EP4079908A1

EP4079908A1 - Leaf spring steel sheet having excellent fatigue life, and manufacturing method therefor

Info

Publication number: EP4079908A1
Application number: EP20902722.6A
Authority: EP
Inventors: Sun-Mi Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2019-12-20
Filing date: 2020-12-10
Publication date: 2022-10-26
Also published as: CN114846170A; KR102327929B1; KR20210079746A; WO2021125688A1

Abstract

A leaf spring steel sheet according to one aspect of the present invention comprises, by wt%, 0.7-1.0% of carbon (C), 0.1-0.4% of silicon (Si), 0.2-1.0% of manganese (Mn), 0.05-2.0% of chromium (Cr), 0.07-0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S) and the balance of Fe and inevitable impurities, wherein the steel sheet comprises, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and the average particle size of prior austenite can be 28 µm or less.

Description

Technical Field

The present disclosure relates to a steel sheet for a part material used in an environment in which a constant load is repeated at regular intervals, and a method for manufacturing the same.

Background Art

For a part used in an environment in which a load is repeatedly applied at regular intervals, it is required to have an excellent fatigue lifespan in terms of securing durability of the part. The seat belt of a vehicle has a structure in which a belt and a leaf spring are connected to each other to assist a user in easy attachment and detachment, and the fatigue lifespan of the leaf spring is an important factor influencing the durability of the seat belt. In particular, since the seat belt is a part directly related to the safety of passengers, the leaf spring provided for the seat belt should have excellent durability so as not to reach a fatigue limit during a life cycle of the vehicle, but development of a material thereof has not been undertaken.
Patent Document 1 proposes a steel sheet for a spring with improved strength using upper bainite. However, there is an issue in that the physical properties of the steel sheet are varied by the introduction of upper bainite, and thus the durability of the spring is rather reduced.

(Prior Art Document)

(Patent Document 1) Korean Patent Application Publication No. 10-2008-0060619 (July 2, 2008 )

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a turbine. According to an aspect of the present disclosure, a steel material having excellent workability and a method for manufacturing the same may be provided.
The object of the present disclosure is not limited to the above description. A person skilled in the art would have no difficulty in understanding additional objects of the present disclosure from overall aspects of the present specification.

Solution to Problem

According to an aspect of the present disclosure, a steel sheet for a leaf spring may include, by wt%, 0.7-1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities. The steel sheet may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and an average prior austenite grain size may be 28 µm or less.
A content (Vs) of vanadium (V) of the steel sheet and a content (Vp) of vanadium (V) of the pearlite may satisfy Formular 1 below.

$Vs > Vp$
In Formula 1 above, Vs denotes a content (wt%) of vanadium (V) of a steel sheet, and Vp denotes a content (wt%) of vanadium (V) of a pearlite.
A fraction of the vanadium carbide (VC) may be 0.002 area% or more.
An average interlamellar spacing of the pearlite may be 0.09 - 0.12 µm.
The average prior austenite grain size may be 2 µm or more.
A surface hardness of the steel sheet may be 410 (HB) or more based on Brinell hardness.
A thickness of the steel sheet is 3 mm or less (excluding 0 mm).
According to another aspect of the present disclosure, a method for manufacturing a steel sheet for a leaf spring, the method may include providing a hot-rolled material including, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities, isothermally holding the hot-rolled material by charging the hot-rolled material in a salt bath of 400 - 600°C after reheating the hot-rolled material, and providing a cold-rolled material by cold rolling the isothermally held hot-rolled material at a cumulative reduction ratio of 50% or more. The hot-rolled material may be reheated in a temperature range of 900°C or higher and less than 1000°C when a content of vanadium (V) of the hot-rolled material is less than 0.15%, and the hot-rolled material may be reheated in a temperature range of 900 - 1050°C when the content of vanadium (V) of the hot-rolled material is 0.15% or more.
The providing of the hot-rolled material may include an operation of reheating a slab to a temperature range of 1000 - 1300°C, a rough rolling operation of providing an intermediate material by rough rolling the reheated slab at a cumulative reduction ratio of 60% or more, a finishing rolling operation of rolling the intermediate material at a cumulative reduction ratio of 60% or more, and providing a hot-rolled material by rolling the intermediate material at an exit temperature of 700 - 1000°C, a first cooling operation of performing cooling to a first cooling end temperature of 500 - 700°C by applying a slow cooling condition, a first holding operation of isothermally holding the hot-rolled material at the first cooling end temperature, and a second cooling operation of cooling the isothermally held hot-rolled material to room temperature by furnace cooling.

