DE102016107787A1 - Ultrahigh-strength spring steel - Google Patents

Abstract

An ultra-high strength spring steel for use as a valve spring steel in a vehicle engine comprises 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn , 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% by weight or less of CU (but more than 0%), 0.0001 to 0.3% by weight of Al, 0.03% by weight or less of N (but more than 0%), 0.0001 to 0.003% by weight of O and a balance of Fe or other unavoidable impurities based on 100% by weight of the ultra high strength spring steel.

Description

TECHNICAL AREA

The present disclosure relates to an ultra high strength spring steel, and more particularly to an ultra high strength spring steel having improved tensile strength and fatigue life for use as a motor valve spring.

BACKGROUND

Significant attention is given to improving the fuel consumption of vehicles. So far, reducing vehicle weight or reducing power loss by reducing friction is important. In addition, the output efficiency maximization by increasing the dynamic characteristics in controlling the combustion of an engine itself is also important. Further, attempts have been made to improve fuel efficiency through weight reductions of components that undergo dynamic behavior patterns in an engine head section.

Since engine valve springs, which are among the components having dynamic characteristics that directly control the dynamic load, a high fuel efficiency enhancement effect can be recognized when their weight is reduced. As known valve spring materials, CrSi steel having a tensile strength of about 1900 MPa and CrSiV steel having a tensile strength of about 2100 MPa are commonly used. Therefore, there have been attempts to develop high strength spring steel having a tensile strength of about 2100 MPa or more by adding alloying elements to the known CrSiV steel.

The above-disclosed prior art has been provided to assist in understanding the present disclosure and should not be interpreted as a known technology known to those skilled in the art.

SUMMARY OF THE REVELATION

The present disclosure is made in view of the above problems and an object of the present disclosure is an ultra-high strength spring steel having improved tensile strength as compared to known spring steels by optimizing the contents of Mo, Ni, V, Nb and Ti and fatigue life by controlling inclusions, which improve the fatigue life, provide.

According to one aspect of the present disclosure, the above and other objects can be achieved by providing an ultra high strength spring steel for use as a valve spring steel in a vehicle engine comprising 0.5 to 0.7% by weight of C, 1, 2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0, 05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3 weight% or less of Cu (but more than 0%), 0.0001 to 0.3 weight% of Al, 0.03 weight% or less of N (but more than 0%), 0 , 0001 to 0.003% by weight of O and a balance of Fe or other unavoidable impurities based on 100% by weight of the ultra high-strength spring steel.

The spring steel may have a tensile strength of 2300 MPa or more.

The spring steel may have a fatigue life of 1100 MPa or more.

The spring steel may have a yield strength of 2800 MPa or more.

The spring steel may have a hardness of 710 HV or more.

The sizes of the shots present in the spring steel may be 15 μm or less.

In the inclusions, a content of inclusions having sizes of 10 to 15 μm may be 10% or less, and a content of inclusions having sizes of 10 μm or less may be 90% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present disclosure can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

1 Fig. 10 is a table showing ingredients of ingredients of Examples and Comparative Examples;

2 is a table showing characteristics and performances of Examples and Comparative Examples;

3 FIG. 10 is a graph showing calculation results for a phase change according to temperatures of ultra high strength spring steel according to an embodiment of the present disclosure; FIG. and

4 FIG. 10 is a graph showing calculation results for a phase transformation according to temperatures in the cementite fabric of the ultra-high-strength spring steel according to an embodiment of the present disclosure. FIG.

DETAILED DESCRIPTION

Reference will now be made in detail to preferred embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

3 FIG. 12 is a graph showing calculation results for phase transformations according to a temperature of the ultra high strength spring steel according to an embodiment of the present invention; and FIG 4 FIG. 12 is a graph showing calculation results for phase transformations according to a temperature in the cementite fabric of an ultra high-strength spring steel according to an embodiment of the present invention. FIG.

