as quenching and tempering to secure the performance and durability of the spring. Various types of spring steel are applied according to the magnitude of the force acting on the applied structure.

SUP9 that satisfies the conditions of a tensile strength of 135±5kg/㎟, a yield strength of 125±5kg/㎟, and an elongation of 11±2% in order to maintain the performance and durability of the spring in the case of springs used in automobile suspensions, etc. The series of spring steel was mainly used.

However, in the case of heat treated steel including the conventional tempering process, it is difficult to efficiently use the space of the factory as the production line of the spring production plant is lengthened by the heating furnace that is required in the reheating process for the tempering process. There was a problem that additional installation cost and maintenance cost for the furnace occurred.

In particular, in recent years, a smart factory is being built on the extension of the concept of unmanned production facilities and management automation according to factory automation, but the smart factory is a step away from the factory automation concept of optimizing each unit process in the past, and the whole process is organically linked to each other. A production process suitable for multi-species complex production by sharing data between each process through the Internet of Things so that it can occur, and delivering active decision-making generated by comprehensive analysis of the collected data to the devices of each process in real time. You get flexibility.

Smart Factory exchanges data between processes and In order to allow the analysis to occur smoothly, reduction of the factory size is important, and it is inevitable to reduce the number of production lines and processes to reduce the factory scale.

In addition, when tempering a steel made of high carbon steel, oil is applied to the steel surface to prevent decarbonization during reheating and deteriorating surface hardness and corrosion resistance.These oils are vaporized during heating and are toxic. It discharged gas and caused environmental pollution along with treatment of waste oil.

At this time, if the tempering process is omitted, the production line of the spring production plant is reduced to improve the efficiency of the plant's space utilization, and the oil used for tempering can be heated to prevent environmental pollution that occurs during the treatment of steam or waste oil. In addition to being able to save electricity or chemical energy used for heating, production costs can be reduced, and the production speed can be further improved by reducing the number of processes, enabling a step forward in smart factory construction.

Due to the above advantages, steel for springs that can omit the heat treatment process has been developed, and the development process of the steel material omitting the heat treatment process is largely divided into four stages.

The first-generation heat treatment-omitted steel is a medium carbon steel with vanadium (V) added, and is air-cooled after hot forming to form a ferrite-pearlite structure, and its impact toughness is lower than that of heat-treated steel having the same tensile strength. , It is applied to crankshafts or connecting rods for automobiles that do not apply excessive shock and load during operation.

The second-generation heat treatment-omitted steel lowers the carbon content to compensate for the low impact toughness of the first-generation heat treatment-omitted steel, while increasing the silicon (Si) content, and performing strong cooling to form a needle-shaped ferrite or ferrite-pearlite structure. The cooling rate was increased, and the impact toughness was improved by adding molybdenum (Mo) and titanium (Ti) to refine the grains of pearlite.

In the 3rd generation heat treatment-omitted steel, niobium (Nb) and molybdenum (Mo) are added to further improve impact toughness and strength, increasing the martensite end temperature to 200°C by mass effect, and through controlled cooling immediately after hot forming. The carbide was uniformly dispersed to form a composite structure of bainite and martensite.

In order to improve the impact toughness, strength, and formability of the 4th generation heat treated steel at a level similar to that of heat treatment steel, development is underway in the direction of increasing the structure fraction of bainite through controlled cooling immediately after hot forming.

As a steel material forming the structure of such bainite, there is a high strength bainite-based non-tempered steel for automobile chassis parts of Korean Patent Laid-Open Publication No. 10-2003-0008852 (published on January 29, 2003), but in a high manganese (Mn) content. Accordingly, there is a problem in that the formation rate of martensite, which adversely affects the improvement of toughness, is increased, and boron (B), which can delay ferrite formation, is not contained, making it difficult to form a uniform bainite structure.

In addition, Korean Patent Publication No. 10-0908624 (registered on July 14, 2009) and Korean Patent Publication No. 10-1766567 (registered on August 02, 2017) contain pre-hardened steel with improved machinability and toughness, and a method for manufacturing the same, respectively. And, although it is posted in the hot-rolled steel sheet and its manufacturing method, since it corresponds to the manufacturing method of heat-treated steel including tempering, the disadvantage of heat-treated steel is reduced space efficiency in production plants, environmental pollution caused by oil, and heating. There was a problem that the problem of decreasing production speed due to an increase in production cost and an increase in the number of processes due to a large amount of energy consumed for harm was not solved.

Korean Patent Application Publication No. 10-2003-0008852 (published on January 29, 2003) Korean Registered Patent Publication No. 10-0908624 (registered on July 14, 2009) Republic of Korea Patent Publication No. 10-1766567 (registered on Aug. 2, 2017)

In an embodiment of the present invention, by applying a heat treatment-omitted steel material that omits a tempering process in the production process of a steel material for a spring, the production efficiency of the spring is improved, and the impact toughness, strength, and formability of the steel material without heat treatment are similar to those of the heat treatment steel. It aims to improve by.

In an embodiment of the present invention, it is an object to improve the toughness of the manufactured spring steel by preventing an increase in the martensitic structure generation rate.

In an embodiment of the present invention, it is an object to improve the toughness of the manufactured spring steel by increasing the structure fraction of bainite to reduce or minimize the amount of pearlite produced.

In an embodiment of the present invention, an object of the present invention is to further improve the toughness, strength, and hardness of the manufactured spring steel by suppressing the increase in the upper bainite structure fraction.

According to an embodiment of the present invention, in the spring steel material manufactured so that the tempering process can be omitted during the heat treatment process consisting of quenching and tempering, nickel (Ni) 0.003 to 0.2 wt%, copper (Cu) 0.005 to 0.2 wt%, Molybdenum (Mo) 0.01 to 0.5 wt%, titanium (Ti) 0.01 to 0.04 wt%, vanadium (V) 0.01 to 0.04 wt%, niobium (Nb) 0.001 to 0.2 wt%, aluminum (Al) 0.001 to 0.01 wt% Contains at least one selected, carbon (C) 0.1 to 0.4 wt%, silicon (Si) 0.1 to 1.0 wt%, manganese (Mn) 0.1 to 1.5 wt%, chromium (Cr) 0.1 to 0.7 wt%, boron (B) 0.001 to 0.004 wt%, nitrogen (N) 0.004 to 0.015 wt%, the balance is composed of iron and other inevitable impurities, and the bainite structure fraction is formed more than 90%.