Advantageous Effects of Invention

According to a preferred aspect of the present disclosure, it is possible to provide a steel sheet for a leaf spring having excellent durability by effectively improving a fatigue lifespan thereof.

Brief Description of Drawings

FIGS. 1 to 3 are photographic images illustrating prior austenite grains of specimens 4, 5, and 6 observed using an optical microscope, respectively.
FIGS. 4 to 6 are photographic images illustrating pearlite structures of specimens 4, 5, and 6 observed using a scanning electron microscope (SEM).

Best Mode for Invention

The present disclosure relates to a steel plate for a leaf spring having an excellent fatigue lifespan, and a method for manufacturing the same. Hereinafter, preferred embodiments of the present disclosure will be described. The embodiments of the present disclosure can be modified to have various other forms, and the scope of the present disclosure should not be limited to the embodiments described below. In addition, the embodiments of the present disclosure are provided to more fully explain the present disclosure to those skilled in the art.
The present inventors conducted an in-depth study on a method for improving a fatigue lifespan of a steel part used in an environment in which a constant load is repeatedly applied at regular intervals, and recognized that a fatigue lifespan of a steel material can be improved by suppressing generation and propagation of fatigue cracks when physical properties of the steel material itself are strengthened, and thus the present disclosure was derived.
As a method of increasing strength and hardness of a steel material, mechanisms such as solid solution strengthening, work hardening, structure refinement, and precipitation hardening are representatively known. The present inventors studied a method for improving strength and hardness of a steel material by adding a large amount of carbon (C), which is a solid solution strengthening element, and confirmed that, when an amount of carbon (C) added exceeds a predetermined level, a large amount of hard, coarse carbide is formed along a grain boundary, which rather promotes generation and propagation of fatigue cracks. That is, it was found that a desired level of fatigue lifespan improvement effect cannot be expected only with the solid solution strengthening. In addition, the present inventors studied a method for improving strength and hardness of a steel material using work hardening. However, it was confirmed that, as an amount of processing increases, strength and hardness of the steel material increase to create an environment favorable for suppression of generation and propagation of fatigue cracks, whereas a fatigue lifespan is rather lowered by inducing generation of defects inside the steel material when the amount of processing exceeds a predetermined level.
Therefore, the present inventors conducted an in-depth study of a method for effectively securing strength and hardness of a steel material through precipitation hardening and grain refinement while securing an effect of solid solution strengthening by including a predetermined level of carbon (C) or more, and confirmed that, when an appropriate level of carbon (C) is added to the steel material while vanadium carbide (VC) is used, the hardness and strength of the steel material may be effectively improved by maximizing effects of precipitation hardening and grain refinement, and thus the present disclosure was derived.
Hereinafter, a steel material for a leaf spring according to an aspect of the present disclosure will be described in more detail.
A steel sheet for a leaf spring according to an aspect of the present disclosure may include, by wt%, 0.7-1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities. The steel sheet may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and an average prior austenite grain size may be 28 µm or less.
Hereinafter, an alloy composition of the present disclosure will be described in more detail. Hereinafter, unless otherwise specified, % and ppm related to a content of the alloy composition are based on weight.
The steel material for a leaf spring according to an aspect of the present disclosure may include, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0%, of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities.

