JP4869051B2 - Spring steel, spring manufacturing method using this steel, and spring obtained from this steel - Google Patents

Abstract

A spring steel has the following composition (by wt.%): (A) Carbon (C) = 0.45 - 0.70; (B) Silicon (Si) = 1.65 - 2.50; (C) Manganese (Mn) = 0.20 - 0.75; (D) Chromium (Cr) = 0.60 - 2; (E) Nickel (Ni) = 0.15 - 1; (F) Molybdenum (Mo) = traces - 1; (G) Vanadium (V) = 0.003 - 0.8; (H) Copper (Cu) = 0.10 - 1; (I) Titanium (Ti) = 0.020 - 0.2; (J) Niobium (Nb) = traces - 0.2; (K) Aluminum (Al) = 0.002 - 0.050; (L) Phosphorus (P) = traces - 0.015; (M) Sulfer (S) = traces - 0.015; (N) Oxygen (O) = traces - 0.0020; (O) Nitrogen (N) = 0.0020 - 0.0110; (P) Remainder iron (Fe) and production impurities; (Q) a calculated carbon equivalent (C eq). Independent claims are also included for: (1) the fabrication of this spring steel; (2) a spring made from this steel.

Description

Technical field:
Generally speaking, as the fatigue stress applied to the spring increases, the spring hardness and tensile strength continue to increase. As a result, susceptibility to breakage caused by defects (such as inclusions or surface defects occurring during spring manufacture) increases and fatigue strength tends to reach its limit. On the other hand, springs used in severe corrosive environments, such as suspension springs, must exhibit at least equivalent (and better) corrosion fatigue properties when using steel with higher hardness and tensile strength. Thus, such springs are prone to breakage on defects immediately upon repeated fatigue in the atmosphere or repeated under corrosive media. For example, in corrosion fatigue, defects can arise from corrosion holes. Furthermore, the effect of stress concentration in corrosion holes, in spring surface defects that may occur during spring coiling or other operations, or in non-metallic inclusions becomes more critical as spring stiffness increases. Considering this, it is more difficult to improve the corrosion fatigue life or maintain it at the same level as the load stress increases.

  According to previous state of the art, French Patent No. 2740476 and (Japan) Patent No. 3474373 describe spring steel grades with good hydrogen-induced brittleness resistance and fatigue resistance, among which Ti, “Carbonitride sulfide” inclusions composed of at least one element of Nb, Zr, Ta or Hf have an average particle size smaller than 5 μm in diameter and very large number (more than 10,000 on the cut surface) ) To be controlled.

However, this type of steel, after quenching and tempering by an industrial spring manufacturing method, reaches a level of hardness of only 50 HRC or slightly higher (which corresponds to a tensile strength of 1700 MPa or slightly higher). It does not greatly exceed 1900 MPa, that is, 53.5 HRC. At this moderate hardness level, the steel exhibits only moderate sag resistance. In order to improve sag resistance, a steel solution with higher tensile strength is required. That is why, as required for the springs that are the object of this patent application, high strength exceeding 2100 MPa, hardness exceeding 55 HRC, high fatigue resistance in the atmosphere, at least equivalent and better A good balance between good corrosion fatigue resistance is not ensured with the aforementioned steels.
French Patent No. 2740476 Japanese Patent No. 3474373

Technical problems:
The purpose of this patent application is to determine the stiffness and tensile strength of the spring, higher fatigue properties in the atmosphere, at least equivalent and higher corrosion fatigue properties, higher spring sag resistance, and spring coiling or other It is to propose a means to collectively achieve less sensitivity to surface defects that may occur during the operation.