According to an embodiment of the present invention, it contains molybdenum (Mo), nickel (Ni) and copper (Cu), and is expressed in the formula of C + Mn/6 + (Cr + Mo)/5 + (Ni + Cu)/15 The carbon equivalent (equivalent) is formed within the range of 0.2 to 0.6.

According to an embodiment of the present invention, a trace alloy of titanium (Ti), vanadium (V), niobium (Nb) and aluminum (Al) is included, and (1.15Ti + 0.99V + 0.58Nb + 1.99Al) / 7.66N The trace alloy addition constant expressed by the formula is formed within the range of 0.8 to 1.0.

According to an embodiment of the invention _{[{5.25B - (7.66N 2 -} 1.15Ti + 0.99V + 0.58Nb + 1.99Al) /5.25}] effective amount of boron, which is represented by the formula: x 10000 is 7 ~ 20 ppm range Is formed from

According to an embodiment of the present invention, the steel material contains 0.003 wt% or less of oxygen (O), 0.01 wt% or less of phosphorus (P), and 0.01 wt% or less of sulfur (S).

According to an embodiment of the present invention, austenite has a crystal grain size of 15 to 70 μm.

According to an embodiment of the present invention, a tissue fraction of lower-bainite formed at a relatively low temperature is formed at a rate of 60% or more than that of upper-bainite formed by austempering.

According to an embodiment of the present invention, the formed steel has a hardness of Hv410-525, of 135-185kg/mm ² Tensile strength, yield strength of 110~155kg/mm ² , yield ratio of 0.81~0.88, reduction ratio of 30~50%, elongation of 11~16%, and fatigue limit ratio range of 0.5~0.6 are satisfied.

According to an embodiment of the present invention, by omitting the tempering process in the manufacturing process of spring steel, the production speed of the spring is improved and production cost is reduced, and the scale of the factory is reduced by reducing the heating furnace installation line for reheating the spring. Thus, there is an effect of optimizing the flow of the production line.

According to an embodiment of the present invention, the impact toughness, strength, and molding processability of a spring manufactured by omitting the tempering process in the production process can be improved to a level similar to that of the tempering process heat-treated steel.

According to an embodiment of the present invention, by providing an appropriate carbon equivalent to adjust the time axis in which the bainite nose of the CCT curve is formed, there is an effect of increasing the bainite structure fraction.

According to an embodiment of the present invention, there is an effect of improving the toughness of the manufactured spring steel by preventing an increase in the martensitic structure generation rate.

According to an embodiment of the present invention, there is an effect of improving the toughness of the manufactured spring steel by increasing the texture fraction of bainite to reduce or minimize the amount of pearlite produced.

According to an embodiment of the present invention, there is an effect of suppressing an increase in the upper bainite structure fraction to further improve the toughness, strength, and hardness of the manufactured spring steel.

According to an embodiment of the present invention, there is an effect of preventing a nozzle clogging phenomenon when casting a steel material for an automobile spring.

1A and 1B are graphs showing a cooling rate of a spring made of a conventional steel without heat treatment and a cooling process of a spring made of a spring steel for omitting the tempering process according to the present invention, respectively.
2 is a graph showing a continuous cooling transformation (CCT) curve showing a phase transformation according to a cooling rate of a spring steel material.
3 is a graph showing a phenomenon in which the starting temperature of the phase transition temperature changes while the CCT curve moves according to the carbon equivalent, FIG. 3A is a carbon equivalent of 0.2 or less, FIG. 3B is a carbon equivalent of 0.2 to 0.6, and FIG. 3C is a carbon equivalent of 0.6 The above shows the CCT curve shift.
4 is a graph showing the range of the volume ratio of retained austenite that does not affect the yield strength according to the carbon content of the spring steel material.
5 is a graph showing an ideal effective boron range for securing an ideal bainite structure by delaying ferrite transformation.
6 is a graph showing the hardness value of a spring formed according to the cooling rate in the quenching process during the manufacturing process of the spring.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

It will be described in detail focusing on the parts necessary to understand the operation and operation according to the present invention.

In describing the embodiments of the present invention, descriptions of technical contents that are well known in the technical field to which the present invention pertains and are not directly related to the present invention will be omitted.

This is to more clearly convey the gist of the present invention by omitting unnecessary description.

In addition, in describing the constituent elements of the present invention, different reference numerals may be assigned to constituent elements of the same name according to the drawings, and the same reference numerals may be denoted even in different drawings.

However, even in such a case, it does not mean that the corresponding component has different functions according to the embodiment, or that it has the same function in different embodiments, and the function of each component is the corresponding embodiment. It should be determined based on the description of each component in

In addition, the technical terms used in the present specification should be interpreted in the meaning generally understood by those of ordinary skill in the technical field to which the present invention belongs, unless otherwise defined in this specification. It should not be construed as a meaning or an excessively reduced meaning.

In addition, the singular expression used in the present specification includes a plurality of expressions unless the context indicates otherwise.

In the present application, terms such as "consist of" or "include" should not be construed as necessarily including all of the various elements or various steps described in the specification, and some of the elements or some steps It may not be included, or it should be interpreted that it may further include additional elements or steps.

Spring steel for omitting the tempering process according to an embodiment of the present invention is 0.003 to 0.2 wt% nickel (Ni), 0.005 to 0.2 wt% copper (Cu), 0.01 to 0.5 wt% molybdenum (Mo), A component selected from 0.01 to 0.04% by weight of titanium (Ti), 0.01 to 0.04% by weight of vanadium (V), 0.001 to 0.2% by weight of niobium (Nb), and 0.001 to 0.01% by weight of aluminum (Al) contains the above content. At least one or more is contained, 0.1 to 0.4% by weight of carbon (C), 0.1 to 1.0% by weight of silicon (Si), 0.1 to 1.5% by weight of manganese (Mn), 0.1 to 0.7% by weight of chromium ( Cr), 0.001 to 0.004% by weight of boron (B), 0.004 to 0.015% by weight of nitrogen (N), balance iron and other inevitable impurities.

Hereinafter, the action and content of essential components excluding iron and other inevitable impurities constituting a spring steel material for omitting the tempering process according to an embodiment of the present invention are as follows.

