JP2005350736A - High-strength steel having superior corrosion resistance and fatigue characteristics for spring, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel for a spring, which is manufactured by using mainly scrap for a raw material, and has superior corrosion resistance and fatigue characteristics; and a manufacturing method therefor. <P>SOLUTION: This high-strength steel comprises, by mass%, 0.45-0.7% C, 0.15-2.5% Si, 0.3-1% Mn, 0.015% or less P, 0.015% or less S, 0.3-2% Cu, 0.01-0.1% sol.Al, 0.1% to less than 0.5% Cr, 0.005-0.1% Ti, 0.1% or less Sn+Sb+As while satisfying (Sn+Sb+As)/Cu < 0.05, one or more elements among Ni, Mo, V and Nb while satisfying Ceq ≥ 0.90%, and the balance iron with unavoidable impurities. The manufacturing method comprises: heating the above steel at such a temperature as to satisfy 950≤T<T', wherein T'=1083-ä1509*(([%Sn]+[%Sb]+[%As])/[%Cu])<SP>2</SP>+535*(([%Sn]+[%Sb]+[%As])/[%Cu])}, while [%Sn], [%Sb], [%As] and [%Cu] represent mass%; hot-rolling it; and then quenching and tempering it. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a steel for a spring, which is a raw material for automobile suspension springs, torsion bars, stabilizers, and the like, which is manufactured from a melted raw material mainly composed of scrap, and a method for manufacturing the same, and particularly to high strength and corrosion resistance and fatigue characteristics. The present invention relates to an excellent vehicle suspension part.

  In recent years, the weight reduction of automobiles has been demanded for the purpose of improving fuel efficiency. In particular, there is a strong demand for weight reduction of suspension springs, which are one of the underbody parts, and the strength after quenching and tempering is increased to 2000 MPa or more. High stress design using the selected material is applied.

  General-purpose spring steel has a strength after quenching and tempering of 1600 to 1800 MPa and JIS G 4801 is specified. After hot-rolling into a predetermined wire, hot-formed spring is hot-formed into a spring shape. In the case of a cold formed spring, after quenching, a quenching and tempering process is performed to form the spring.

  In the case of a material whose strength after quenching and tempering is increased to 2000 MPa or more, the susceptibility to cracking is increased, so when corrosion resistance is inferior, parts exposed to the outside such as suspension springs of automobile undercarriage parts were peeled off with pebbles etc. Corrosion pits are formed at locations, and there is concern that parts may be damaged by the propagation of fatigue cracks starting from the corrosion pits.

  Recently, during winter, snow melting agent is often sprayed to prevent freezing, and the corrosion environment of suspension springs for automobile undercarriage parts has become harsher, so there is a demand for spring steel with high strength and excellent corrosion resistance. Some are strongly suggested.

  Patent Document 1 is a spring steel whose corrosion resistance is improved using Cu as a corrosion resistance element, Patent Document 2 is a spring steel whose corrosion resistance is improved using Si, Ni, Cr, Cu, and Patent Document 3 is Cu in a spring steel. It is described that corrosion resistance is improved by utilizing surface enrichment of Ni.

JP 59-173246 A JP-A-7-1753577 JP 2000-26938 A

  Cu and Ni are effective elements for improving the corrosion resistance, but steel containing Cu is likely to generate surface flaws during hot rolling, and is unavoidably mixed from the melting raw material mainly composed of scrap. Hot embrittlement is promoted and surface properties are reduced.

  Patent Documents 1 and 2 do not describe surface defects due to Cu, and Patent Document 3 describes that Ni / Cu is defined within a predetermined range in order to suppress hot embrittlement due to Cu. There is no description about the influence of Sn, and it is an issue to improve both corrosion resistance and surface properties in high strength spring steel using a melting raw material mainly composed of scrap.

  An object of the present invention is to provide a high-strength spring steel material that is excellent in corrosion resistance and fatigue characteristics using a melting raw material mainly composed of scrap, and a method for producing the same.

  The present inventors have studied in detail a steel material using a melting raw material mainly composed of scraps, and obtained the following knowledge.