For this purpose, unexpectedly, exactly the following elements in mass%:
C: 0.45-0.7%,
Si: 1.65 to 2.5%,
Mn: 0.20 to 0.75%,
Cr: 0.60 to 2%,
Ni: 0.15 to 1%,
Mo: Trace amount to 1%,
V: 0.003-0.8%,
Cu: 0.10 to 1%,
Ti: 0.020 to 0.20%,
Nb: Trace amount to 0.2%,
Al: 0.002 to 0.050%,
P: Trace amount to 0.015%,
S: Trace amount to 0.015%,
O: Trace amount to 0.0020%,
N: 0.0020 to 0.0110%
Or the balance is substantially Fe and inevitable impurities,
Or preferably:
C: 0.45-0.65%,
Si: 1.65 to 2.2%,
Mn: 0.20 to 0.65%,
Cr: 0.8 to 1.7%,
Ni: 0.15-0.80%,
Mo: Trace amount to 0.80%,
V: 0.003-0.5%,
Cu: 0.10-0.90%,
Ti: 0.020 to 0.15%,
Nb: Trace amount to 0.15%,
Al: 0.002 to 0.050%,
P: Trace amount to 0.010%,
S: Trace amount to 0.010%,
O: Trace amount to 0.0020%,
N: 0.0020 to 0.0110%
And the balance is substantially Fe and inevitable impurities,
The following formula:
Ceq. = [C] +0.12 [Si] +0.17 [Mn] -0.1 [Ni]
+0.13 [Cr] -0.24 [V]
[In the formula, the amount in parentheses [X] indicates the element content in mass%. ]
All the inclusions in the 100 mm ² cut surface at a position of 1.5 mm ± 0.5 mm below the surface of the steel bar or wire rod for spring are included. and, when the square root of the surface area of the inclusions were considered inclusions size, Ti nitrides (or optionally carbonitrides) spring steel maximum size of inclusions is is less than 20μm is, certain steelmaking, Casting and rolling conditions, followed by precise quenching and tempering treatment, can ensure hardness exceeding 55HRC, while long life fatigue in the atmosphere, corrosion fatigue, sag resistance, coiling of springs or other It has been found to ensure excellent harmony with less sensitivity to surface stress concentrations that can be encountered with surface defects caused by operation.

Effects of the present invention:
Through the means given in the technology described in this patent application, the addition of trace alloying elements, the reduction of residual elements and strict control of the steel analysis described above, and the manufacturing path, improved atmospheric fatigue and sag. Highly improved hardness, along with properties, at least equivalent (and better) corrosion fatigue properties, and less sensitivity to stress concentrations caused by surface defects that can occur during spring coiling or other operations Spring steel having both tensile strength is obtained.

Explanation of chemical analysis of steel justified in the present invention S: Trace amount up to a maximum of 0.015% Sulfur is an unavoidable impurity of steel. Its content is as small as possible, and is trace amount to 0.015% (preferably 0.010%). This is to prevent the presence of sulfides that are undesirable for the corrosion fatigue resistance and high atmospheric fatigue resistance of the steels described in the present invention.

N: 0.0020 to 0.0110%
Nitrogen must be adjusted in the range of 0.0020-0.011%. This is because, in combination with Ti, Nb, Al or V, a sufficient number of very fine and ultra-microscopic nitrides, carbides or carbonitrides are formed that significantly refine the crystal grains. Therefore, for this purpose, the minimum content must be 0.0020%. Its maximum content should not exceed 0.011%. This is because the size definition described above is 1.5 mm ± 0.5 mm from the surface of the steel bar or wire used for manufacturing the spring (the position from the surface of the steel bar or wire is an important position regarding the fatigue load of the spring). This is to avoid the formation of coarse and hard Ti larger than 20 μm and possibly (V, Ti) nitride or carbonitride. In fact, in a fatigue test in the atmosphere or in a corrosive medium, if such large inclusions are present around the spring surface, considering that the spring breaks on such large inclusions, Such large nitrides or carbonitrides are highly undesirable for atmospheric fatigue resistance and corrosion fatigue in the high strength and hardness steels described in this invention.

・ V: 0.003-0.8%
Vanadium is an element that can improve the hardenability of steel, the tensile stress and the hardness after quenching and tempering. Furthermore, in combination with nitrogen, V can form many fine and microscopic V or (V, Ti) nitrides that refine grain size and improve tensile strength levels and hardness through structural hardening. Vanadium should be present at a minimum of 0.003% to form fine and microscopic (V, Ti) nitrides for grain refinement. However, since vanadium is an expensive element, it should be close to this lower limit in consideration of cost reduction as well as crystal grain refinement. On the other hand, vanadium should not exceed 0.8% (preferably 0.5%). Beyond these values, the coarse and very hard special V-containing carbide precipitates, along with Cr and Mo, may remain undissolved during the austenitization stage performed prior to quenching, Steels with high strength and hardness values described in the present invention may be highly undesirable for fatigue life in air or in a corrosive medium. Finally, the extra addition of V above 0.8% unnecessarily increases the cost of the steel.

