Yttrium-based heavy rare earth spring steel and preparation method thereof

Abstract

The invention discloses yttrium-based heavy rare earth spring steel and a preparation method thereof, and belongs to the technical field of spring steel materials, components of existing 51CrV4 spring steel are improved, and the yttrium-based heavy rare earth spring steel prepared by adding yttrium-based composite rare earth alloys Y, Ce and La has finer and dispersively distributed inclusions and better fatigue performance. In the rolling process of the heavy rare earth alloy and other components in the 51CrV4, fine carbides, nitrides or carbonitrides formed by Cr, V, yttrium-based heavy rare earth Y, Ce, La and the like are separated out, austenite recrystallization can be inhibited, and therefore austenite grains are prevented from growing continuously and become fine, and the structure morphology is fine and dense.

Description

Technical Field

[0001] The invention relates to the technical field of spring steel materials, and in particular to a yttrium-based heavy rare earth spring steel and a preparation method thereof. Background Art

[0002] As the country revitalizes the high-end equipment manufacturing industry, Chinese companies have higher requirements for the manufacturing of key components and basic parts needed for high-end equipment. Springs are key basic parts for the equipment manufacturing industry. They are large in quantity, wide in scope, and diverse in variety. They are used to control the movement of parts, mitigate impact or vibration, store energy, measure the size of force, etc. They are widely used in various fields of the national economy such as automobiles, railways, engineering machinery, and electronic appliances. At present, my country's high-performance spring manufacturing guarantee capacity is insufficient, and high-end springs for industries such as high-end automobiles and heavy-duty railways still need to be imported. With the lightweighting of automobiles, the large-scale development of major technical equipment, and the extreme parameters, the demand for high-end springs is growing rapidly. At the same time, the increasing demand for high-performance spring manufacturing has also put forward higher and higher requirements on the variety and performance of spring steel.

[0003] 51CrV4 spring steel has good mechanical properties and process properties, and has high hardenability. However, the existing 51CrV4 spring steel contains inclusions such as oxygen and sulfur, and the particle size of the inclusions is large, resulting in poor fatigue performance of 51CrV4 spring steel. Adding a small amount of rare earth metals to spring steel can play a role in deoxidation and desulfurization, and has a small amount of high efficiency. In addition, a large number of high-melting-point, dispersed, fine, spherical oxides / oxysulfides / sulfides can be generated. At the same time, the rare earth elements in the molten steel can also change the chemical properties of aluminum oxide inclusions and generate dispersed aluminate rare earth inclusions, thereby improving the performance of the steel. From a thermodynamic perspective, the solubility product of rare earth inclusions in steel decreases as the temperature decreases, indicating that new fine inclusions will continue to be generated in the steel as the temperature decreases during the solidification and cooling process after adding rare earths, which can meet the requirements of oxide metallurgy for the dispersed distribution of fine inclusions.

[0004] Spring steel liquid treated with yttrium element is easier to reduce the oxygen and sulfur content in the steel and purify the steel liquid. At the same time, since the atomic radius of yttrium is smaller than that of light rare earth, it can more effectively reduce the inclusion content, improve the inclusion distribution, reduce the inclusion size, refine and spheroidize the inclusions, and thus improve the steel performance. However, since the price of heavy rare earth elements such as yttrium is higher than that of light rare earth, the cost of industrial production of rare earth yttrium steel is increased, and the market competitiveness of yttrium steel is reduced, so the use of rare earth yttrium in industrial production is limited. For this reason, the present invention provides a yttrium-based heavy rare earth spring steel and a preparation method thereof. Summary of the invention

[0005] The invention provides a yttrium-based heavy rare earth spring steel and a preparation method thereof, which effectively solves the technical problems of large inclusion particle size and poor fatigue performance of the existing 51CrV4 spring steel, and also provides a yttrium-based heavy rare earth composite spring steel which adopts a yttrium-based heavy rare earth alloy to purify spring steel liquid, strengthen inclusion denaturation, and refine steel structure.