Carbon (C): 0.7 - 1.0%

Carbon (C) is not only a representative hardenability improving element, but also an element effectively contributing to improvement of strength and hardness of steel by solid solution strengthening. Accordingly, the present disclosure may include 0.7% or more of carbon (C) for such an effect. A preferable content of carbon (C) may be 0.75% or more, and a more preferable content of carbon (C) may be 0.8% or more. Conversely, carbon (C) may have a low solid solution limit in ferrite, and thus may react with a carbideforming element to form a precipitate or may combine with Fe to form cementite (Fe₃C). When an amount of carbon (C) added is excessive, a large amount of hard carbide is formed, which may adversely affect a fatigue lifespan. Accordingly, the present disclosure may limit an upper limit of a content of carbon (C) to 1.0%. A preferable content of carbon (C) may be 0.95% or less, and a more preferable content of carbon (C) may be 0.9% or less.

Silicon (Si): 0.1 - 0.4%

Silicon (Si) is not only a ferrite stabilizing element, but also an element effectively contributing to improvement of strength and hardness of steel by refining an interlamellar spacing of pearlite by delaying a transformation rate of the pearlite. Accordingly, the present disclosure may include 0.1% or more of silicon (Si) for such an effect. A preferable content of silicon (Si) may be 0.15% or more, and a more preferable content of silicon (Si) may be 0.2% or more. However, when silicon (Si) is excessively added, hot workability and toughness may be lowered as well as surface quality may be lowered, so the present disclosure may limit an upper limit of the content of silicon (Si) to 0.4%. The preferable content of silicon (Si) may be 0.35% or less, and the more preferable content of silicon (Si) may be 0.3% or less.

Manganese (Mn): 0.2 - 1.0%

Manganese (Mn) is an element not only contributing to improvement of hardenability of steel, but also an element effectively contributing to securing cleanliness of steel by deoxidation and desulfurization. Accordingly, the present disclosure may include 0.2% or more of manganese (Mn). A preferable content of manganese (Mn) may be 0.3% or more, and a more preferable content of manganese (Mn) may be 0.4% or more. However, when manganese (Mn) is excessively added, a segregation layer may be formed in a central portion of a steel sheet to lower workability, so that the present disclosure may limit an upper limit of the content of manganese (Mn) to 1.0%. The preferable content of manganese (Mn) may be 0.8% or less, and the more preferable content of manganese (Mn) may be 0.6% or less.

Chromium (Cr): 0.05 - 2.0%

Chromium (Cr) is an element contributing to improvement of hardenability of steel. In addition, chromium (Cr) is an element not only forming fine carbide, but also an element effectively contributing to improvement of strength and hardness of steel by refining the interlamellar spacing of the pearlite. Accordingly, the present disclosure may include 0.05% or more of chromium (Cr). A preferred content of chromium (Cr) may be 0.07% or more. However, when chromium (Cr) is added excessively, there is a concern that toughness may be lowered due to excessive hardenability or heat treatment properties may be lowered due to stabilization of carbide. The present disclosure may limit an upper limit of the content of chromium (Cr) to 2.0%. The preferred content of chromium (Cr) may be 1.5% or less, and a more preferred content of chromium (Cr) may be 1.0% or less.

Vanadium (V): 0.07 - 0.2%

In the present disclosure, vanadium (V) is an element essentially added to improve strength and hardness of steel. Vanadium (V) is not only an element having easy heat treatment properties and having low reactivity with oxygen, but also an element reacting with carbon (C) in steel to precipitate fine vanadium carbide (VC) and effectively contribute to grain refinement of austenite. Accordingly, the present disclosure may include 0.07% or more of vanadium (V). A more preferable content of vanadium (V) may be 0.1% or more. However, when a content of vanadium (V) exceeds a predetermined level, an effect of adding vanadium (V) may be saturated, whereas vanadium (V), a relatively expensive element, may not be preferred in terms of economic feasibility. The present disclosure may limit an upper limit of the content of vanadium (V) to 0.2%. The more preferable content of vanadium (V) may be 0.18% or less.