  1 Addition of Cu is indispensable for improving corrosion resistance, but Sn inevitably mixed from the melting raw material is less likely to be oxidized than iron like Cu, and is concentrated on the surface during heating to form a Cu-Sn alloy. Is formed, and the melting point of Cu is lowered to promote hot embrittlement. Therefore, in order to improve the surface properties that greatly affect the fatigue characteristics of the spring, it is necessary to strictly control the mixing level.

  2 Further, Sb and As inevitably mixed from the melting raw material not only promote hot embrittlement and damage surface properties, but these elements promote temper embrittlement and reduce the toughness of steel. It is very harmful to spring steel.

  3 When Sn, Sb and As in steel inevitably mixed from the melting raw material are not more than a specific value, and the ratio of the total amount and Cu is not more than a specific value, hot embrittlement Is suppressed.

  4 Even when the heating temperature at the time of heating the steel slab before the hot rolling is specified below a specific value in relation to Sn, Sb, As and Cu in the steel, hot embrittlement is suppressed and the fatigue of the spring Improved characteristics.

The present invention has been made on the basis of the above findings and further studies, that is, the present invention,
1% by mass, C: 0.45-0.7%, Si: 0.15-2.5%, Mn: 0.3-1%, P ≦ 0.015%, S ≦ 0.015%, Cu: 0.3-2%, sol. Al: 0.01 to 0.1%, Cr: 0.1 to less than 0.5%, Ti: 0.005 to 0.1%, (Sn + Sb + As) / Cu <0.05, Sn + Sb + As ≦ 0.1% Further, Ni: 0.1 to 2%, Mo: 0.01 to 1%, V: 0.05 to 0.5%, Nb: 0.01 to 0.5%, or one or more of them are contained. , Ceq ≧ 0.90%, high strength spring steel excellent in corrosion resistance and fatigue properties, comprising the balance iron and inevitable impurities.
However, Ceq = C + Si / 7 + Mn / 6 + Cu / 7 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14

2% by mass, C: 0.45-0.7%, Si: 0.15-2.5%, Mn: 0.3-1%, P ≦ 0.015%, S ≦ 0.015%, Cu: 0.30 to 2%, sol. Al: 0.01 to 0.1%, Cr: 0.1 to less than 0.5%, Ti: 0.005 to 0.1%, Ni: 0.1 to 2%, Mo: 0.01 to Steel containing 1%, V: 0.05 to 0.5%, Nb: 0.01 to 0.5%, or two or more, Ceq ≧ 0.90%, balance iron and inevitable impurities Is heated at a heating temperature T (° C.) that satisfies the following formula (1), and then hot-rolled: A method for producing a high-strength spring steel excellent in corrosion resistance and fatigue characteristics.
950 ≦ T <T ′ (1)
However, T ′ = 1083- {1509 * (([% Sn] + [% Sb] + [% As]) / [% Cu]) ² +535 * (([% Sn] + [% Sb] + [% As]) / [% Cu])}
In the formula (1), [% Sn], [% Sb], [% As], and [% Cu] indicate mass% in steel content.

3% by mass, C: 0.45-0.7%, Si: 0.15-2.5%, Mn: 0.3-1%, P ≦ 0.015%, S ≦ 0.015%, Cu: 0.3-2%, (Sn + Sb + As) / Cu <0.05, Sn + Sb + As ≦ 0.1%, sol. Al: 0.01 to 0.1%, Cr: 0.1 to less than 0.5%, Ti: 0.005 to 0.1%, Ni: 0.1 to 2%, Mo: 0.01 to Steel containing 1%, V: 0.05 to 0.5%, Nb: 0.01 to 0.5%, or two or more, Ceq ≧ 0.90%, balance iron and inevitable impurities Is heated at a heating temperature T (° C.) that satisfies the following formula (1), and then hot-rolled: A method for producing a high-strength spring steel excellent in corrosion resistance and fatigue characteristics.
950 ≦ T <T ′ (1)
However, T ′ = 1083- {1509 * (([% Sn] + [% Sb] + [% As]) / [% Cu]) ² +535 * (([% Sn] + [% Sb] + [% As]) / [% Cu])}
In the formula (1), [% Sn], [% Sb], [% As], and [% Cu] indicate mass% in the steel content.