・ C: 0.45-0.70%
Carbon can improve the tensile strength and hardness of steel after quenching and tempering. When the carbon content is less than 0.45%, the steel having high strength and hardness values described in the present invention cannot be obtained by quenching and tempering treatment in the temperature range normally used for spring production. On the other hand, if the C content exceeds 0.70% (preferably 0.65%), the coarse and very hard special carbides combined with Cr, Mo and V are not treated during the austenitizing process before quenching. May melt away. This can have a significant impact on atmospheric fatigue life, corrosion fatigue resistance, and also toughness. Therefore, the C content should not exceed 0.70%.

・ Si: 1.65 to 2.5%
Silicon is an important element that can ensure high resistance and hardness level, as well as Ceq value and sag resistance when dissolved. Due to the tensile strength and hardness values of the steel described in this invention, the Si content should not be less than 1.65%. Furthermore, silicon contributes at least partially to the deoxidation of the steel. If its content exceeds 2.5% (preferably 2.2%), the oxygen content of the steel may exceed 0.0020% (or even 0.0025%) by thermodynamic reaction ( Including the formation of oxides of various compositions), which is detrimental to fatigue resistance in the atmosphere. Furthermore, at Si contents above 2.5%, different combinations of elements such as Mn, Cr or others can segregate during the solidification stage after casting. These segregations are very detrimental to atmospheric fatigue behavior and corrosion fatigue resistance. Finally, at Si contents greater than 2.5%, decarburization on the steel bar or wire surface for the spring becomes too critical for the properties of the spring in use. For this reason, the Si content should not exceed 2.5% (preferably 2.2%).

・ Al: 0.002 to 0.050%
Aluminum can be added to complete the deoxidation of the steel and to achieve an oxygen content of as low as 0.0020% in all cases of the steel described in the present invention as low as possible. Furthermore, Al combined with nitrogen contributes to crystal grain refinement through the formation of ultramicroscopic nitrides. In order to ensure these two characteristics, an Al content of 0.0020% or more is required. On the other hand, at an Al content greater than 0.05%, there may be large, separated aluminum oxide inclusions, or harder, angular aluminate inclusions in the form of finer but long veins. . These are detrimental to atmospheric fatigue life and steel cleanliness. For this reason, the Al content should not exceed 0.05%.

・ Mn: 0.20 to 0.75%
Manganese must be added at a content of at least 10 times the S content in order to combine with trace amounts up to 0.015% residual S and avoid the formation of iron sulfide, which is extremely harmful to steel rolling. I must. Therefore, 0.20% is required as the minimum Mn content. Further, Mn contributes to solid solution hardening during quenching of the steel in the same manner as Ni, Cr, Mo and V in order to obtain the high tensile strength and hardness and Ceq value of the steel described in the present invention. At Mn content above 0.75% (preferably 0.65%), in combination with Si content, segregation can occur during steelmaking and post-casting solidification procedures. These segregations are detrimental to the operational properties of the steel and the homogeneity of the steel. For this reason, the Mn content should not exceed 0.75%.

Cr: 0.60% to 2% (preferably 0.80 to 1.70%)
Cr is added to obtain high tensile strength and hardness values by solid solution after austenitizing and quenching and tempering, and to contribute to the Ceq range, and to improve corrosion fatigue resistance. In order to ensure these properties, the Cr content should be at least 0.60% (preferably 0.80%). Beyond 2% (preferably 1.7%), together with V and Mo, coarse and very hard special Cr carbides may be present after the austenitizing treatment performed before quenching. Such carbides greatly affect the fatigue resistance in the atmosphere. For this reason, the Cr content should not exceed 2%.