Phosphorus (P): 0.03% or less (including 0%) and Sulfur (S): 0.03% or less (including 0%)

Phosphorus (P) and sulfur (S) are representative impurity elements, and the present disclosure seeks to minimize contents of the elements in terms of securing cleanliness of steel. However, in consideration of economic feasibility in a conventional steelmaking process, the present disclosure may limit upper limits of contents of phosphorus (P) and sulfur (S) to 0.03%, respectively.
The steel sheet for a leaf spring according to an aspect of the present disclosure may include a balance of Fe and other inevitable impurities in addition to the above-described elements. However, unintended impurities from a raw material or a surrounding environment may be inevitably mixed therewith in a general manufacturing process, so that the unintended impurities may not be entirely excluded. The impurities are known to those skilled in the art, and thus all contents thereof are not specifically mentioned in the present specification. In addition, the addition of effective elements other than the above-described composition is not excluded.
The steel sheet for a leaf spring according to an aspect of the present disclosure may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC). A lower limit of a preferable fraction of vanadium carbide (VC) may be 0.002 area% or more. That is, the steel sheet for a leaf spring according to an aspect of the present disclosure may have a microstructure including a predetermined level of vanadium carbide (VC) in a pearlite single-phase structure.
In addition, in the steel sheet for a leaf spring according to an aspect of the present disclosure, a content (Vs, wt%) of vanadium (V) included in the steel sheet and a content (Vp, wt%) of vanadium (V) included in the pearlite may satisfy Formula 1 below.

$Vs > Vp$
In Formula 1 above, Vs denotes a content (wt%) of vanadium (V) included in the steel sheet, and Vp denotes a content (wt%) of vanadium (V) included in the pearlite. A person skilled in the art may determine Vp and Vs without a particular technical difficulty through an analysis method generally performed in the art.
That is, the steel sheet for a leaf spring according to an aspect of the present disclosure does not aim for complete solid solution of vanadium (V) in the pearlite structure, and thus may mean that some vanadium (V) is precipitated as vanadium carbide (VC). Accordingly, in the steel sheet for a leaf spring according to an aspect of the present disclosure, an effect of improving strength and hardness of the steel sheet by precipitation of vanadium carbide (VC) may be expected.
In the steel sheet for a leaf spring according to an aspect of the present disclosure, an average prior austenite grain size (average grain size at an austenitization temperature) may be 28 µm or less, and a preferred average prior austenite grain size may be 2 µm or more. In addition, a preferred average interlamellar spacing of the pearlite may be 0.09 - 0.12 µm. That is, the steel sheet for a leaf spring according to an aspect of the present disclosure may be manufactured by heat-treating a hot-rolled material with added vanadium (V), and thus it can be seen that a structure is refined by vanadium carbide (VC) present in a state of the hot-rolled material. Accordingly, in the steel sheet for a leaf spring according to an aspect of the present disclosure, an effect of improving strength and hardness of the steel sheet by refining the structure may be expected.
A thickness of the steel sheet for a leaf spring according to an aspect of the present disclosure is not particularly limited, but may preferably have a thickness of 3 mm or less (excluding 0 mm).
A surface hardness of the steel sheet for a leaf spring according to an aspect of the present disclosure may be greater than or equal to 410 (HB) based on Brinell hardness. In the case of a leaf spring manufactured using the steel sheet of the present disclosure, an expected fatigue lifespan calculated through Formula 2 below may be 15*10⁴ times or more. $0.35 = \{(4.25 * HB + 225) * {(2 Nf)}^{- 0.09}\} / E + (\begin{array}{l} 0.32 * {HB}^{2} - \\ 487 * HB + 19.1 * 10^{3} \end{array}) * \{{(2 Nf)}^{- 0.56}\} / E$
In Formula 2 above, HB denotes a Brinell hardness of a steel sheet surface, and 2Nf denotes a fatigue lifespan. In addition, E in Formula 2 above denotes an elastic modulus, and a fixed value of 210 GPa is applied in the present disclosure.
Accordingly, according to an aspect of the present disclosure, it is possible to provide a steel sheet for a leaf spring having an excellent fatigue lifespan even in an environment in which a constant load is continuously repeated at regular intervals.
Hereinafter, a method for manufacturing a steel sheet for a spring according to an aspect of the present disclosure will be described in more detail.
According to an aspect of the present disclosure, a method for manufacturing a steel sheet for a spring, the method may include providing a hot-rolled material including, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities, isothermally holding the hot-rolled material by charging the hot-rolled material in a salt bath of 400 - 600°C after reheating the hot-rolled material, and providing a cold-rolled material by cold rolling the isothermally held hot-rolled material at a cumulative reduction ratio of 50% or more. The hot-rolled material may be reheated in a temperature range of 900°C or higher and less than 1000°C when a content of vanadium (V) of the hot-rolled material is less than 0.15%, and the hot-rolled material may be reheated in a temperature range of 900 - 1050°C when the content of vanadium (V) of the hot-rolled material is 0.15% or more.
The providing of the hot-rolled material may include an operation of reheating a slab to a temperature range of 1000 - 1300°C, a rough rolling operation of providing an intermediate material by rough rolling the reheated slab at a cumulative reduction ratio of 60% or more, a finishing rolling operation of rolling the intermediate material at a cumulative reduction ratio of 60% or more, and providing a hot-rolled material by rolling the intermediate material at an exit temperature of 700 - 1000°C, a first cooling operation of performing cooling to a first cooling end temperature of 500 - 700°C by applying a slow cooling condition, a first holding operation of isothermally holding the hot-rolled material at the first cooling end temperature, and a second cooling operation of cooling the isothermally held hot-rolled material to room temperature by furnace cooling.