  According to the present invention, a high-strength spring steel having a tensile strength of 2000 MPa or more, which is excellent in corrosion resistance and surface properties, and a method for producing the same are obtained by using a melting raw material mainly composed of scrap, which is extremely useful industrially.

  The present invention is a steel in which the total amount of Sn, Sb and As contained in the inevitable impurities of the steel as the specific component and the ratio of the total amount of these inevitable impurities and the Cu amount are specified.

  In addition, when the steel having the specific component is hot-rolled, the heating temperature before hot rolling is defined according to the ratio of the total amount of Sn, Sb and As and the amount of Cu.

  Furthermore, hot rolling a steel having the specific component and defining the total amount of Sn, Sb and As contained in the unavoidable impurities in the steel, and the ratio of the total amount of these unavoidable impurities and the Cu amount This is a manufacturing method in which the previous heating temperature is specified. In the present invention, the specific component refers to a component composition other than inevitable impurities in steel. However, in the present invention, P and S are specified components.

  The reasons for limiting the components of the steel having a specific component according to the present invention will be described in detail below.

C
C is contained in order to improve the strength. If it is less than 0.45%, a predetermined strength as a spring cannot be obtained, and if it exceeds 0.7%, ductility is reduced and embrittlement occurs, so 0.45 to 0.7% is set.

Si
Si is indispensable as a deoxidizing element at the time of steelmaking, and improves the temper softening resistance and increases the strength of the steel. If it is less than 0.15%, the effect cannot be obtained, and if it exceeds 2.5%, the ductility is lowered, so 0.15 to 2.5%.

Mn
Mn is added because it improves the hardenability of the steel and increases the strength. If it is less than 0.3%, the effect cannot be obtained. If it exceeds 1%, the hardenability becomes excessive and the ductility is lowered, so the content is made 0.3 to 1%.

P, S
P and S segregate at the grain boundaries and degrade the ductility of the steel, so both are managed with 0.015% as the upper limit. The lower limit is not particularly specified as long as it is managed in consideration of economy.

Cu
Cu is added to improve the corrosion resistance of the steel and improve the strength by precipitation hardening. If it is less than 0.3%, the effect cannot be obtained, and if it exceeds 2%, the effect is saturated and the alloy cost increases, so 0.3 to 2%, preferably 0.5 to 2%.

sol. Al
Al is indispensable as a deoxidizer, and N is segregated at the grain boundary, and is contained in order to fix the grain boundary strength as AlN. sol. If Al (acid-soluble Al) is less than 0.01%, the effect cannot be obtained, and if it exceeds 0.1%, the cleanliness of the steel deteriorates and the ductility decreases. .

Cr
Cr is added to improve the hardenability of the steel and increase the strength. If it is less than 0.1%, the effect cannot be obtained, and if it exceeds 0.5%, the hardenability becomes excessive and the toughness decreases, so the content is made 0.1 to 0.5%.

Ti
Ti is contained in order to form carbonitrides and to refine crystal grains. If it is less than 0.005%, the effect cannot be obtained, and if it exceeds 0.1%, coarse carbonitrides are produced and the fatigue characteristics of the “spring” are deteriorated, so 0.005 to 0.1%.

One or more of Ni, Mo, V, Nb In order to improve the strength, toughness and corrosion resistance of the steel, one or more of Ni, Mo, V, Nb is contained. Ni is contained because it improves the toughness of the steel after quenching and tempering and improves the corrosion resistance. If it is less than 0.15%, the effect cannot be obtained, and if it exceeds 2%, the effect is saturated. Therefore, when such an effect is required, 0.15 to 2% is contained.

  Mo is included to improve hardenability, form carbides, have temper softening resistance, and improve corrosion resistance. If it is less than 0.05%, the effect cannot be obtained, and if it exceeds 1%, an overcooled structure is likely to be generated during hot rolling, so when such an effect is required, 0.05 to 1% is contained. Let

  V and Nb are refined by quenching and tempering to improve ductility. Moreover, precipitation strengthening in tempering improves the strength. If each of V and Nb is less than 0.05%, the effect cannot be obtained, and if it exceeds 0.5%, coarse carbides are precipitated and the ductility is impaired, so when such an effect is required, V: 0.05-0.5%, Nb: 0.05-0.5% is contained.