・ Ni: 0.15 to 1%
Nickel is added to improve the hardenability of the steel and the tensile strength and hardness after quenching and tempering. Since no carbide is formed, Ni contributes to hardening of the steel. This is similar to Cr, Mo and V which do not dissolve during austenitization performed prior to quenching and do not form coarse, hard special carbides that can be detrimental to fatigue resistance in the atmosphere. Also, Ni can adjust Ceq between 0.8 and 1% in the steel described in the present invention. Like elements that are not oxidized, Ni improves corrosion fatigue resistance. In order to ensure the effectiveness of these actions, the Ni content should not be less than 0.15%. On the other hand, above 1% (preferably 0.80%), Ni can induce excessive retained austenite, the presence of which is very detrimental to corrosion fatigue resistance. The higher Ni content significantly increases the cost of the steel. For all these reasons, the Ni content should not exceed 1%.

・ Mo: Trace amount ~ 1%
Molybdenum, like chromium, improves the hardenability of the steel and the strength and hardness of the steel after quenching and tempering. Furthermore, Mo is an element having a low oxidation potential. For these two reasons, Mo is preferred for atmospheric fatigue and corrosion fatigue life. On the other hand, at values above 1% (preferably 0.80%), special carbides that are coarse and very hard can be present alone or in combination with V and Cr after the austenitizing treatment performed before quenching. . These special carbides are very detrimental to fatigue resistance in the atmosphere. Finally, the extra addition of Mo with a content above 1% unnecessarily increases the cost of the steel. For this reason, the Mo content should not exceed 1%.

Cu: 0.10 to 1%
Copper is a hardening element of steel by solid solution after quenching and tempering treatment. Therefore, copper can be added together with other elements that contribute to improving the strength and hardness of the steel. Copper can harden steel if it does not bond with carbon and does not form hard and coarse carbides that are detrimental to fatigue resistance in the atmosphere. From an electrochemical point of view, its passive potential is higher than that of iron and as a result is preferred for the corrosion fatigue resistance of steel. In order to ensure the effectiveness of these actions, the Cu content should not be less than 0.10%. On the other hand, at a content exceeding 1% (preferably 0.9%), Cu has a very detrimental effect on hot rolling properties. For this reason, the Cu content should not exceed 1%.

・ P: Trace amount to 0.015%
Phosphorus is an inevitable impurity in steel. Phosphorus segregates with elements such as Cr or Mn or others at the previous austenitic grain boundaries during the quenching and tempering process. As a result, the agglomeration of crystal grain boundaries is reduced, and embrittlement between crystal grains that is extremely harmful to toughness and fatigue resistance in the atmosphere occurs. These effects are even more detrimental to steels with high tensile strength and hardness values as described in the present invention. In order to simultaneously obtain high tensile strength and hardness in spring steel and good atmospheric fatigue resistance and corrosion fatigue resistance, the P content should be as low as possible, 0.015% (preferably 0.010 %) Must not be exceeded.

・ Oxygen: Trace amount ~ 0.0020%
Oxygen is also an inevitable impurity in steel. In combination with the deoxidizing element, oxygen can separate to yield very hard and angular inclusions in a coarse form or fine but long vein form. These are very detrimental to fatigue resistance in the atmosphere. These effects are even more detrimental when the steels described in the present invention have high tensile strength and hardness values. For these reasons, in the steel described in the present invention, the oxygen content is 0.0020% in order to ensure a good balance between high strength / high hardness and resistance to atmospheric fatigue and corrosion fatigue. Must not be exceeded.

Ti: 0.020 to 0.2%
Titanium is added with nitrogen and / or carbon and / or vanadium to form fine, microscopic nitrides or carbides, which are austenitic grain sizes during the austenitizing process performed prior to quenching. Can be refined. This increases the grain boundary area in the steel, which in turn leads to a reduction in the amount of inevitable impurities (eg P) that segregate at the grain boundaries. Such segregation between grains is very detrimental to toughness and fatigue resistance in the atmosphere when present at high concentrations per unit area of grain boundaries. In addition, titanium can combine with C and N, optionally with V and Nb, to form other fine nitrides or carbonitrides, which are some elements that can be extremely detrimental to corrosion fatigue resistance It causes an irreversible action to trap (eg hydrogen formed during the corrosion reaction). For good efficacy, the Ti content should not be less than 0.020%. On the other hand, Ti exceeding 0.2% (preferably 0.15%) can form coarse and hard nitrides or carbonitrides that are very detrimental to fatigue resistance in the atmosphere. This latter action is even more detrimental to steels with high levels of tensile strength and hardness as described in the present invention. For these reasons, the Ti content should not exceed 0.2%.