Provision of Hot-Rolled Material

After a slab having a predetermined alloy composition is prepared, reheating and hot rolling of the slab may be performed. The slab of the present disclosure may have an alloy composition corresponding to the alloy composition of the steel sheet described above, and thus descriptions of the alloy composition of the slab of the present disclosure are replaced with descriptions of the alloy composition of the steel sheet described above.
A slab reheating temperature of the present disclosure is not particularly limited, but reheating of the slab may be performed at 1000 - 1300°C in consideration of uniformity of a material and a rolling load of a subsequent hot-rolling process. An intermediate material may be provided by rough rolling the reheated slab at a cumulative reduction ratio of 60% or more, and then finishing rolling may be performed to provide a hot-rolled material by rolling the intermediate material at a cumulative reduction ratio of 60% or more. In terms of suppressing formation of a coarse structure, an upper limit of an exit temperature of finishing rolling may be limited to 1000°C, and a lower limit of the exit temperature may be limited to 700°C in consideration of the rolling load. An upper limit of a preferable exit temperature may be 920°C, and a lower limit of the preferable exit temperature may be 830°C.
After the hot-rolled material is wound, two-stagecondition cooling may be performed. That is, after first cooling is performed to a first cooling end temperature of 500 - 700°C by applying a slow cooling condition, isothermal holding may be performed at the first cooling end temperature, and then secondary cooling of the hot-rolled material held isothermally by furnace cooling may be performed to room temperature. A cooling rate of the first cooling may be 20 - 100°C/s. The isothermal holding may be performed in a state of a hot-rolled coil, and isothermal holding time may be 30 - 90 minutes. In addition, a cooling rate of the secondary cooling may be 5°C/ s or less.