Ceq
Ceq is quenched, and after tempering at 400 ° C. or higher and below the Ac1 transformation point, in order to obtain a tensile strength of 2000 MPa or higher, it is set to 0.90% or higher. However, Ceq = C + Si / 7 + Mn / 6 + Cu / 7 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14, and elements not contained are 0.

  The above is the specific component of the steel of the present invention. Further, in the present invention, the total amount of Sn, Sb and As, which are inevitable impurities in the steel, and the ratio between the total amount and the Cu amount in the steel are specified.

(Sn + Sb + As) / Cu, Sn + Sb + As
These parameters make the steel composed of the above-mentioned specific components excellent in corrosion resistance and fatigue characteristics and high strength spring steel.

  When (Sn + Sb + As) / Cu is 0.05 or more and Sn + Sb + As is contained more than 0.1%, hot embrittlement is promoted, and minute defects are easily generated on the steel surface after hot rolling. When the part is placed in a corrosive environment, it generates corrosion pits and deteriorates fatigue characteristics.

  The spring steel according to the present invention is manufactured through a predetermined spring manufacturing process by melting a raw material mainly composed of scrap using an electric furnace, and performing conventional steelmaking-continuous casting-bundling rolling-bar wire rolling. Is done.

  In the present invention, instead of defining the total amount of Sn, Sb and As in the unavoidable impurities of the steel of the specific component described above and the ratio of the total amount to Cu, Sn, Sb and As in the unavoidable impurities of the steel are used. It also includes manufacturing by specifying the heating temperature before hot rolling according to the amount. In this case, the heating temperature T (° C.) at the time of hot rolling is defined so as to satisfy the following formula.

950 ≦ T ≦ T ′, more preferably 950 ≦ T ≦ T′−15
T ′ = 1083- {1509 * (([% Sn] + [% Sb] + [% As]) / [% Cu]) ² +535 * (([% Sn] + [% Sb] + [% As]) ) / [% Cu])}
However, [% Sn], [% Sb], [% As], and [% Cu] represent mass% in terms of the content in the inevitable impurities of the steel.

  In the case of hot rolling by applying the above formula, the steel of the specific component further defines the total amount of Sn, Sb and As in the inevitable impurities and the ratio of the total amount of the element and Cu. In this case, particularly excellent surface properties are obtained, and fatigue characteristics are improved, which is preferable.

  In the present invention, the tempering temperature is preferably 400 ° C. or higher in order to prevent the toughness from being lowered due to the low temperature temper brittleness.

  Hereinafter, the present invention will be described by way of examples. After melting steel of various chemical components shown in Table 1 in a 150 kg vacuum melting furnace, it is made into a steel ingot, forged to Φ25 mm by hot forging, allowed to cool, and quenched and tempered. did.

  Quenching was heated to 875 ° C. for 30 minutes, followed by oil cooling, and the tempering temperature was adjusted so that the tensile strength was 2000 to 2100 MPa.

  Steel No. Nos. 1 to 17 have the specific components of the present invention, and Sn, Sb and As in the unavoidable impurities in the steel are steels satisfying the provisions of the present invention. Nos. 18 to 26 and 29 to 33 are steels outside the specific component range of the present invention. Although 27 and 28 have the specific component of this invention, Sn, Sb, and As in an unavoidable impurity in steel do not satisfy the prescription | regulation of invention of Claim 1, and show the invention steel of Claim 2. Steel 33 is a conventional steel and exhibits characteristics that serve as a basis for corrosion fatigue characteristics and spring fatigue characteristics.

  The strength, ductility, corrosion resistance, corrosion fatigue properties, and spring fatigue properties of each steel were investigated. For the strength and ductility, a tensile test (based on JIS Z 2201 and using No. 4 test piece) was performed to obtain the tensile strength and the drawing. Corrosion resistance was obtained by cutting a plate-shaped test piece having a width of 15 mm * 40 mm from steel and performing a corrosion test in a dry / wet combined cycle.