The ultra-high-strength spring steel according to the present disclosure may be a valve spring steel used in a vehicle engine. In addition, the ultra-high-strength steel may be a spring steel having improved tensile strength, fatigue life and the like according to the optimization of the main alloy components. In particular, the ultra-high-strength spring steel may comprise 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1 , 2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% by weight or less of CU (but more than 0), 0.0001 to 0.3% by weight Al, 0.03 wt% or less of N (but not 0), 0.0001 to 0.003 wt% of O, and a balance of Fe and other unavoidable impurities based on 100 wt% of the ultra high-strength spring steel.

In the present disclosure, the alloying ingredients and composition ranges thereof are limited for the following reasons. Hereinafter, "%" as a unit of the composition indicates ranges in terms of weight% unless otherwise specified.

The content of carbon (C) is preferably 0.5 to 0.7%. An increase in the carbon content in the steel provides a proportional increase in strength. When the content of carbon is less than 0.5%, the increase in strength is small due to lack of hardenability during the heat treatment. When the content of carbon is larger than 0.7%, a martensite fabric is formed during curing and the fatigue life and hardness are reduced. Within the range, high strength and ductility can be ensured.

The proportion of silicone (Si) is preferably 1.2 to 1.5%. Silicone increases extensibility, heat resistance, and hardenability and improves shape memory (permanent adjustability) by inhibiting a shape change. In addition, silicon cures iron and martensite fabrics and increases strength and resistance during heat treatment and softening when trapped in ferrite. When the content of silicon (Si) is less than 1.2%, the resistance to heat treatment and softening is low. When the content of silicon (Si) is more than 1.5%, the heat resistance increases, but the material becomes susceptible to decarburization and decarburization occurs during the heat treatment.

The proportion of manganese (Mn) is preferably 0.6 to 1.2%. When manganese (Mn) is used as an element improving the hardenability and strength in a matrix, the bending fatigue life is improved and the hardenability is improved. In addition, manganese (Mn) as a deoxidizer which generates an oxide inhibits the formation of inclusions such as Al ₂ O ₃ . When the content of manganese (Mn) is less than 0.6%, it is difficult to ensure the hardenability. When the content of manganese (Mn) is more than 1.2%, the durability deteriorates.

The proportion of chromium (Cr) is preferably 0.6 to 1.2%. Chromium for ensuring the durability can form a failure product during the heat treatment, improve the hardenability, increase the strength by suppressing the softening and contribute to the refining of crystal grains and an increase in strength. When the content of chromium (Cr) is 0.6% or more, improved tempering and softening properties result, and decarburization, curability and corrosion resistance are improved. When the proportion of chromium (Cr) is larger than 1.2%, intergranular carbides are excessively produced and strength reduction and embrittlement can be caused.

The proportion of molybdenum (Mo) is preferably 0.1 to 0.5%. Similar to chromium, molybdenum forms minute carbide precipitates, improving strength and fracture toughness. In particular, TiMoC having a size of 1 to 5 nm is uniformly formed to ensure tempering resistance, heat resistance and high strength. If the content of molybdenum (Mo) is less than 0.1%, it may be possible to produce a carbide and the strength is not sufficiently ensured. When the content of molybdenum (Mo) is more than 0.5%, precipitation and strength increase effects become saturated, and hence it may be unnecessary to increase the proportion in terms of a cost aspect.

The proportion of nickel (Ni) is preferably 0.05 to 0.8%. Nickel, as an element that helps to increase corrosion resistance, improves heat resistance, prevents brittleness at low temperatures, and improves hardenability, dimensional stability, and adjustability. When the content of nickel (Ni) is less than 0.05%, corrosion resistance and high temperature stability are lowered. If the proportion of nickel (Ni) is greater than 0.8%, a Rotbruchanfälligkeit may occur.