Nb: Trace amount to 0.2%
Niobium is added to combine with C and N to form very fine and microscopic nitrides and / or carbides and / or carbonitride precipitates. In particular, when the Al content is low (for example, 0.002%), the austenite crystal grain size can be refined during the austenitizing treatment performed before quenching. Therefore, Nb, like Ti, increases the grain boundary area in steel and is preferable for grain boundary embrittlement due to inevitable impurities such as P (its action is harmful to toughness and corrosion fatigue resistance). Contributes to action. Furthermore, very fine Nb nitrides or carbonitride precipitates can contribute to the hardening of the steel by structural hardening. However, the Nb content is kept so that the nitride or carbonitride remains very fine to ensure refinement of the austenitic grain size and to avoid cracking or crack formation during hot rolling. Should not exceed 0.2% (preferably 0.15%). For these reasons, the Nb content should not exceed 0.2%.

Process Path Description According to the Invention A non-limiting description of the steel making process and implementation according to the invention is described below.

  Steel is produced in either a converter or an electric arc furnace, followed by a ladle refining stage during which alloying elements are added, and also deoxidation, and more generally any secondary refining The operation is performed. Thus, accurate steel analysis according to the invention can be achieved and the formation of large and complex sulfides or “carbonitride sulfides” of elements such as Ti and / or Nb and / or V in the liquid can be avoided. In order to prevent such coarse precipitates from forming in the steelmaking process, unexpectedly all elemental contents, and especially Ti, N, V and S contents, are probably current levels according to the state of the art. It has been found that compared to steel analysis, it must be carefully controlled within the above-mentioned compositional limitations of this patent application. After the steel making process described above, the steel of the present invention is then either continuously cast in the form of a bloom or bloom-billet or cast via an ingot path. However, formation of coarse titanium nitride (optionally titanium carbonitride) or agglomeration of fine titanium nitride or carbonitride of these products (bloom, bloom-billette or ingot) formed during and after the solidification stage In order to avoid or reduce as much as possible, the average cooling rate of these products after casting is at least equal to 0.3 ° C./s over the exact temperature range of 1450 to 1300 ° C. I found out that this should be the case. When operating at these conditions during the solidification and cooling phase, it was unexpectedly found that the maximum size of titanium nitride or carbonitride observed in the spring was always finer than 20 μm. The location and size of these Ti precipitates will be described later. After returning to room temperature, the product (bloom, bloom-billet or ingot) consistent with the exact steel grade analysis described in the present invention is reheated in the range of 1200/800 ° C. in single or double heating and rolling rows and Roll to form wire or bar. The rods, bars or slags or optionally springs made from these wires or bars are then austenitized in the temperature range between 850 and 1000 ° C. in order to obtain the special properties of the steels described in this invention, Subject to water or polymer or oil quenching. This is to achieve a fine austenitic grain size without some grains coarser than the ASTM grain size scale N ° 9. After this quenching treatment, a tempering treatment is clearly performed between 300 and 550 ° C. This is to obtain a high level of tensile strength and hardness, and on the one hand to avoid the microstructure leading to tempered martensite embrittlement and on the other hand excessive residual austenite. It has been found that the presence of tempered martensite embrittlement and excess retained austenite is extremely detrimental to the corrosion fatigue resistance of the steels described in the present invention. When springs are manufactured from non-heat treated steel bars or wires or slag obtained from these bars or wires, the aforementioned treatment (quenching and tempering) must be carried out on the springs under the same conditions as described above. If the springs are manufactured via cold forming, these heat treatments must be performed on the steel bars, wires or slugs obtained from these bars or wires prior to spring manufacture.

means:
Follow the table and drawings below.