[0006] The first object of the present invention is to provide a yttrium-based heavy rare earth spring steel, which is composed of the following raw materials in percentage by mass: 0.46% to 0.54% C, 0.17% to 0.39% Si, 0.50% to 0.90% Mn, 0.80% to 1.10% Cr, 0.10% to 0.20% V, P≤0.025%, O≤0.025%, S≤0.025%, 0.005% to 0.010% Y, 0.003% to 0.006% Ce, 0.002% to 0.005% La, and the remainder is Fe, totaling 100%.

[0007] As a preferred embodiment, the yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: 0.49% to 0.52% C, 0.25% to 0.37% Si, 0.60% to 0.85% Mn, 0.90% to 1.05% Cr, 0.13% to 0.17% V, P≤0.015%, O≤0.015%, S≤0.015%, Y+Ce+La≤0.015%, and the remainder is Fe, totaling 100%.

[0008] As a preferred embodiment, Y, Ce and La in the yttrium-based heavy rare earth spring steel exist in the form of rare earth composite inclusions.

[0009] As a preferred embodiment, the rare earth composite inclusions are composite inclusions formed by combining Y, Ce and La with O and S respectively.

[0010] As a preferred implementation, the rare earth composite inclusions are dispersedly distributed, and the particle size of the rare earth composite inclusions is 1 μm to 10 μm.

[0011] The second object of the present invention is to provide a method for preparing yttrium-based heavy rare earth spring steel, comprising the following steps:

[0012] Weigh the raw materials according to mass percentage: rare earth raw materials: 0.005% to 0.010% Y, 0.003% to 0.006% Ce and 0.002% to 0.005% La; alloy raw materials: 0.46% to 0.54% C, 0.17% to 0.39% Si, 0.50% to 0.90% Mn, 0.80% to 1.10% Cr, 0.10% to 0.20% V, P≤0.025%, O≤0.025%, S≤0.025%, and the balance Fe;

[0013] The alloy raw materials are melted and then cast to obtain a cast alloy; the melting includes converter melting, refining outside the furnace and vacuum degassing; rare earth raw materials are added during the refining outside the furnace;

[0014] continuously hot-rolling the cast alloy to obtain a rolled alloy;

[0015] The rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0016] As a preferred implementation manner, the off-furnace refining includes LF furnace refining and VD furnace vacuum refining, and rare earth raw materials are added during the VD furnace vacuum refining process.

[0017] As a preferred embodiment, the continuous hot rolling includes rough rolling and finish rolling; the starting temperature of the rough rolling is 1000℃~1050℃, and the total deformation of the rough rolling is 60%~66%; the starting temperature of the finish rolling is 850℃~950℃, and the total deformation of the finish rolling is 20%~22%.

[0018] As a preferred embodiment, the heat treatment includes quenching and medium-temperature tempering; the quenching temperature is 830°C to 880°C, the medium-temperature tempering temperature is 350°C to 500°C, the medium-temperature tempering time is 40min to 60min, and the medium-temperature tempering cooling method is air cooling.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention provides a yttrium-based heavy rare earth spring steel and a preparation method thereof. The composition of the existing 51CrV4 spring steel is improved, and the yttrium-based heavy rare earth spring steel prepared by adding yttrium-based composite rare earth alloys Y, Ce and La has more finely dispersed inclusions and better fatigue performance. The reaction products of Y, La and Ce elements in molten steel are all rare earth oxysulfides. Yttrium is mainly YOS composite inclusions with a density of about 4.25, while La and Ce are mainly (Ce, La)-OS composite inclusions with a density of about 6.0. According to the Stock formula, the floating terminal velocity of YOS particles in steel is twice that of (Ce, La)-OS particles. During the rolling process of heavy rare earth alloy and other components in 51CrV4, fine carbides, nitrides or carbonitrides formed by Cr, V, yttrium-based heavy rare earths Y, Ce and La are precipitated, which can inhibit the recrystallization of austenite, thereby hindering the continuous growth of austenite grains, making the grains smaller and the organizational morphology fine and dense. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1The surface inclusions diagram of the yttrium-based heavy rare earth spring steel prepared in Example 5 of the present invention and its corresponding energy spectrum diagram; wherein, Figure A is a morphology diagram of the surface inclusions of the yttrium-based heavy rare earth spring steel; Figure B is a corresponding energy spectrum diagram;