1) carbon (C)

The steel material containing the above components and contents has a bainite structure fraction of 90% or more, and the steel material without heat treatment forming the existing bainite structure undergoes a process from p0 to p1 as shown in FIG. A steel material heated to an austenitization temperature between °C is hot-formed while passing through the p1 to p2 process, and the hot-formed spring is subjected to constant temperature transformation in the p3a to p4 process, or the bainite structure is formed through controlled cooling in the p3a to p4 process. Is done.

In contrast, in the embodiment of the present invention, the bainite structure is formed through rapid cooling in the p2 to p3c process as shown in FIG. 1B, and both the steel material that formed the existing bainite structure and the heat treatment omitted steel material and the embodiment of the present invention Although there is a common point that a separate tempering process is not required, when the rapid cooling process is performed, there is an advantage that the time required to complete the process can be significantly reduced compared to the controlled cooling or constant temperature transformation process.

At this time, when the bainite structure fraction is less than 90%, it becomes difficult to secure the mechanical properties and durability characteristics of the manufactured spring, and thus the bainite structure fraction is preferably formed to be 95% or more.

Bainite is formed within a temperature range of 400 to 650°C, which is the temperature at which pearlite and martensite are generated, as shown in the continuous cooling transformation curve (CCT curve) of FIG. 2 according to the content of the alloy and the cooling rate. As the microstructure of steel, it is one of the decomposition products formed when austenite is cooled beyond the critical temperature of 727℃, and the microstructure shape and hardness characteristics of bainite are similar to those of martensite after a tempering process. Have a structure

In addition, bainite has a fine non-lamellar structure and is made of cementite and dislocation-rich ferrite. Due to the high density dislocations contained in ferrite, it has a higher hardness than conventional ferrite. Will have.

In addition, the bainite microstructure has a two-phase structure composed of ferrite and iron carbide, and at a relatively low temperature compared to upper-bainite and upper bainite, depending on the composition and cooling rate of austenite. The resulting lower-bainite structure is created.

Upper bainite is an aggregate of ferrite laths forming a plate-shaped region by forming parallel groups, and carbides precipitated at the austenite grain boundary (inter-lath region) before the formation of the bainite structure are between the lath boundaries depending on the carbon content. It may be a cause of lowering the toughness of bainite steel while forming a complete carbide film.

On the other hand, the lower bainite is composed of ferrite and carbide aggregates of fine scale, and the carbide precipitated inside the ferrite plate has a rod or blade shape, so even if the tempering process is omitted, sufficient strength and toughness are secured. To be able to do it.

Therefore, by forming the lower bainite structure fraction larger than the upper bainite structure fraction, it is possible to improve the strength, hardness and toughness characteristics of the manufactured spring, and it is preferable to form the lower bainite structure fraction of 60% or more.

If the lower bainite structure fraction is less than 60%, the upper bainite structure fraction is relatively high and the manufactured spring's toughness decreases, and accordingly, a softening heat treatment similar to the tempering process must be added in the spring processing process. This is because the process shortening effect by omission of the process cannot be obtained.

The tissue fraction of the lower bainite is maximized at the cooling rate approaching the nose of the bainite diagram as shown in the continuous cooling transformation curve shown in FIG. 2, and is increased at the cooling rate away from the nose. In order to increase the structure fraction of bainite, rapid cooling must be performed, and it is preferable to cool the molded spring at a cooling rate of 20 to 150°C/sec until it reaches room temperature.

In this case, the cooling rate of 20 to 150°C/sec may be applied differently according to the volume of the spring for rapid cooling.

The bainite structure fraction of the spring steel is greatly influenced by the carbon equivalent as shown in FIG. 3, but when the carbon equivalent is low, the bainite nose of the CCT curve moves to the short-time side as shown in FIG. 3A. While the upper bainite tissue fraction is relatively increased, and if the carbon equivalent is high, the amount of martensite produced increases as the bainite nose of the CCT curve moves to the long side as shown in FIG. 3C.

Therefore, the steel material for a spring according to an embodiment of the present invention is limited to contain 0.1 to 0.4% by weight of carbon.

When the carbon content is less than 0.1% by weight, sufficient strength of the spring cannot be secured, and the hardenability (quenching property) of the spring steel itself is insufficient, causing difficulty in forming a bainite structure.

If the carbon content exceeds 0.4% by weight, the strength of the manufactured spring can be improved, but the martensite generation rate increases sharply and the toughness is greatly reduced, which is liable to cause damage, so the carbon content is less than 0.4% by weight. To meet the minimum standards required to form an appropriate level of strength.

2) Silicon (Si)

The spring steel material for omitting the tempering process according to an embodiment of the present invention contains 0.1 to 1.0% by weight of silicon, and when the silicon content is less than 0.1% by weight, the decarburization and solid solution strengthening effect cannot be sufficiently generated.

In addition, when the silicon content exceeds 1.0% by weight, the toughness and plastic workability of the spring steel with bainite structure decreases, and the surface hardness and durability of the manufactured spring due to excessive decarburization of the material surface of the spring steel material. This is greatly degraded.

3) Manganese (Mn)

The spring steel material for omitting the tempering process according to an embodiment of the present invention contains 0.1 to 1.5% by weight of manganese, and manganese increases the strength of the spring steel by causing solid solution strengthening of ferrite contained in bainite, The bainite structure is refined to improve the toughness of spring steel.

At this time, when the manganese content is less than 0.1% by weight, it is insufficient to promote the hardenability and the formation of bainite structure, and when the manganese content is more than 1.5% by weight, the hardenability is greatly increased and the martensite structure production rate is increased. It adversely affects the toughness of the spring, and it becomes difficult to ensure the homogeneity of the bainite structure.

4) Chrome (Cr)

The spring steel material for omitting the tempering process according to an embodiment of the present invention contains 0.1 to 0.7% by weight of chromium, which improves the strength, fatigue strength, and abrasion resistance of the steel material for spring, and improves hardenability to improve bainite. It generates a structure stably and increases impact resistance by forming a composite carbide with molybdenum and vanadium.

At this time, when the chromium content is less than 0.1% by weight, it becomes difficult to obtain the effect of the chromium content, and when the chromium content exceeds 0.7% by weight, martensite structure is generated and the brittleness of the manufactured spring increases, resulting in the risk of damage to the spring. Will increase.