Measurement of Ti Nitride or Carbonitride Size The maximum size of inclusions that are Ti nitride or carbonitride is measured as follows:

Observation is performed at a position of 1.5 mm ± 0.5 mm below the surface of the steel bar or wire, with a surface area of 100 mm ² of the cut surface of the steel bar or wire obtained from predetermined steel heating. After these observations, the inclusions are square and each size of these inclusions including inclusions with the largest surface area is considered to be equal to the square root of their surface area and Ti nitrides with the largest surface area or The size of inclusions, which are carbonitrides, was determined. Observe all inclusions at 100 mm ² of spring bar or cut surface. If the maximum size of the above-mentioned inclusions at 100 mm ² at 1.5 mm ± 0.5 mm below the surface is less than 20 μm, the steel heating is consistent with the present invention. The corresponding results thus obtained with the present invention and comparative steel are shown in Table 3.

Fatigue test:
The specimen for the fatigue test was obtained with a rod, and the final diameter of the gauge cross section of the specimen was 11 mm.

The process of manufacturing a specimen for fatigue testing consists of rough machining, austenitizing, oil quenching, tempering, polishing and shot peening. These specimens were subjected to torsional fatigue tests in the atmosphere. The applied shear stress was 856 ± 494 MPa, and the number of cycles until breakage was counted. If the specimen was not broken, the test was stopped at 2 × 10 ⁶ cycles.

Corrosion fatigue test The specimen of the corrosion fatigue test was obtained as a rod, and the final diameter of the gauge cross section of the test piece was 11 mm. The process for producing corrosion fatigue test specimens consists of rough machining, austenitizing, oil quenching, tempering, polishing and shot peening. These specimens were subjected to a corrosion fatigue test in which corrosion was applied in the same time as the fatigue load. The fatigue load was a shear stress equal to 856 ± 300 MPa. The added corrosion was repeated corrosion in two alternating stages:
The first stage is a wet stage for 5 minutes at 35 ° C. by spraying with a salt solution containing 5% NaCl. The second stage is without spraying, has a duration of 30 minutes and a temperature of 35 A drying stage maintained at 0C.
The number of cycles until breakage was counted up to breakage like the corrosion fatigue life.

Sag test:
The sag resistance was measured using a cycle compression test with a cylindrical pin. The pin had a diameter of 7 mm and a height of 12 mm. They were obtained from steel bars.

  The process of making a test piece for sag testing consists of rough machining, austenitizing, oil quenching, tempering, and final fine polishing. The pin height was accurately measured with 1 μm accuracy using a comparator before the test was started. In order to simulate a spring preset, a preload of 2200 MPa compressive stress was applied.

  A cycle fatigue load was then applied. This stress was 1270 ± 730 MPa. Pin height loss was measured during loads up to 1 million cycles. After the test was completed, the total height was determined by accurately measuring the residual height compared to the initial height. The greater the decrease in height (%) from the initial height, the lower the sag resistance.

Table 4 shows the results of fatigue, corrosion fatigue, and sag test in the present invention and comparative steel.

Comments on Table 4 and FIGS. 1-3 As shown in FIG. 1, the sag resistance is higher in the steel of the present invention than in the comparative steel. From Table 4, according to the sag measurement described above, the sag value is at least 32% in the worst case of the steel of the present invention (invention steel 1) compared to the best case of the comparative steel (comparative steel 1). Clearly low.

  The fatigue life in the air of the steel of the present invention is considerably longer than that of the comparative steel (FIG. 2). It is due to increased hardness (as shown in FIG. 2). But it's not just about increasing hardness. In fact, generally speaking, higher hardness steels are more sensitive to defects (eg, inclusions, surface defects,...) As the hardness increases. So, considering that the present invention avoids such coarse inclusion generation (Table 3), the steel according to the present invention is resistant to defects, and especially Ti nitride or carbonitride such as Less sensitive to large inclusions. As shown in Table 3, the size of the maximum inclusion found in the steel of the present invention does not exceed 14.1 μm, but in the comparative steel 2, inclusions larger than 20 μm are found. Furthermore, the low susceptibility to surface defects (as may occur during spring coiling or other operations when using steel according to the present invention) has been heat treated for hardness above 55 HRC. This can be explained through an impact test performed on the present invention and comparative steel (FIG. 3). The Charpy U-notch impact value measured with the steel of the present invention is higher than that measured with the comparative steel (FIG. 3). (Note that the notch in the Charpy U-notch impact value is measured during coiling of the spring or other operations. For simulating stress concentrations like other stress concentrations that may be encountered with the surface defects created). This highlights that the inventive steel is less sensitive to such surface stress concentrations at the defects than the comparative steel according to the prior art.