[0022] Figure 2 The surface inclusions diagram of the yttrium-based heavy rare earth spring steel prepared in Example 1 of the present invention and its corresponding energy spectrum diagram; wherein, Figure A is a morphology diagram of the surface inclusions of the yttrium-based heavy rare earth spring steel; Figure B is a corresponding energy spectrum diagram;

[0023] Figure 3 The surface inclusions diagram of the yttrium-based heavy rare earth spring steel prepared in Example 2 of the present invention and its corresponding energy spectrum diagram; wherein, Figure A is a morphology diagram of the surface inclusions of the yttrium-based heavy rare earth spring steel; Figure B is a corresponding energy spectrum diagram;

[0024] Figure 4 The typical morphology of inclusions in 51CrV4 spring steel of comparative example 1 of the present invention and its corresponding energy spectrum diagram; wherein, Figure A is a morphology diagram of inclusions on the surface of yttrium-based heavy rare earth spring steel; Figure B is a corresponding energy spectrum diagram;

[0025] Figure 5 The typical morphology of inclusions in 51CrV4 spring steel of comparative example 1 of the present invention; wherein, Figure A is 80 μm to 140 μm, Figure B is 140 μm to 300 μm, and Figure C is >300 μm;

[0026] Figure 6 The following are typical morphology diagrams of inclusions in the yttrium-based heavy rare earth spring steel of Example 5 of the present invention; wherein, Figure A is <80 μm, Figure B is 80 μm to 140 μm, Figure C is 140 μm to 300 μm, and Figure D is >300 μm;

[0027] Figure 7 This is a scanning electron microscope image of the yttrium-based heavy rare earth spring steel of Example 5 of the present invention;

[0028] Figure 8 This is a scanning electron microscope image of the 51CrV4 spring steel of Comparative Example 1 of the present invention;

[0029] Fig. 9 The graph is a change in tensile strength of the yttrium-based heavy rare earth spring steel and 51CrV4 spring steel according to the present invention with increasing tempering temperature;

[0030] Fig.10 The graph is a change diagram of the yield strength of the yttrium-based heavy rare earth spring steel and 51CrV4 spring steel of the present invention with the increase of tempering temperature;

[0031] Fig.11 The figure is a graph showing the variation of the cross-sectional expansion ratio of the yttrium-based heavy rare earth spring steel and the 51CrV4 spring steel according to the present invention with the increase of the tempering temperature;

[0032] Fig.12 It is a graph showing the change of total elongation of the yttrium-based heavy rare earth spring steel and 51CrV4 spring steel according to the present invention with the increase of tempering temperature;

[0033] Fig.13 This is a comparison chart of the impact toughness of the yttrium-based heavy rare earth spring steel of Example 5 of the present invention and the 51CrV4 spring steel of Comparative Example 1;

[0034] Fig.14 The scanning electron microscope images of the impact fracture of the yttrium-based heavy rare earth spring steel of Example 5 of the present invention and the 51CrV4 spring steel of Comparative Example 1; wherein, Figure A is Example 5, and Figure B is Comparative Example 1;

[0035] Fig.15 It is a comparison diagram of the hardenability of the yttrium-based heavy rare earth spring steel of Example 5 of the present invention and the 51CrV4 spring steel of Comparative Example 1; wherein, Figure A is Example 5, and Figure B is Comparative Example 1;

[0036] Fig.16 It is a comparison diagram of the hardenability of the yttrium-based heavy rare earth spring steel of Example 1 of the present invention and the 51CrV4 spring steel of Comparative Example 1; wherein, Figure A is Example 1, and Figure B is Comparative Example 1;

[0037] Fig.17 900-1050° C. for the yttrium-based heavy rare earth spring steel of Example 5 of the present invention;

[0038] Fig.18 900-1050° C. for the 51CrV4 spring steel of Comparative Example 1 of the present invention;

[0039] Fig.19 This is a fatigue performance diagram of the yttrium-based heavy rare earth spring steel of Example 5 of the present invention;

[0040] Fig. 20 This is a fatigue performance diagram of the 51CrV4 spring steel of Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0041] In order to enable those skilled in the art to better understand the technical solution of the present invention and implement it, the present invention is further described below in conjunction with specific examples, but the examples are not intended to limit the present invention. The following test methods and detection methods, unless otherwise specified, are conventional methods; the reagents and raw materials, unless otherwise specified, are commercially available.