5) Boron (B)

The spring steel material for omitting the tempering process according to the embodiment of the present invention contains 0.001 to 0.004% by weight of boron. If boron is contained, the CCT curve is shifted to the long side by delaying the precipitation of cornerstone ferrite. The boron contained in the atomic state is segregated at the austenite grain boundary, thereby lowering the grain boundary free energy, thereby inhibiting the formation of the foundation stone ferrite, thereby promoting the formation of the bainite structure.

When the boron content is less than 0.001% by weight, the effect of promoting bainite structure formation becomes insignificant, and when the boron content exceeds 0.004% by weight, boron having a high affinity with nitrogen and oxygen forms oxides and nitrides during the dissolution process. Since borocarbide such as M ₂₃ (CB) ₆ or Fe ₂ B is formed at the rolling or forging temperature, the formation of cornerstone ferrite is promoted, making it difficult to secure the bainite structure fraction.

In particular, if the cooling rate is slow when the spring is quenched, boron nitride is formed at the grain boundaries, and the generated nitride acts as a nucleation site of ferrite, thereby reducing the strength and toughness of the spring.

Therefore, in order to prevent the formation of boron nitride, titanium, vanadium, niobium, aluminum, and the like must be added to appropriately secure the amount of effective boron that does not combine with nitrogen and exists in a single atomic state.

6) nitrogen (N)

The spring steel material for omitting the tempering process according to an embodiment of the present invention contains 0.004 to 0.015% by weight of nitrogen, and nitrogen is combined with titanium, aluminum and vanadium contained in the spring steel to form carbonitrides, and formed carbonitrides Cargo refines austenite grains to improve the strength and toughness of bainite steel.

At this time, when the nitrogen content is less than 0.004% by weight, it is difficult to obtain the effect of improving the strength and toughness of the bainite steel, and when the nitrogen content is more than 0.015% by weight, carbonitrides are coarsened, which does not contribute to the grain coarsening. .

In addition, when the content of titanium, aluminum, and vanadium is insufficient compared to the amount of nitrogen contained, nitrogen is dissolved in the spring steel material to greatly reduce toughness.

In the spring steel material for omitting the tempering process according to an embodiment of the present invention, in addition to the carbon, silicon, manganese, chromium, boron, nitrogen, in order to omit the tempering process in the spring manufacturing process, the characteristics of the steel material structure for the spring may be changed or , In order to improve the performance of the manufactured spring, such as strength or toughness, one or more of nickel, copper, molybdenum, titanium, vanadium, niobium and aluminum may be optionally added and contained, and the action and content of the optional additional component It is as follows.

1) Nickel (Ni)

The spring steel material for omitting the tempering process according to an embodiment of the present invention may additionally contain 0.003 to 0.2% by weight of nickel. Nickel improves the hardenability of the steel to stably form a bainite structure, and manufacture Strength can be increased without reducing the toughness of the spring, and corrosion resistance is improved.

At this time, if the nickel content is less than 0.003% by weight, the effect of improving the strength and corrosion resistance of the spring material and the effect of securing toughness at low temperatures can be obtained.If the nickel content exceeds 0.2% by weight, the critical point of the effect is reached, resulting in a more improved effect. It cannot be obtained, and manufacturing cost can be increased.

2) Copper (Cu)

The spring steel for omitting the tempering process according to an embodiment of the present invention may additionally contain 0.005 to 0.2% by weight of copper, improving the tensile and yield strength of the spring according to the solid solution strengthening and precipitation strengthening effect, and corrosion resistance. Improves.

At this time, when the copper content is less than 0.005% by weight, the effect of improving corrosion resistance is insufficient, and when the copper content exceeds 0.2% by weight, the critical point of the effect is reached and no improved effect is obtained, and the melting point is lowered during intergranular segregation. When charging into a heating furnace for rolling, grain boundary embrittlement may cause surface flaws or decrease in toughness of the manufactured spring.

3) Titanium (Ti)

The steel material for automobile springs according to an embodiment of the present invention may additionally contain 0.01 to 0.04% by weight of titanium, and titanium is combined with nitrogen in the bainite steel to which boron is added to fix nitrogen, thereby suppressing the formation of boron nitride. Thus, the effective amount of boron in the atomic state can be secured.

At this time, TiN, which is the most stable of the grain refining element compounds, has a low solubility at high temperature and a slow grain growth rate, which can contribute to grain refinement, and reduces austenite grain size by inhibiting austenite grain growth by immobilizing austenite grain boundaries. .

As the austenite grain size decreases, the bainite transformation initiation temperature decreases, reducing the formation of upper bainite, increasing the fraction of lower bainite, suppressing the formation of martensite, securing the lower bainite structure more easily It is possible to increase the tensile strength and yield strength of the manufactured spring.

At this time, when the titanium content is less than 0.01% by weight, the above effect becomes insignificant, and when the titanium content exceeds 0.04% by weight, the improvement effect reaches saturation, so that the improved effect is not exhibited, and the toughness of the manufactured spring is deteriorated. .

4) Molybdenum (Mo)

The spring steel material for omitting the tempering process according to the embodiment of the present invention may additionally contain 0.01 to 0.5% by weight of molybdenum, and molybdenum increases the hardenability of the spring steel to stably obtain a bainite structure, and molybdenum Carbide refines the grain size and improves the strength and toughness of the manufactured spring.

If the cooling rate is slow during the manufacturing process of the spring, the toughness of the spring produced by dispersing the coarse precipitates of molybdenum may be lowered.Thus, the transformation temperature of bainite and martensite may be reduced through rapid cooling, resulting in microstructure of carbides and It can increase the toughness of the spring by generating densification.

At this time, when molybdenum is contained together with boron, the quenching property is controlled during cooling to optimize the balance between tensile strength and toughness. When the molybdenum content is less than 0.01% by weight, the effect of containing it becomes insignificant.

In addition, when the molybdenum content exceeds 0.5% by weight, the hardenability increases more than necessary, increases the martensite production rate, and leads to a saturated state in which the improvement effect does not increase any more.

Particularly, since boron and molybdenum are expensive elements, if more than necessary content is contained, the production cost of the spring increases significantly, so it is preferable not to contain more than the required amount.