Corrosion conditions include salt spray (5% NaCl solution, 25 ° C.) for 8 hours, then left in the atmosphere (humidity 60%, 25 ° C.) for 16 hours, 7 cycles. Rust was removed by immersing it in a 20% aqueous solution of ammonium hydrogen citrate at 80 ° C., and then the weight was measured. Corrosion weight loss [g / mm ² ] (weight difference before and after corrosion test / corrosion area (mm ² )) Asked. After the rust removal, the depth of the corrosion pits on the surface of the test piece was measured using a laser microscope to obtain the maximum depth.

  Corrosion fatigue characteristics are measured for some specimens with a tensile strength of 2000-2100 MPa. Rotating bending fatigue test specimens are collected from a Φ25 material, and 5% NaCl solution is dropped at the center of the parallel part while rotating. A fatigue test was performed. The surface of the test piece was subjected to shot peening.

  Spring fatigue characteristics are: material diameter: Φ11.5, coil average diameter (mm): 115.0, total number of turns: 6.0, effective number of turns: 4.5, free height (mm): 320, spring constant (N / mm): A spring of 25.2 was manufactured, and a fatigue test was performed under the conditions of average stress (MPa): 650 and amplitude stress (MPa): 550. For the evaluation of spring fatigue, a fatigue test was performed at N = 10 for each steel.

  The spring was manufactured in the steps of test melting, wire rod (hot rolling), heat treatment, pickling, wire drawing, OT treatment, coiling, low temperature annealing, shot peening, and setting.

  Table 2 shows the evaluation test results of heat treatment conditions and strength, ductility and corrosion resistance.

[Strength / Ductility]
Steels 1 to 17 all have high tensile strength (TS) of 2000 MPa or higher and squeezing (RA%) of 41.4% even when the tempering temperature is 400 ° C. or higher which does not cause low temperature temper brittleness. It was excellent in the above and ductility.

  On the other hand, the steel 18 is C, the steel 20 is Si, the steel 22 is Mn, and the steel 29 is low in Cr outside the scope of the present invention, and the Ceq is also low outside the scope of the present invention, so that the tensile strength is 2000 MPa or more. The steel has a low tempering temperature of less than 400 ° C. and low ductility due to low temperature tempering brittleness, making it unsuitable as a spring steel.

  Steel 19 is C, V, Steel 21 is Si, Mo, Steel 23 is Mn, Nb, Steel 30 is excessive in Cr outside the scope of the present invention, Steel 24, Steel 25 is P, S is the present invention. Since it was excessive outside the range, segregation at the grain boundary occurred, and in all cases, the ductility was lowered and the drawing was 30% or less.

  Steel 31 is sol. Since the Al content is low outside the scope of the present invention, deoxidation is insufficient, and grain boundary segregation of N cannot be suppressed, and the steel 32 has a high Al content outside the scope of the present invention, so the cleanliness is low, and the ductility of each is low. The decrease was significant.

[Corrosion resistance], [Corrosion fatigue properties]
As a result of the corrosion test, steels 1 to 17 were confirmed to have good corrosion resistance with a corrosion pit depth of 75 μm or less and little weight loss due to corrosion. On the other hand, since the steel 26 has a low Cu content outside the range of the present invention, the corrosion pit depth is 100 μm or more and is inferior in corrosion resistance.

  In addition, the corrosion fatigue characteristics of the steel 33 with a fatigue strength of x1.5 was good (◯ in the table). Steel 33 is a steel showing the fatigue characteristics of the conventional steel and used as a reference steel. Steels 1, 8, 10, 11, 13, 14, and 15 all had good results with respect to steel 26 having a high Cu content and a low Cu content.

[Spring fatigue test]
The spring fatigue test was performed using springs manufactured from wire rods obtained by hot rolling the materials of steels 1, 10, 11, 15, and steels 27, 28, and 33 at various heating temperatures. Steel 33 is a steel showing the fatigue characteristics of the conventional steel and used as a reference steel. The evaluation of fatigue characteristics is based on the test result No. More than 1.8 times the number of repetitions of 10 is long and excellent (◎), 1.5 times or more is good (◯), and a material with an equivalent life is (×).

  Table 3 shows the test results. Test result No. 1 to 5 are examples of the invention according to claim 3, and steels 1, 10, 11, and 15 of the test steel have specific components according to the present invention, and the amount of Sn, Sb, As in impurities is also specified. This is a satisfactory steel and the heating temperature before hot rolling is within the range of the present invention. Compared to 10, excellent fatigue characteristics were obtained.