[0042] The 51CrV4 spring steel mentioned in the background technology of the present invention contains inclusions, such as oxygen, sulfur, etc. The inclusions have a large particle size, resulting in poor fatigue performance of the 51CrV4 spring steel. Although the addition of yttrium elements to the spring steel can purify the molten steel, thereby avoiding the influence of inclusions in the spring steel on the performance of the spring steel, there is a technical problem that the price of yttrium-based heavy rare earth elements is higher than that of light rare earths, thereby increasing the cost of industrial production of rare earth yttrium steel and reducing the market competitiveness of yttrium-containing steel. Based on the above-mentioned practical problems of 51CrV4 spring steel, the present invention provides a yttrium-based heavy rare earth spring steel and a preparation method thereof, and the technical solution of the present invention is analyzed and explained in detail below.

[0043] The invention firstly provides a yttrium-based heavy rare earth spring steel, namely RE51CrV4, which is composed of the following raw materials in percentage by mass: 0.46% to 0.54% C, 0.17% to 0.39% Si, 0.50% to 0.90% Mn, 0.80% to 1.10% Cr, 0.10% to 0.20% V, P≤0.025%, O≤0.025%, S≤0.025%, 0.005% to 0.010% Y, 0.003% to 0.006% Ce, 0.002% to 0.005% La, and the remainder is Fe, which is 100% in total. Yttrium-based composite rare earth alloy Y, Ce and La are added to the components of 51CrV4 spring steel. The structure of the obtained yttrium-based heavy rare earth spring steel includes a matrix and rare earth composite inclusions distributed on the matrix. The matrix is ​​tempered troostite and a small amount of residual austenite, and has more finely dispersed inclusions. The formed fine carbides, nitrides or carbonitrides precipitate, which can inhibit the recrystallization of austenite, thereby hindering the continuous growth of austenite grains, making the grains fine and the organizational morphology fine and orderly, thereby effectively improving the various properties of the spring steel.

[0044] Furthermore, the yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: 0.49% to 0.52% C, 0.25% to 0.37% Si, 0.60% to 0.85% Mn, 0.90% to 1.05% Cr, 0.13% to 0.17% V, P≤0.015%, O≤0.015%, S≤0.015%, Y+Ce+La≤0.015%, and the balance is Fe, which is 100% in total. The amount of each component of the yttrium-based heavy rare earth spring steel is limited to a smaller amount range, and the yttrium-based heavy rare earth spring steel obtained within this range has a more delicate microstructure.

[0045] In order to further improve various properties of yttrium-based heavy rare earth spring steel, the rare earth composite inclusions are dispersedly distributed, and the particle size of the rare earth composite inclusions is 1 μm to 10 μm.

[0046] The present invention also provides a method for preparing yttrium-based heavy rare earth spring steel, comprising the following steps:

[0047] Weigh the raw materials according to mass percentage: rare earth raw materials: 0.005% to 0.010% Y, 0.003% to 0.006% Ce and 0.002% to 0.005% La; alloy raw materials: 0.46% to 0.54% C, 0.17% to 0.39% Si, 0.50% to 0.90% Mn, 0.80% to 1.10% Cr, 0.10% to 0.20% V, P≤0.025%, O≤0.025%, S≤0.025%, and the balance Fe;

[0048] The alloy raw materials are melted and then cast to obtain a cast alloy; the melting includes converter melting, refining outside the furnace and vacuum degassing; rare earth raw materials are added during the refining outside the furnace;

[0049] continuously hot-rolling the cast alloy to obtain a rolled alloy;