Reheating and Isothermally Holding Hot-Rolled Material

After the hot-rolled material is reheated in a predetermined temperature range for microstructure control, a process of isothermal holding may be performed by charging the hot-rolled material in a salt bath set to a pearlitization temperature. A hot-rolled material reheating temperature may be selectively applied according to a content of vanadium (V) included in the hot-rolled material. Here, the content of vanadium (V) included in the hot-rolled material may be interpreted as meaning corresponding to that of a content of vanadium (V) included in the slab.
When the content of vanadium (V) of the hot-rolled material is less than 0.15%, reheating of the hot-rolled material may be performed in a temperature range of 900°C or higher and less than 1000°C. In addition, when the content of vanadium (V) of the hot-rolled material is 0.15% or more, reheating of the hot-rolled material may be performed in a temperature range of 900 - 1050°C. That is, in order to allow a predetermined level of vanadium carbide (VC) to remain in a final steel sheet, the present disclosure may selectively apply a reheating temperature range according to the content of vanadium (V).
The present disclosure does not particularly limit hot-rolled material reheating time, but the hot-rolled material reheating time may be 1 minute to 10 minutes in consideration of effects of preventing coarsening of a microstructure and preventing complete solid solution of a carbide. A lower limit of preferred hot-rolled material reheating time may be 3 minutes, and an upper limit of the preferred hot-rolled material reheating time may be 5 minutes.
After the reheated hot-rolled material is charged into a salt bath of 400 - 600°C, the reheated hot-rolled material may be isothermally held to provide a microstructure of the final steel sheet as a single-phase pearlite structure. The present disclosure does not particularly limit isothermal holding time, but in order to prevent a fatigue lifespan from being lowered due to local formation of a lowtemperature structure, isothermal holding may be performed for 30 seconds or more. In addition, preferred isothermal holding time may be 60 to 150 seconds in consideration of implementation and economic feasibility of a desired final structure.
Cold rolling may be performed on the isothermally held hot-rolled material under a condition of a cumulative reduction ratio of 50% or more, and a steel sheet having a final thickness of 3 mm or less may be provided.
The steel sheet for a leaf spring manufactured by the above-described manufacturing method may include, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and a content (Vs, wt%) of vanadium (V) of the steel sheet and a content (Vp, wt%) of vanadium (V) of the pearlite may satisfy Formula 1 below.

$Vs > Vp$
In Formula 1 above, Vs denotes a content (wt%) of vanadium (V) included in a steel sheet, and Vp denotes a content (wt%) of vanadium (V) included in pearlite.
In addition, in the steel sheet for a leaf spring manufactured by the above-described manufacturing method, an average prior austenite grain size (average grain size at an austenitization temperature) may be 28 µm or less, and an average interlamellar spacing of the pearlite may be 0.09 - 0.12 µm or more.
In addition, the steel sheet for a leaf spring manufactured by the above-described manufacturing method may have a hardness of 410 (HB) or more based on Brinell hardness. In the case of a leaf spring manufactured using the steel sheet, an expected fatigue lifespan calculated through Formula 2 below may satisfy 15*10⁴ times or more. $0.35 = \{(4.25 * HB + 225) * {(2 Nf)}^{- 0.09}\} / E + (\begin{array}{l} 0.32 * {HB}^{2} - \\ 487 * HB + 19.1 * 10^{3} \end{array}) * \{{(2 Nf)}^{- 0.56}\} / E$
In Formula 2 above, HB denotes a Brinell hardness of a steel sheet surface, and 2Nf denotes a fatigue lifespan. In addition, E in Formula 2 above denotes an elastic modulus, and a fixed value of 210 GPa is applied in the present disclosure.

Mode for Invention

Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the examples described below are for illustrative purposes only and not for limiting the scope of the present disclosure.

(Example)

After a slab having an alloy composition of Table 1 was reheated in a temperature range of 1200°C, rough rolling was performed at a cumulative reduction ratio of 60%, and finish rolling was performed at a cumulative reduction ratio of 80% to prepare a hot-rolled material specimen. In this case, an exit temperature of the finishing rolling was applied with a condition of 850°C. Each hot-rolled material specimen was reheated under conditions shown in Table 2, and then was charged into a salt bath at 600°C or lower to perform isothermal holding, and a cold-rolled reduction ratio of 80% was applied to prepare a final cold-rolled material specimen. A microstructure and carbide of each cold-rolled material specimen were observed, and results thereof are shown in Table 2. In addition, a surface hardness of each of the hot-rolled material specimen and the final cold-rolled material specimen before and after isothermal heat treatment was measured, and results thereof are shown in Table 3.