The proportion of vanadium (V) is preferably 0.05 to 0.5%. Vanadium, as an element that improves fabric refinement, tempering resistance, dimensional stability and adjustability, and that ensures heat resistance and high strength, forms VC as a minute precipitate to increase fracture toughness. In particular, VC as a minute precipitate prevents the movement of a grain boundary. In addition, V is dissolved and used during austenitization and precipitates during tempering, resulting in secondary hardening. If the proportion of V is less than 0.05%, the prevention effect of the fracture toughness reduction may be reduced. If the content of vanadium (V) is larger than 0.5%, the size of the precipitates may be coarse, and after quenching, the hardness may be lowered.

The proportion of niobium (Nb) is preferably 0.05 to 0.5%. Niobium refines the fabric, cures a surface by nitrification and improves dimensional stability and adjustability. In addition, the strength can be increased by forming NbC, and the formation rates of other carbides such as CrC, VC, TiC, and MoC are controlled. If the content of niobium (Nb) is less than 0.05%, the strength may be lowered and the carbide may be heterogenized. If the content of niobium (Nb) is larger than 0.5%, generation of other carbides may be suppressed.

The proportion of titanium (Ti) is preferably 0.05 to 0.3%. Titanium prevents recrystallization of crystal grains such as Nb and Al and suppresses the growth thereof. In addition, titanium forms nanoscale carbides, such as TiC and TiMoC, reacts with nitrogen, suppresses crystal grain growth through the generation of TiN, and minimizes the decrease in BN hardenability by disrupting the bonding of B to N through the formation of TiB ₂ . When the proportion of titanium (Ti) is less than 0.05%, other inclusions such as Al ₂ O _{3 are} generated, and hence the fatigue life is lowered. If the content of titanium Ti is more than 0.3%, the functions of other alloying elements may be disturbed and the manufacturing cost may increase.

The content of copper (Cu) is preferably 0.3% or less (but not 0). Copper increases the cooling properties or strength after tempering and, similar to Ni, increases the corrosion resistance of the steel. However, if the proportion of copper (Cu) becomes too high, the alloying cost may increase. Therefore, the content of copper (Cu) can be limited to 0.3% or less.

The content of aluminum (Al) is preferably 0.0001 to 0.3%. Aluminum reacts with nitrogen, refines the Austentit by the formation of AlN and increases the strength and impact resistance. In particular, by adding Nb, Ti, and Mo, the addition amount of vanadium for refining the crystal grains and nickel for ensuring the toughness as high-priced elements can be reduced. When the content of aluminum (Al) is less than 0.0001%, the effects due to the addition of aluminum (Al) can not be expected. When the content of aluminum (Al) is larger than 0.3%, large square inclusions (Al ₂ O ₃ ) are generated. Such large, square inclusions can act as fatigue points that reduce durability by weakening the steel.

The proportion of nitrogen (N) is preferably 0.03% or less (but not 0). Nitrogen forms AlN and TiN by reaction with Al and Ti to form the crystal grain refining effects, and maximizes boron hardenability by forming TiN. However, if the proportion of nitrogen (N) is too large, the hardenability of the steel may be weakened by the reaction with boron. Therefore, the content of nitrogen (N) may preferably be limited to 0.03% or less.

The proportion of oxygen (O) is preferably 0.0001 to 0.003%. Since oxygen binds with Si or Al, forming hard oxygen-based non-metallic inclusions and causing a decrease in fatigue life properties, it is preferable to keep the content of oxygen (O) as small as possible. However, with respect to a steelmaking technology aspect, it is difficult to keep the content of oxygen (O) less than 0.0001%. Accordingly, in the present disclosure, the lowest amount of oxygen (O) is 0.003%.

Thus, components other than the aforementioned ingredients such as Fe and other unavoidable impurities remain.

In the following, the present disclosure will be described in more detail with reference to the following Examples and Comparative Examples.