[0050] The rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0051] In order to make the yttrium-based heavy rare earth spring steel have more finely dispersed inclusions, the out-of-furnace refining includes LF furnace refining and VD furnace vacuum refining, and rare earth raw materials are added during the VD furnace vacuum refining process. The continuous hot rolling includes rough rolling and finishing rolling; the starting rolling temperature of the rough rolling is 1000℃~1050℃, and the total deformation of the rough rolling is 60%~66%; the starting rolling temperature of the finishing rolling is 850℃~950℃, and the total deformation of the finishing rolling is 20%~22%. The heat treatment includes quenching and medium-temperature tempering; the quenching temperature is 830℃~880℃, the medium-temperature tempering temperature is 350℃~500℃, the medium-temperature tempering time is 40min~60min, and the cooling method of the medium-temperature tempering is air cooling.

[0052] The technical effects of the present invention are described below in conjunction with specific embodiments and comparative examples.

[0053] Example 1

[0054] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.005% Y, 0.003% Ce, 0.002% La, alloy raw materials: 0.46% C, 0.17% Si, 0.50% Mn, 0.80% Cr, 0.10% V, 0.025% P, 0.015% O, 0.025% S, and the balance is Fe, which is 100% in total.

[0055] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0056] Weigh each raw material according to the above mass percentage;

[0057] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0058] The invention adopts converter smelting and external refining: the converter smelting temperature is 1600°C and the time is 40min; the LF furnace refining temperature is 1700°C and the time is 9min; the vacuum degassing time is 15min; the rare earth raw materials are added after the VD furnace refining breaks the vacuum; the external refining is subjected to soft argon blowing treatment, and then continuous casting is performed, the ingot is heated to 1150°C with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1000°C, the deformation is 66%, the finishing rolling temperature is 850°C, the deformation is 22%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875°C, and air cooling is performed after rolling.

[0059] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 830°C, the heat preservation time was 40 min, the tempering temperature was 350°C, the medium-temperature tempering time was 40 min, and the heat preservation time was 1 hour before air cooling.

[0060] Example 2

[0061] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.01% Y, 0.006% Ce, 0.005% La, alloy raw materials: 0.54% C, 0.39% Si, 0.90% Mn, 1.1% Cr, 0.20% V, 0.015% P, 0.025% O, 0.01% S, and the balance is Fe, which is 100% in total.

[0062] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0063] Weigh each raw material according to the above mass percentage;

[0064] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0065] The invention adopts converter smelting and external refining: the converter smelting temperature is 1600°C and the time is 40min; the LF furnace external refining temperature is 1700°C and the time is 9min; the vacuum degassing time is 15min; the rare earth raw materials are added after the VD furnace refining breaks the vacuum; the external refining is subjected to soft argon blowing treatment, and then continuous casting is performed, the ingot is heated to 1150°C with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1050°C, the deformation is 62%, the finishing rolling temperature is 950°C, the deformation is 24%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875°C, and air cooling is performed after rolling.

[0066] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 880°C, the heat preservation time was 40 min, the tempering temperature was 500°C, the medium-temperature tempering time was 60 min, and the heat preservation time was 1 hour before air cooling.

[0067] Example 3

[0068] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.008% Y, 0.004% Ce, 0.003% La, alloy raw materials: 0.49% C, 0.25% Si, 0.60% Mn, 0.90% Cr, 0.13% V, 0.02% P, 0.01% O, 0.015% S, and the balance is Fe, which is 100% in total.

[0069] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0070] Weigh each raw material according to the above mass percentage;

[0071] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0072] The invention adopts converter smelting and external refining: the converter smelting temperature is 1600°C and the time is 40min; the LF furnace external refining temperature is 1700°C and the time is 9min; the vacuum degassing time is 15min; the rare earth raw materials are added after the VD furnace refining breaks the vacuum; the external refining is subjected to soft argon blowing treatment, and then continuous casting is performed, the ingot is heated to 1150°C with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1020°C, the deformation is 64%, the finishing rolling temperature is 900°C, the deformation is 24%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875°C, and air cooling is performed after rolling.

[0073] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 860°C, the heat preservation time was 40 min, the tempering temperature was 400°C, the medium-temperature tempering time was 50 min, and the heat preservation time was 1 hour before air cooling.