5) Vanadium (V)

The spring steel for omitting the tempering process according to an embodiment of the present invention may additionally contain 0.01 to 0.04% by weight of vanadium, and the vanadium improves the strength of the spring steel through fine carbide formed by bonding with carbon in the spring steel. In addition, it plays an important role in controlling the hardenability to secure an appropriate bainite structure by forming vanadium carbonitride (VC,VCN) at a temperature of 900°C or higher to prevent grain growth of austenite phase.

Vanadium forms fine precipitates of vanadium carbonitride in the ferrite structure while the spring steel is cooled, and improves the strength of the manufactured spring through precipitation and dispersion strengthening.

In particular, since the fine precipitates of vanadium carbonitride are unstable at high temperatures, it is very important to control the cooling rate in order to form the fine precipitates of vanadium carbonitride, and the degree of strength improvement of the spring steel varies depending on the carbon content and cooling rate.

At this time, if the content of vanadium is lower than 0.01% by weight, the effect of the vanadium content is insignificant, and when the content of vanadium exceeds 0.04% by weight, the improvement effect reaches a saturation state in which no further increase is made.

In particular, when coarse carbonitrides are formed according to the excessive vanadium content, the toughness of the manufactured spring decreases and the steel material is embrittled, so it is preferable to contain an appropriate amount so as not to deviate from the appropriate content range.

6) Niobium (Nb)

The spring steel material for omitting the tempering process according to an embodiment of the present invention may additionally contain 0.01 to 0.04% by weight of niobium, and niobium is niobium carbonitride (NbC, NbN) during hot forming processes such as rolling or forging of the spring. As it precipitates at this grain boundary, a fixing effect appears, thereby miniaturizing the crystal grains and improving the strength and toughness of the bainite structure.

At this time, if the content of niobium is less than 0.01% by weight, it is difficult to compensate for the fixing effect due to niobium carbonitride and the effect of improving the hardenability accompanying lowering the carbon content, and the bainite transformation becomes difficult, and the content of niobium When the content exceeds 0.04% by weight, the toughness of the bainite structure may be lowered while the coarse niobium carbonitride is formed, so it is preferable to contain an appropriate amount of niobium.

7) Aluminum (Al)

In the spring steel material for omitting the tempering process according to an embodiment of the present invention, aluminum may additionally contain 0.001 to 0.01 wt%, and aluminum is a strong deoxidizing agent that removes oxygen contained in the spring steel material while forming aluminum oxide. It acts and combines with nitrogen to refine bainite grains.

At this time, if the content of aluminum is less than 0.001% by weight, the deoxidation effect or the effect of the bainite grain refining effect is reduced, which is not preferable, and when the content of aluminum exceeds 0.01% by weight, the amount of aluminum oxide non-metallic inclusions increases. It may cause a decrease in the toughness of the manufactured spring or clogging of the nozzle when casting steel for spring.

In addition, the steel material for spring for omitting the tempering process according to an embodiment of the present invention includes 0.003 wt% or less of oxygen (O), 0.01 wt% or less of phosphorus (P), and 0.01 wt% or less of sulfur ( S).

Specifically, phosphorus is segregated at the grain boundary (inter-rath region) to reduce the toughness of the spring steel.

In addition, sulfur combines with manganese and iron during the steelmaking of spring steels to form emulsions (MnS) and irons (FeS) that lower the toughness of bainite steels. The emulsions are elongated during hot processing and increase the anisotropy of the steel. The mechanical properties of the molten steel are deteriorated, and the iron product becomes a path for surface defects due to inclusions (foreign substances) contained in the composition during hot or cold working due to its low melting point.

In addition, oxygen is combined with oxidizing elements of spring steel to form non-metallic inclusions, thereby impairing the mechanical properties and fatigue properties of bainite steel, so it should not be contained in excess of the above content, and the content of oxygen, phosphorus and sulfur should be minimized. It is more preferable to do it.

In the case of containing molybdenum (Mo), nickel (Ni), and copper (Cu) in the spring steel for omitting the tempering process according to an embodiment of the present invention, a carbon equivalent satisfying the range of 0.25 to 0.6 wt% It is desirable to form.

The carbon equivalent is derived by the formula below.

Carbon equivalent (wt%) = C + Mn/6 + (Cr + Mo)/5 + (Ni + Cu)/15

When the carbon equivalent is less than 0.25 (wt%), especially in the case of rapid cooling in the production process of the spring at a carbon equivalent of 0.20 (wt%) or less, the structure of the steel material for the spring is transformed into ferrite and pearlite to secure a bainite structure. It is difficult and, as shown in FIG. 3A, the CCT curve moves to the short-time side, and the upper bainite tissue fraction increases.

In addition, when the carbon equivalent exceeds 0.6 (wt%), as shown in FIG. 3C, the CCT curve moves to the long-term side, increasing the amount of martensite, reducing the amount of bainite, reducing the effect of the manganese content. do.

At this time, among the constituent factors of the bainite microstructure, the residual austenite distributed at the grain boundaries after phase transformation greatly decreases the yield strength of the manufactured spring. When quenching by continuous cooling of the spring, the yield strength of the manufactured spring decreases. As a result of studying the correlation between the amount of carbon equivalent and the amount of retained austenite, the limit conditions as shown in the graph of FIG. 4 were obtained.

Specifically, when the residual austenite amount of 5 vol% or less and the carbon equivalent of 0.25 to 0.6 (wt%) that falls within the range of the square box indicated by the broken line in the graph of FIG. 4 are maintained, the yield strength of the manufactured spring is not reduced. Does not.

In the spring steel for omitting the tempering process according to an embodiment of the present invention, the addition constant of a trace alloy consisting of titanium (Ti), vanadium (V), niobium (Nb), and aluminum (Al) is within a range of 0.8 to 1.0. It is desirable to be satisfied, and the microalloy addition constant can be expressed by the following formula.

Addition constant of trace alloy = (1.15Ti + 0.99V + 0.58Nb + 1.99Al) / 7.66N

If the microalloy addition constant is less than 0.8, the effect of the addition of boron decreases, making it difficult to secure the bainite structure.If the microalloy addition constant exceeds 1.0, the effect of the addition of boron reaches saturation, and coarse carbonitrides While this is formed, the toughness of the manufactured spring may be reduced.