Examples of producing a spring steel according to each of Examples and Comparative Examples were conducted under commercial spring steel manufacturing conditions. The proportion of each ingredient, as in 1 summarized, has been changed to produce ingot steel. A wire rod formed of the ingot steel was made into a steel wire by isothermal heat treatment, wire drawing, hardening treatment and hardening in a solder bath in this order. Specifically, the wire rod was held at 940 to 960 ° C for 3 to 5 minutes and then cooled at 640 to 660 ° C, followed by cooling at 18 to 22 ° C for 0.5 to 1.5 minutes. Such an isothermal heat treatment was performed to help a subsequent cable pulling process to be easily performed. By this heat treatment, pearlite was produced in the wire rod.

The wire rod subjected to the isothermal heat treatment was processed into a wire rod having a desired diameter by some wire drawing steps. In the present disclosure, the cable drawing was carried out so that a wire rod has a diameter of 3.3 mm.

The drawn wire rod was reheated and held at 940 to 960 ° C for 3 to 5 minutes. Subsequently, rapid cooling to 45 to 55 ° C was carried out to be tempered for 0.5 to 1.5 minutes. Subsequently, the wire rod was heated to 440 to 460 ° C and held at the temperature for 2 to 4 minutes. Thereafter, the wire rod was subjected to curing in a solder bath for rapid cooling. By hardening and tempering, martensite was formed in the wire rod to ensure strength. In addition, tempered martensite was formed on a surface of the wire rod by hardening in a solder bath to ensure strength and toughness.

Next, test examples to confirm the properties of the spring steels according to the examples and comparative examples are examined.

The spring steels according to the Examples and Comparative Examples were subjected to tensile strength, yield strength, hardness, fatigue strength, moldability and fatigue life test tests, and one test was to control the inclusions. Results are in 2 summarized.

Here, the yield strength and the tensile strength using samples with a wire diameter of 3.3 mm were carried out by means of a 20-ton tester according to KS B 0802. The hardness was carried out at 300 gf loads by means of a micro Vickers hardness tester according to KS B 0811. The fatigue strength and the fatigue life of the samples became measured by a rotational bending fatigue test according to KS B ISO 1143. To measure the formability, were Valve springs with a diameter / cable diameter of 6.5 and a winding number of 8 made and it was determined that the normal state is present when no break occurred in the manufacture of 10,000 valve springs.

In order to test for inclusion control, each sample was rolled in parallel, followed by a cut along a central line and collected. The maximum sizes of B-based and C-based inclusions present in a surface to be tested were measured by a Max.t. method with respect to an area of 60 mm ² . Here, a microscope magnification was 400 to 500 times and it was found to be normal when the inclusions having a particle size larger than 15 μm were not present, the proportion of inclusions having a particle size of 10 to 15 μm was 10% or less, and the Proportion of inclusions with a particle size of 10 microns 90% or more. Here, the B-based inclusions formed in a group in a processing direction, and granular inclusions collected intermittently and lined up in rows. For example, the B-based inclusions may be aluminum-based inclusions (Al ₂ O ₃ ), and the C-based inclusions may be silicate-based inclusions (SiO ₂ ) that have not viscously deformed and are irregularly distributed.

As in 2 As known steels do not include Mo, Ni, V, Nb, and Ti, they are formability and inclusion controlled, but do not meet the stated requirements of the present disclosure in terms of tensile strength, yield strength, hardness, fatigue strength, and fatigue life ,

Comparative Examples 1 to 12 do not satisfy the alloy content compositions according to the present disclosure. Comparative Examples 1 to 12 partially represent improved tensile strength, tensile strength, hardness, fatigue strength, moldability and fatigue life as compared to However, they do not meet the regulated requirements of the present disclosure.

In particular, since Comparative Example 1 comprises a small amount of Mo, the yield strength is not sufficiently ensured. Consequently, the tensile strength and yield strength are slightly increased and hardness, fatigue strength, formability and fatigue life are reduced as compared with the known steel.

The contents of Mo, Ni, V, and Ti in each of Comparative Examples 2, 3, 6, 9, and 10 do not meet the specified requirements, thereby failing to meet the inclusions regulations. It was confirmed that the inclusions became coarse or not uniform in the ingot steel, thereby affecting the formation of the inclusions during the steelmaking process and thus did not satisfy the inclusions regulations.