  Increasing hardness is known to reduce corrosion fatigue resistance. Clearly, therefore, the advantage of the steel according to the invention is that its corrosion fatigue resistance is higher than that of the comparative steel according to the state of the art, in particular with a hardness higher than 55HRc (FIG. 2).

  Thus, the present invention provides a better balance between fatigue life in the atmosphere, strongly increased sag resistance, and better corrosion fatigue life (Figure 2) than the comparative steel according to the prior art, and more High hardness can be achieved. In addition, a low sensitivity to surface defects that can occur, inter alia, during spring coiling or other operations is also obtained (FIG. 3).

The Mn, Ni, Cr, Ti and S content is outside the exact composition range of the steel according to the invention,
-S content is too high to ensure a good balance between atmospheric fatigue, corrosion fatigue and sensitivity to possible spring surface defects,
-Mn content is too high, causing segregation that is detrimental to steel homogeneity, atmospheric fatigue and corrosion fatigue properties,
Since -Ti is too low to ensure high fatigue corrosion properties, Comparative Steel 1 is not compatible with the present invention.

-C and Mn contents are outside the exact composition range of the steel according to the invention, Ceq. Is obviously low,
-C content and Ceq. Is too low to ensure that high hardness and tensile strength are obtained,
The tensile strength is too low to obtain good fatigue resistance in the atmosphere,
-Chemical analysis is not appropriate for good sag resistance,
The comparative steel 2 is not compatible with the present invention because the maximum size of Ti nitride or carbonitride found is too high to ensure fatigue resistance and corrosion fatigue resistance.

The Si, Mn, Cr, Ni and Ti content is outside the exact composition range of the steel according to the invention,
The Si content is too low to obtain the required high hardness, good sag resistance and also good fatigue behavior in the atmosphere,
Since -Ti is too low to ensure high corrosion fatigue properties, comparative steel 3 is not compatible with the present invention.

  Other arguments that are discussed as needed can be mentioned in the properties of the three comparative steels.

Summary of sag resistance of the present invention and comparative steel. Results of fatigue and corrosion fatigue of the present invention and comparative steel (number of cycles to failure). Comparison of toughness between the present invention and the comparative steel for hardness (Charpy impact value measured with U-notch specimen).

Claims (9)