Effective boron satisfying the range of 7 to 20 ppm in the case of including titanium (Ti), vanadium (V), niobium (Nb), and aluminum (Al) in the spring steel for omitting the tempering process according to an embodiment of the present invention It is desirable to form an effective Boron.

The effective boron amount is derived by the formula below.

Effective boron amount (ppm) = [{5.25B-(7.66N-1.15Ti + 0.99V + 0.58Nb + 1.99Al)/5.25}] x 10000

The relationship between the effective amount of boron in the spring steel and the ideal bainite grain size of the manufactured spring is shown in the graph of Fig. 5, and it is stable bainite transformation by most effectively delaying the ferrite transformation at the effective boron content within the box range indicated by the broken line When the effective boron content according to the effective boron content formed in the broken line box of FIG. 5 is obtained through Equation 3, a value in the range of 7 to 20 ppm can be obtained.

At this time, when the effective boron amount is less than 7 ppm, the effect of adding boron becomes insignificant, and when the effective boron amount exceeds 20 ppm, the hardenability of the spring steel decreases, making it difficult to secure a bainite structure.

When manufacturing a spring using a spring steel for omitting the tempering process according to an embodiment of the present invention, the steel for the spring is reheated and then rolled or forged to be hot-formed into a spring shape, and a hot-formed automobile Quenching is performed by rapidly cooling the dragon spring.

At this time, the reheating of the spring material for hot forming is preferably performed by atmosphere heating or induction heating within a temperature range of 1050° C. or less from the A3 transformation point.

When the reheating temperature for hot forming is less than the Ac3 transformation point, the fraction of bainite produced decreases as the abnormal ferrite precipitates, and the fraction occupied by the upper bainite in the generated bainite increases, and the reheating temperature exceeds 1050℃. In this case, it becomes difficult to secure the toughness of the manufactured spring due to the coarse grains.

In addition, in the quenching process, the spring is cooled to room temperature at a cooling rate of 20 to 150°C/sec. As shown in the graph of FIG. 6, ferrite or pearlite transformation occurs at a cooling rate of less than 20°C/sec. It is difficult to secure a structure of high toughness bainite in the spring.

In addition, at a cooling rate exceeding 150°C/sec, the effect of improving the structure of bainite reaches saturation, and the spring produced by the difference in the cooling rate between the rapidly cooled spring surface and the relatively slowly cooled spring center It is not desirable because shape deformation may occur.

In addition, the austenite grain size of the spring steel is preferably formed in a size in the range of 15 ~ 70㎛.

When the austenite grain size is less than 15㎛, the upper bainite fraction that decreases the toughness of the manufactured spring increases as the hardenability decreases. If the austenite crystal grain size exceeds 70㎛, the improvement effect due to the increase in the lower bainite fraction The saturation state is reached, and the toughness of the manufactured spring may be lowered.

The table below shows the mechanical properties of spring steels according to the composition of the components.

Each of the spring steels according to the test example was hot-rolled and cooled after casting with an ingot, but a different rolling temperature was applied to generate a change in the austenite grain size, and the rolling ratio of all steels was 80% or more. I did it.

In addition, Test Examples 1 to 12 were heated at a temperature of 900 to 1050°C for 1 hour and then cooled at a rate of 20 to 150°C/sec, and Test Examples 13 to 24 were heated at a temperature of 900 to 1050°C for 1 hour. After heating, it was cooled at a rate of 70° C./sec.

Component composition of spring steel for each test example Test example Ingredient composition (%) C Si Mn Cr B Ni Cu Mo Ti V Nb Al N One 0.1 One One 0.4 0.0018 0.05 0.019 0.5 0.025 0.025 0 0 0.007 2 0.12 0.6 0.3 0.1 0.0025 0.012 0.016 0.3 0.02 0.01 0.01 0.005 0.007 3 0.1 0.1 0.3 0.7 0.0038 0.011 0.021 0.2 0.031 0.03 0.029 0.009 0.015 4 0.2 0.2 0.49 0.31 0.002 0.15 0.09 0.1 0.034 0.014 0.01 0.001 0.008 5 0.17 0.6 0.5 0.3 0.0015 0.15 0.09 0.05 0.015 0.01 0.01 0.01 0.007 6 0.23 0.6 0.5 0.7 0.0013 0.015 0.02 0.1 0.03 0.024 0.02 0.01 0.0118 7 0.3 One 0.1 0.1 0.0015 0.013 0.018 0.05 0.015 0.008 0.008 0 0.004 8 0.25 0.6 0.7 0.1 0.0015 0.09 0.05 0.05 0.01 0.02 0 0.01 0.007 9 0.3 0.1 0.1 0.2 0.004 0.08 0.004 0.1 0.03 0.02 0.02 0.01 0.013 10 0.33 0.1 0.8 0.6 0.002 0 0 0.05 0.02 0.01 0.01 0.005 0.007 11 0.34 0.15 1.29 0.13 0.002 0.05 0.019 0.08 0.029 0.02 0 0 0.007 12 0.4 0.1 0.5 0.5 0.0035 0.003 0.008 0.01 0.019 0.01 0.009 0.009 0.009 13 0.1 One 0.2 0.1 0.003 0.01 0.005 0.05 0.03 0.015 0.015 0.001 0.008 14 0.1 0.6 0.2 0.1 0.002 0.03 0.019 0.05 0.022 0.02 0.02 0 0.008 15 0.1 0.1 0.2 0.1 0.002 0.04 0.019 0.1 0.02 0.01 0.01 0.003 0.007 16 0.2 0.7 0.1 0.1 0.004 0.009 0.019 0.05 0.02 0.01 0.01 0 0.005 17 0.13 0.6 0.2 0.5 0.001 0.009 0.019 0.3 0.01 0.01 0.01 0.004 0.005 18 0.35 0.1 0.6 0.5 0.003 0.009 0.019 0.1 0.01 0 0.01 0.007 0.006 19 0.35 One 1.3 0.7 0.0035 0.009 0.019 0.05 0.03 0.015 0.015 0.001 0.008 20 0.33 0.6 One 0.8 0.0019 0.009 0.019 0.1 0.026 0.01 0.001 0.002 0.007 21 0.4 0.1 One 0.4 0.002 0.009 0.019 0.4 0.03 0.01 0.015 0.002 0.008 22 0.1 One 0.2 0.1 0.0033 0.01 0.005 0.05 0.03 0.03 0.02 0.01 0.008 23 0.2 0.7 0.1 0.1 0.004 0.009 0.019 0.05 0.03 0.02 0.02 0 0.005 24 0.33 0.6 One 0.8 0.0035 0.009 0.019 0.1 0.03 0.02 0.02 0.01 0.007