In addition, it can be confirmed that in Comparative Example 9, in which the content of Ti is less than a specified requirement, the generation of other inclusions such as Al ₂ O _{3 is} promoted, and hence the fatigue strength is reduced, thereby providing a comparison with the known Steel fatigue resistance and fatigue life are similar or slightly reduced.

On the other hand, all Examples 1 to 3 meeting all the specified requirements of the present disclosure achieved a tensile strength of about 2300 MPa or more, a yield strength of about 2800 MPa or more, and a hardness of about 710 HV or more. In addition, a fatigue strength of about 1100 MPa or more is achieved, and the formability and inclusions requirements are satisfied. Furthermore, a fatigue life of 400,000 or more is achieved.

Incidentally, shows 3 Graphs showing calculation results for phase transformations according to temperatures of the ultra-high-strength spring steel according to an embodiment of the present publication, and 4 FIG. 12 is a graph showing calculation results for phase transformations according to temperatures in cementite fabric of ultra-high-strength spring steel according to an embodiment of the present disclosure. FIG.

3 Fig. 12 is a graph showing calculation results for phase changes according to temperatures of an example with an alloy composition such as Fe-1.4Si-0.7Mn-0.7Cr-0.55C-0.3Ni-0.1Mo-0.1V. 3 shows that various carbide types such as CrC and VC are produced except for FCC-A1 (austenite), BCC-A2 (ferrite), cementite, etc., when an alloy composition according to the present disclosure is satisfied, and hence the increase in strength and the Fatigue life improvement can be expected.

Additionally shows 4 10 is a graph showing calculation results for phase transformations according to temperatures in the cementite fabric of spring steel according to an example with an alloy composition such as Fe-1.4Si-0.7Mn-0.7Cr-0.55C-0.3Ni-0.1Mo-0.1V. 4 shows that it can be predicted that the compositional properties of the seventh to eighth groups occur in the cementite, and thus it can be expected that minute carbides are evenly distributed.

As apparent from the above description, the present invention provides an ultra high strength spring steel having a high tensile strength of 2300 MPa or more and an improved fatigue life of 1100 MPa by refining inclusions by optimizing the ingredients of the main alloy composition according to an embodiment of the present invention ,

Although the preferred embodiments of the present invention have been disclosed for purposes of illustration, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope of the disclosure as disclosed in the appended claims.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• measured by a rotational bending fatigue test according to KS B ISO 1143. To measure formability, [0042]

Claims (7)

Ultra-high strength spring steel for use as a valve spring steel in a vehicle engine, comprising: 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight % of Nb, 0.05 to 0.3% by weight of Ti, 0.3% by weight or less of CU (but more than 0%), 0.0001 to 0.3% by weight of Al, O, 03% by weight or less of N (but more than 0%), 0.0001 to 0.003% by weight of O and a balance of Fe or other unavoidable impurities based on 100% by weight of the ultra high-strength spring steel. The ultra-high-strength spring steel according to claim 1, wherein the spring steel has a tensile strength of 2300 MPa or more. The ultra-high-strength spring steel according to claim 1 or 2, wherein the spring steel has a fatigue strength of 1100 MPa or more. An ultra-high-strength spring steel according to any one of the preceding claims, wherein the spring steel has a yield strength of 2800 MPa or more. An ultra-high strength spring steel according to any one of the preceding claims, wherein the spring steel has a hardness of 710 HV or more. An ultra-high strength spring steel according to any one of the preceding claims, wherein the sizes of the inclusions present in the spring steel are 15 μm or less. The ultra-high-strength spring steel according to any one of the preceding claims, wherein in the inclusions, a content of the inclusions having sizes of 10 to 15 μm is 10% or less and a content of the inclusions having sizes of less than 10 μm is 90% or more.

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