  On the other hand, test result No. Nos. 6 and 8 show comparative examples of the invention according to claim 2, and the steel 27 of the sample steel has a component composition within the specific component range of the present invention, but the heating temperature T ° C. before hot rolling is the test result No. 6 is a heating temperature T ′ or more defined by Sn, Sb, As in the impurities in the steel. Since No. 8 is 950 ° C. or lower, the spring fatigue characteristics of the test result No. 8 with the conventional steel 33 are shown. It was equivalent to 10.

  Test result No. Nos. 7 and 9 are examples of the invention according to claim 2 and the test results No. Although it does not reach 1-5, the fatigue characteristics are the test results No. Compared to 10, good.

Claims (3)

% By mass
C: 0.45-0.7%,
Si: 0.15-2.5%,
Mn: 0.3 to 1%
P ≦ 0.015%,
S ≦ 0.015%,
Cu: 0.3-2%,
sol. Al: 0.01 to 0.1%,
Cr: 0.1 to less than 0.5%,
Ti: 0.005 to 0.1%,
(Sn + Sb + As) / Cu <0.05,
Sn + Sb + As ≦ 0.1%,
Furthermore, Ni: 0.1 to 2%,
Mo: 0.01 to 1%,
V: 0.05-0.5%
Nb: 0.01 to 0.5%
Containing one or more of
Ceq ≧ 0.90%,
High-strength spring steel with excellent corrosion resistance and fatigue properties, consisting of the balance iron and inevitable impurities.
However,
Ceq = C + Si / 7 + Mn / 6 + Cu / 7 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 % By mass
C: 0.45-0.7%,
Si: 0.15-2.5%,
Mn: 0.3 to 1%
P ≦ 0.015%,
S ≦ 0.015%,
Cu: 0.30 to 2%,
sol. Al: 0.01 to 0.1%,
Cr: 0.1 to less than 0.5%,
Ti: 0.005 to 0.1%,
Furthermore, Ni: 0.1 to 2%,
Mo: 0.01 to 1%,
V: 0.05-0.5%
Nb: 0.01 to 0.5%
Containing one or more of
Ceq ≧ 0.90%,
High strength spring steel with excellent corrosion resistance and fatigue characteristics, characterized in that the steel composed of the remaining iron and inevitable impurities is heated at a heating temperature T (° C.) satisfying the following formula (1) and then hot rolled. Manufacturing method.
950 ≦ T <T ′ (1)
However,
T ′ = 1083- {1509 * (([% Sn] + [% Sb] + [% As]) / [% Cu]) ² +535 * (([% Sn] + [% Sb] + [% As]) ) / [% Cu])}
In the formula (1), [% Sn], [% Sb], [% As], and [% Cu] indicate mass% in steel content. % By mass
C: 0.45-0.7%,
Si: 0.15-2.5%,
Mn: 0.3 to 1%
P ≦ 0.015%,
S ≦ 0.015%,
Cu: 0.3-2%,
(Sn + Sb + As) / Cu <0.05,
Sn + Sb + As ≦ 0.1%,
sol. Al: 0.01 to 0.1%,
Cr: 0.1 to less than 0.5%,
Ti: 0.005 to 0.1%,
Furthermore, Ni: 0.1 to 2%,
Mo: 0.01 to 1%,
V: 0.05-0.5%
Nb: 0.01 to 0.5%
Containing one or more of
Ceq ≧ 0.90%,
High strength spring steel with excellent corrosion resistance and fatigue characteristics, characterized in that the steel composed of the remaining iron and inevitable impurities is heated at a heating temperature T (° C.) satisfying the following formula (1) and then hot rolled. Manufacturing method.
950 ≦ T <T ′ (1)
However,
T ′ = 1083- {1509 * (([% Sn] + [% Sb] + [% As]) / [% Cu]) ² +535 * (([% Sn] + [% Sb] + [% As]) ) / [% Cu])}
In the formula (1), [% Sn], [% Sb], [% As], and [% Cu] indicate mass% in steel content.