Structure formation according to heating and cooling conditions of spring steel for each test example Carbon equivalent Addition constant of trace alloy Effective boron
(ppm) Heating temperature
(℃) Cooling rate
(℃/sec) Bainite
(vol%) bottom
Bainite
(vol%) Austenite
Average grain size
(㎛) One 0.45 1.00 17.8 1050 150 95 75 70 2 0.25 0.91 15.5 970 150 95 80 30 3 0.33 0.87 9.8 900 150 95 85 15 4 0.38 0.99 19.0 1000 100 95 75 50 5 0.34 0.99 13.5 970 70 95 70 30 6 0.48 0.99 11.8 1000 50 95 65 25 7 0.35 0.97 13.4 950 100 95 75 30 8 0.41 0.95 10.4 950 100 95 80 30 9 0.38 0.86 13.8 1000 100 95 75 25 10 0.59 0.91 10.5 950 20 95 75 30 11 0.60 0.99 19.1 1050 20 95 70 70 12 0.59 0.80 9.3 970 20 50 75 30 13 0.16 0.98 27.6 970 70 50 10 30 14 0.17 0.93 11.3 900 70 70 30 10 15 0.18 0.83 3.0 950 70 50 20 30 16 0.25 0.91 33.7 1050 70 30 10 70 17 0.33 0.92 4.0 950 70 30 10 30 18 0.57 0.68 1.9 950 70 30 10 30 19 0.72 0.98 32.6 950 70 30 10 30 20 0.68 0.83 1.4 950 70 30 10 30 21 0.73 0.93 12.0 950 70 10 5 30 22 0.16 1.56 33.0 900 70 30 15 15 23 0.25 1.72 40.0 900 70 30 15 15 24 0.68 1.60 35.0 900 70 30 15 15

Mechanical properties and durability characteristics of spring steel by test example Residual
Austenite
(vol%) Mechanical properties Durability characteristics Hardness
(Hv) The tensile strength
(kg/㎟) Yield strength
(kg/㎟) Yield
(YS/TS) Section reduction rate
(%) Elongation
(%) cycle
(1000) fatigue
Limit Permanent deformation
(mm) One 4.5 440 150 125 0.83 50 15 150 0.5 3 2 0.8 410 135 110 0.81 50 16 140 0.55 3 3 1.3 430 145 120 0.83 50 15 150 0.55 3 4 1.8 450 151 132 0.87 45 14 160 0.6 2 5 0.8 435 145 127 0.88 50 15 150 0.55 3 6 4 470 160 137 0.86 40 14 150 0.55 2 7 1.5 430 145 125 0.86 46 13 150 0.55 3 8 2 455 155 135 0.87 46 13 160 0.55 2 9 1.8 440 150 130 0.87 46 13 150 0.55 2 10 1.8 510 181 152 0.84 35 12 170 0.55 One 11 4 525 185 155 0.84 30 11 160 0.6 One 12 4 515 181 152 0.84 30 12 160 0.55 One 13 0 280 90 60 0.67 50 15 20 0.2 ≥5 14 0 310 100 70 0.70 50 15 25 0.3 ≥5 15 0 280 90 60 0.67 50 15 20 0.3 ≥5 16 One 370 125 80 0.64 50 17 30 0.35 ≥5 17 1.3 372 120 80 0.67 50 17 30 0.35 ≥5 18 4.2 372 125 75 0.60 50 17 255 0.35 ≥5 19 5.2 510 120 90 0.75 10 3 20 0.35 ≥5 20 6 570 170 130 0.76 30 15 50 0.3 5 21 8 550 190 180 0.95 10 3 20 0.35 One 22 0 310 100 70 0.70 60 15 20 0.3 ≥5 23 One 315 110 70 0.64 50 17 20 0.3 ≥5 24 6 515 180 150 0.83 10 3 50 0.35 5

The comparative steels for springs in Tables 4 and 5 are generally steels applied to the manufacture of automotive springs, and after heating for 1 hour in the range of 930 to 970°C, quenching, and then tempering at 420 to 470°C for 1 hour. Done.

Composition of comparable spring steel materials Comparative example Ingredient composition (%) C Si Mn Cr B Ni Cu Mo Ti V Nb Al N SUP9 0.56 0.23 0.76 0.75 0 0.009 0.019 0 0.002 0.002 0.002 0.001 0.007 SUP9A 0.59 0.25 0.85 0.86 0 0.003 0.003 0 0.003 0.002 0.003 0.001 0.008 SUP9D 0.58 0.25 0.84 0.84 0 0.009 0.018 0 0.002 0.001 0.032 0.001 0.006 SUP11A 0.60 0.25 0.82 0.86 0.0011 0.09 0.018 0 0.01 0.006 0.005 0.001 0.006

Mechanical properties and durability characteristics of spring steel materials to be compared Residual
Oste
Night
(vol%) Mechanical properties Durability characteristics Hardness
(Hv) The tensile strength
(kg/㎟) Yield strength
(kg/㎟) Yield
(YS/TS) section
Reduction rate
(%) Elongation
(%) cycle
(1000) fatigue
Limit Permanent deformation
(mm) SUP9 4 410 136 128 0.94 23 10 100 0.45 2 SUP9A 3 400 134 124 0.93 29 11 100 0.46 2 SUP9D 4 430 143 1133 0.93 30 13 110 0.45 2 SUP11A 3 435 146 135 0.92 26 10 110 0.48 2

In order to evaluate the mechanical properties and durability properties such as tensile and impact properties from the hot-rolled test materials, test pieces were taken from the rolling direction of the test materials.

The bainite microstructure phase fraction of the above test examples and comparative examples was measured by a point counting method for a 1000 mm2 test surface using an image analyzer after cooling the spring steel, and the austenite grain size was determined by the KS standard ( KS D0205).

In addition, the total residual austenite phase fraction in the bainite structure was measured using X-ray (Cu radiation), and the tensile test was performed using a test piece of KS standard (KS B0801) No. 4 at a cross head speed of 5 mm. Tested at /min.