At the time of observing the microstructure, 2% nital etching was performed on a polished specimen surface, and then a prior austenite grain size was observed by performing 200 times magnification with an optical microscope, and a structure composition and a pearlite lamellar spacing were observed by performing 500 - 50,000 times magnification with a scanning electron microscope (SEM). A hardness of each specimen was measured using a Vickers hardness tester, and a hardness value was measured for 10 points per specimen by applying a 10 kg load, and then an average value was calculated.

[Table 1]

Type of steel	Alloy composition (wt%)
Type of steel	C	Si	Mn	Cr	V	N	P	S
A	0.83	0.2	0.41	0.098	0	0.004	0.013	0.003
B	0.81	0.21	0.405	0.1	0.07	0.0043	0.011	0.003
C	0.81	0.22	0.41	0.101	0.151	0.0038	0.012	0.003

[Table 2]

Spe cim en No.	Typ e of Ste el	Hot-Rolled Materi al Reheat ing Temper ature (°C)	AGS* Averag e Size (um)	Pearlit e Lamella r Spacing (um)	Pearli te Fracti on (area% )	Pearlite Elements		Precipi tate fractio n (area%)	Precipit ate Elements
Spe cim en No.	Typ e of Ste el	Hot-Rolled Materi al Reheat ing Temper ature (°C)	AGS* Averag e Size (um)	Pearlit e Lamella r Spacing (um)	Pearli te Fracti on (area% )	Fe (wt% )	V (wt% )	Precipi tate fractio n (area%)	V (wt% )	C (wt %)
1	A	1000	37.2	0.110	100	98.4 37	-	-	-	-
2	A	950	28.2	0.086	99.993	98.4 4	-	-	-	-
3	A	900	20.6	0.114	99.986	98.4	-	-	-	-
						43
4	B	1000	31.0	0.100	99.996	98.3 79	0.07	-	-	-
5	B	950	26.0	0.097	99.985	98.3 85	0.06 9	0.002	76.3 29	8.3 14
6	B	900	20.0	0.103	99.958	98.4 03	0.05 4	0.029	74.4 68	11. 55
7	C	1000	27.8	0.095	99.971	98.3 03	0.13 3	0.029	76.4 78	10. 077
8	C	950	21.9	0.105	99.916	98.3 39	0.10 4	0.084	74.8 18	13. 132
9	C	900	5.4	0.119	99.843	98.3 85	0.06 7	0.157	72.4 16	14. 608

[0085] *AGS average size denotes an average prior austenite grain size.

[Table 3]

Specime n No.	Type of Steel	Hot-Rolled Material	Isotherma lly Heat-Treated Material	Cold-Rolled Material		[Formula 2] Expected Fatigue Lifespan of Cold-Rolled Material (Times)
Specime n No.	Type of Steel	Vickers Hardness (Hv)	Vickers Hardness (Hv)	Vickers Hardness (Hv)	Brinell hardness (HB)
1	A	216.7	334.6	408.2	386.4	110,000
2	A	216.7	353.5	415	392.5	120,000
3	A	216.7	347.1	431.5	406.5	140,000
4	B	226.0	368.9	421.3	398.0	130,000
5	B	226.0	369.8	440	415.0	150,000
6	B	226.0	361.7	442.5	417.5	160,000
7	C	244.1	372.4	455.6	429.4	190,000
8	C	244.1	370.4	448.6	423.6	180,000
9	C	244.1	351.0	445.4	420.4	170,000