Exactly mass%,
C: 0.45-0.70%,
Si: 1.65 to 2.50%,
Mn: 0.20 to 0.75%,
Cr: 0.60 to 2%,
Ni: 0.15 to 1%,
Mo: Trace amount to 1%,
V: 0.003-0.8%,
Cu: 0.10 to 1%,
Ti: 0.020 to 0.2%,
Nb: Trace amount to 0.2%,
Al: 0.002 to 0.050%,
P: Trace amount to 0.015%,
S: Trace amount to 0.015%,
O: Trace amount to 0.0020%,
N: 0.0020 to 0.0110%
And the balance is Fe and inevitable impurities,
Following formula:
Ceq. = [C] +0.12 [Si] +0.17 [Mn] -0.1 [Ni] +0.13 [Cr] -0.24 [V]
[In the formula, [C], [Si], [Mn], [Ni], [Cr] and [V] indicate amounts of these elements in mass%. ]
Carbon equivalent Ceq. Is 0.80 to 1.00%,
A spring steel having a Rockwell hardness HRC of 55 or more after quenching and tempering and excellent fatigue characteristics and sag resistance under corrosive and non-corrosive conditions. When all the inclusions on the 100 mm ² cut surface at a position of 1.5 mm ± 0.5 mm below the surface of the steel bar or wire for the spring are observed and the square root of the surface area of the inclusion is the inclusion size, Ti The spring steel excellent in fatigue characteristics and sag resistance under corrosive and non-corrosive conditions according to claim 1, wherein the maximum size of inclusions which are nitrides or carbonitrides is less than 20 µm. Exactly mass%,
C: 0.45-0.65%,
Si: 1.65 to 2.20%,
Mn: 0.20 to 0.65%,
Cr: 0.80 to 1.7%,
Ni: 0.15-0.80%,
Mo: Trace amount to 0.80%,
V: 0.003-0.5%,
Cu: 0.10-0.90%,
Ti: 0.020 to 0.15%,
Nb: Trace amount to 0.15%,
Al: 0.002 to 0.050%,
P: Trace amount to 0.010%,
S: Trace amount to 0.010%,
O: Trace amount to 0.0020%,
N: 0.0020 to 0.0110%
The spring steel according to claim 1, wherein the balance is Fe and inevitable impurities. The converter or steel produced in an electric furnace, by subjecting the smelting fine is then exactly following elements in mass percent:
C: 0.45-0.70%,
Si: 1.65 to 2.5%,
Mn: 0.20 to 0.75%,
Cr: 0.60 to 2%,
Ni: 0.15 to 1%,
Mo: Trace amount to 1%,
V: 0.003-0.8%,
Cu: 0.10 to 1%,
Ti: 0.020 to 0.2%,
Nb: Trace amount to 0.2%,
Al: 0.002 to 0.050%,
P: Trace amount to 0.015%,
S: Trace amount to 0.015%,
O: Trace amount to 0.0020%,
N: 0.0020 to 0.0110%
Steel with the balance being Fe and unavoidable impurities,
Following formula:
Ceq. = [C] +0.12 [Si] +0.17 [Mn] -0.1 [Ni]
+0.13 [Cr] -0.24 [V]
[In the formula, [C], [Si], [Mn], [Ni], [Cr] and [V] indicate amounts of these elements in mass%. ]
Carbon equivalent Ceq. A method for producing a spring steel having a hardness of 55 HRC or more, excellent in fatigue characteristics under corrosion and non-corrosion conditions, and sag resistance. Steel is cast via a continuous casting path in the form of a bloom or bloom-billet or via an ingot path, and the product thus obtained has an average in the solidification stage or thereafter in the range of 1450-1300 ° C. After cooling to a cooling rate of 0.3 ° C./s or more, after cooling to room temperature, it is reheated and rolled in the range of 1200 to 800 ° C. by single or double reheating and rolling train, and is used as a spring steel bar Or the method of Claim 4 made into the form of a wire. Wire resulting from wire or bar steel, bar Ma et spring, subjected to heat treatment is austenitizing in the range of 850 to 1000 ° C., then quenched with water, oil or polymer medium, exactly 300 to 550 ° C. The method according to claim 4 or 5, which is subjected to a tempering treatment performed in a range of. When all the inclusions on the 100 mm ² cut surface at a position of 1.5 mm ± 0.5 mm below the surface of the steel bar or wire for the spring are observed and the square root of the surface area of the inclusion is the inclusion size, Ti The method for producing spring steel according to any one of claims 4 to 6, wherein the maximum size of inclusions that are nitrides or carbonitrides is less than 20 µm. The following elements exactly in% by mass:
C: 0.45-0.65%,
Si: 1.65 to 2.2%,
Mn: 0.20 to 0.65%,
Cr: 0.80 to 1.7%,
Ni: 0.15-0.80%,
Mo: Trace amount to 0.80%,
V: 0.003-0.5%,
Cu: 0.10-0.90%,
Ti: 0.020 to 0.15%,
Nb: Trace amount to 0.15%,
Al: 0.002 to 0.050%,
P: Trace amount to 0.010%,
S: Trace amount to 0.010%,
O: Trace amount to 0.002%,
N: 0.0020 to 0.0110%
A method for producing spring steel having excellent fatigue characteristics and sag resistance under corrosive and non-corrosive conditions according to any one of claims 4 to 7, wherein the balance is Fe and Fe is an inevitable impurity steel composition. . A spring manufactured from a steel having excellent fatigue characteristics and corrosion resistance under corrosion and non-corrosion conditions manufactured according to any one of claims 4 to 8, wherein the hardness of the steel is 55 HRC or more, and fatigue. As a result of the test, the number of cycles until breakage is 1742967 or more .

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