In order to impart the Bauschinger effect, which is a phenomenon in which the elastic limit or yield point is lowered when a load in the opposite direction is applied to a metal that has a load above the yield point, the cold setting applies a load up to 85% of the tensile strength. It was carried out under the conditions, and after removing the load, the tensile test was again conducted to measure the yield strength, yield ratio, reduction in section and elongation.

The test pieces for evaluating the durability fatigue characteristics of the above test examples and comparative examples were manufactured in 6mm (D) * 40mm (W) * 545mm (L) and tested five times with a stress of 70±35 kg/mm2, excluding the maximum and minimum values. It was evaluated by the average of the test results.

In the above table, the amount of permanent deformation of Test Examples 1 to 12 and Comparative Examples was measured from the arc height of the test piece after 300,000 fatigue tests with a stress of 70±35 kg/mm2 after shot peening was performed, and tests 13 to 24 In the example, since premature fracture occurred, the fatigue life was measured at 100,000 times.

At this time, the fatigue limit ratio in the above table was derived based on the average test stress that satisfies the normal fatigue life 300,000 times compared to the tensile strength after shot-peening was performed on the spring steel, and the fatigue limit 0.5 was 50 It means that the fatigue life of 300,000 cycles is satisfied at the tensile strength of% level.

As shown in the table above, in Test Examples 1 to 12, the ideal effective boron amount suitable for maximizing the tissue fraction of bainite is formed within the range of 7 to 20 ppm, whereas in Test Examples 13 to 24, the effective boron amount is in the ideal numerical range. As a result, a bainite structure fraction of 90% or more was formed in the spring steel microstructures of Test Examples 1 to 12, and the bainite structure fraction in the range of 10 to 70% in the spring steel microstructures of Test Examples 13 to 24. It can be seen that a tissue fraction is formed.

Therefore, it can be seen that when rapid cooling is performed on the spring steel of the embodiment of the present invention and an appropriate effective amount of boron is secured, the bainite structure fraction can be maximized.

In addition, as shown in the table above, test examples 1 to 12 have a fatigue life of 140,000 to 160,000 times, hardness Hv410 to 525, tensile strength of 135 to 185 kg/㎟, yield strength of 110 to 155 kg after performing shot peening in a state of quenching completed. /㎟, yield ratio 0.81~0.88, cross-sectional reduction ratio 30~50%, elongation 11~16% range of mechanical properties and durability characteristics, these values represent remarkably superior values compared to test examples 13~24 You can see that there is.

In addition, it can be seen that the steel materials for springs according to Test Examples 1 to 12, in which only quenching heat treatment was performed, exhibits equivalent or more mechanical properties and durability characteristics, when compared with the spring steel materials of Comparative Examples in which both quenching and tempering heat treatment were performed.

By omitting the tempering process in the spring manufacturing process through the spring steel material according to the present invention, the production speed of the spring is improved and the production cost is reduced, and the scale of the factory by reducing the heating furnace installation line for reheating the spring It is possible to prepare the basis for constructing a smart factory by optimizing the flow of the production line by reducing it.

Although the embodiments of the present invention have been described with reference to the above, those skilled in the art to which the present invention pertains will understand that the present invention can be implemented in other specific forms without changing the technical spirit or essential features.

Therefore, the embodiments described above are to be understood as illustrative and non-limiting in all respects, and the scope of the present invention described in the detailed description is indicated by the claims to be described later, and the meaning of the claims and It should be construed that all changes or modifications derived from the scope and equivalent concepts are included in the scope of the invention.

T: temperature
t: time
Fs: Ferrite transformation start point
Ps: Pearlite transformation start point
Bs: Bainite transformation start point
Ms: Martensite transformation starting point

Claims (8)

In the steel for springs manufactured so that the tempering process can be omitted among the heat treatment processes consisting of quenching and tempering,
Nickel (Ni) 0.003 to 0.2 wt%, copper (Cu) 0.005 to 0.2 wt%, molybdenum (Mo) 0.01 to 0.5 wt%, titanium (Ti) 0.01 to 0.04 wt%, vanadium (V) 0.01 to 0.04 wt%, Niobium (Nb) 0.001 to 0.2% by weight, aluminum (Al) 0.001 to 0.01% by weight, contains at least one selected from, carbon (C) 0.1 to 0.4% by weight, silicon (Si) 0.1 to 1.0% by weight, manganese (Mn) 0.1 to 1.5% by weight, chromium (Cr) 0.1 to 0.7% by weight, boron (B) 0.001 to 0.004% by weight, nitrogen (N) 0.004 to 0.015% by weight, balance iron and other inevitable impurities And the bainite structure fraction is formed over 90%,
It contains molybdenum (Mo), nickel (Ni) and copper (Cu), and has a carbon equivalent expressed by the formula C + Mn/6 + (Cr + Mo)/5 + (Ni + Cu)/15 It is formed within the range of 0.2 to 0.6,
Contains trace alloys of titanium (Ti), vanadium (V), niobium (Nb) and aluminum (Al), and added a trace alloy expressed by the formula (1.15Ti + 0.99V + 0.58Nb + 1.99Al) / 7.66N A constant is formed within the range of 0.8 to 1.0,
[{5.25B-(7.66N-1.15Ti + 0.99V + 0.58Nb + 1.99Al)/5.25}] The effective boron amount expressed by the formula of x 10000 is formed within the range of 7-20 ppm,
Omits the tempering process, characterized in that the structure fraction of lower-bainite formed at a relatively low temperature is formed at a relatively low temperature of 60% or more compared to upper-bainite formed by austempering. Steel for springs. delete delete delete The method of claim 1,
The steel material for a spring for omitting the tempering process, characterized in that it contains less than 0.003% by weight oxygen (O), less than 0.01% by weight phosphorus (P) and less than 0.01% by weight sulfur (S) . The method of claim 1,
Steel for springs for omitting the tempering process, characterized in that the crystal grain size of austenite is formed in a size of 15 ~ 70㎛. delete The method of claim 1,
The formed steel has a hardness of Hv410~525, 135~185kg/mm ² Tensile strength, a yield strength of 110 to 155 kg/mm ² , a yield ratio of 0.81 to 0.88, a reduction rate of 30 to 50%, an elongation of 11 to 16%, and a fatigue limit ratio of 0.5 to 0.6. Steel for spring to skip the process.

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