As shown in Tables 1 to 3, it can be confirmed that specimens 5 to 9 satisfying an alloy composition and a process condition of the present disclosure satisfied a desired microstructure and physical properties, whereas specimens 1 to 4 that do not satisfy one or more among the alloy composition and the process condition of the present disclosure did not satisfy the desired microstructure and physical properties.
In particular, it can be confirmed that specimen 5 satisfied a content of vanadium (V) limited by the present disclosure, but re-dissolution of vanadium carbide (VC) present in a hot-rolled material specimen occurred due to a hot-rolled material reheating temperature exceeding a range limited by the present disclosure.
Specimens 1 to 3 are photographic images illustrating prior austenite grains of specimens 4, 5, and 6 observed using an optical microscope, respectively, from which, it can be confirmed that specimen 4 had a largest average prior austenite grain size, and specimen 6 had a smallest average prior austenite grain size.
Specimens 4 to 6 are photographic images illustrating pearlite structures of specimens 4, 5 and 6 observed using an SEM, from which, it can be confirmed that average pearlite interlamellar spacings of specimens 5 and 6 were formed narrower than that of specimen 4.
While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.

Claims

A steel sheet for a leaf spring, the steel sheet comprising:
by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities,

wherein the steel sheet comprises, as a microstructure, 99 area% or more of pearlite and 1 area% or less (excluding 0%) of vanadium carbide (VC), and

an average prior austenite grain size is 28 µm or less.
The steel sheet of claim 1, wherein a content (Vs) of vanadium (V) of the steel sheet and a content (Vp) of vanadium (V) of the pearlite satisfy Formular 1 below. $Vs > Vp$
in Formula 1 above, Vs denotes a content (wt%) of vanadium (V) of a steel sheet, and Vp denotes a content (wt%) of vanadium (V) of a pearlite.
The steel sheet of claim 1, wherein a fraction of the vanadium carbide (VC) is 0.002 area% or more.
The steel sheet of claim 1, wherein an average interlamellar spacing of the pearlite is 0.09 - 0.12 µm.
The steel sheet of claim 1, wherein the average prior austenite grain size is 2 µm or more.
The steel sheet of claim 1, wherein a surface hardness of the steel sheet is 410 (HB) or more based on Brinell hardness.
The steel sheet of claim 1, wherein a thickness of the steel sheet is 3 mm or less (excluding 0 mm).
A method for manufacturing a steel sheet for a leaf spring, the method comprising:
providing a hot-rolled material including, by wt%, 0.7 - 1.0% of carbon (C), 0.1 - 0.4% of silicon (Si), 0.2 - 1.0% of manganese (Mn), 0.05 - 2.0% of chromium (Cr), 0.07 - 0.2% of vanadium (V), 0.03% or less of phosphorus (P), 0.03% or less of sulfur (S), a balance of Fe, and inevitable impurities;

isothermally holding the hot-rolled material by charging the hot-rolled material in a salt bath of 400 - 600°C after reheating the hot-rolled material; and

providing a cold-rolled material by cold rolling the isothermally held hot-rolled material at a cumulative reduction ratio of 50% or more,

wherein the hot-rolled material is reheated in a temperature range of 900°C or higher and less than 1000°C when a content of vanadium (V) of the hot-rolled material is less than 0.15%, and

the hot-rolled material is reheated in a temperature range of 900 - 1050°C when the content of vanadium (V) of the hot-rolled material is 0.15% or more.
The method of claim 8, wherein the providing of the hot-rolled material comprises:
an operation of reheating a slab to a temperature range of 1000 - 1300°C;

a rough rolling operation of providing an intermediate material by rough rolling the reheated slab at a cumulative reduction ratio of 60% or more;

a finishing rolling operation of rolling the intermediate material at a cumulative reduction ratio of 60% or more, and providing a hot-rolled material by rolling the intermediate material at an exit temperature of 700 - 1000°C;

a first cooling operation of performing cooling to a first cooling end temperature of 500 - 700°C by applying a slow cooling condition;

a first holding operation of isothermally holding the hot-rolled material at the first cooling end temperature; and

a second cooling operation of cooling the isothermally held hot-rolled material to room temperature by furnace cooling.