[0074] Example 4

[0075] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.006% Y, 0.005% Ce, 0.004% La, alloy raw materials: 0.52% C, 0.37% Si, 0.85% Mn, 1.05% Cr, 0.17% V, 0.01% P, 0.02% O, 0.02% S, and the balance is Fe, which is 100% in total.

[0076] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0077] Weigh each raw material according to the above mass percentage;

[0078] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0079] The invention adopts converter smelting and external refining: the converter smelting temperature is 1600°C and the time is 40min; the LF furnace external refining temperature is 1700°C and the time is 9min; the vacuum degassing time is 15min; the rare earth raw materials are added after the VD furnace refining breaks the vacuum; the external refining is subjected to soft argon blowing treatment, and then continuous casting is performed, the ingot is heated to 1150°C with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1000°C, the deformation is 66%, the finishing rolling temperature is 920°C, the deformation is 26%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875°C, and air cooling is performed after rolling.

[0080] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 840°C, the heat preservation time was 40 min, the tempering temperature was 450°C, the medium-temperature tempering time was 45 min, and the heat preservation time was 1 hour before air cooling.

[0081] Example 5

[0082] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.005% Y, 0.003% Ce, 0.002% La, alloy raw materials: 0.46% C, 0.17% Si, 0.50% Mn, 0.80% Cr, 0.10% V, 0.025% P, 0.015% O, 0.025% S, and the balance is Fe, which is 100% in total.

[0083] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0084] Weigh each raw material according to the above mass percentage;

[0085] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0086] The invention adopts converter smelting and external refining: the converter smelting temperature is 1600°C and the time is 40min; the LF furnace external refining temperature is 1700°C and the time is 9min; the vacuum degassing time is 15min; the rare earth raw materials are added after the VD furnace refining breaks the vacuum; the external refining is subjected to soft argon blowing treatment, and then continuous casting is performed, the ingot is heated to 1150°C with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1050°C, the deformation is 62%, the finishing rolling temperature is 890°C, the deformation is 22%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875°C, and air cooling is performed after rolling.

[0087] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 870°C, the heat preservation time was 40 min, the tempering temperature was 420°C, the medium-temperature tempering time was 55 min, and the heat preservation time was 1 hour before air cooling.

[0088] Example 6

[0089] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.009% Y, 0.0055% Ce, 0.0045% La, alloy raw materials: 0.48% C, 0.20% Si, 0.80% Mn, 0.85% Cr, 0.12% V, 0.012% P, 0.008% O, 0.008% S, and the balance is Fe, which is 100% in total.

[0090] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0091] Weigh each raw material according to the above mass percentage;

[0092] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0093] The invention adopts converter smelting and external refining: the converter smelting temperature is 1600°C and the time is 40min; the LF furnace external refining temperature is 1700°C and the time is 9min; the vacuum degassing time is 15min; the rare earth raw material is added after the VD furnace refining breaks the vacuum; the external refining is subjected to soft argon blowing treatment, and then continuous casting is performed, the ingot is heated to 1150°C with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1000°C, the deformation is 60%, the finishing rolling temperature is 900°C, the deformation is 23%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875°C, and air cooling is performed after rolling.

[0094] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 830°C, the heat preservation time was 40 min, the tempering temperature was 480°C, the medium-temperature tempering time was 48 min, and the plate was air-cooled after heat preservation for 1 hour.

[0095] Example 7

[0096] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.006% Y, 0.0045% Ce, 0.0025% La, alloy raw materials: 0.51% C, 0.38% Si, 0.65% Mn, 0.95% Cr, 0.16% V, 0.024% P, 0.006% O, 0.014% S, and the balance is Fe, which is 100% in total.

[0097] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0098] Weigh each raw material according to the above mass percentage;

[0099] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0100] The invention adopts converter smelting and external refining: the converter smelting temperature is 1600°C and the time is 40min; the LF furnace external refining temperature is 1700°C and the time is 9min; the vacuum degassing time is 15min; the rare earth raw materials are added after the VD furnace refining breaks the vacuum; the external refining is subjected to soft argon blowing treatment, and then continuous casting is performed, the ingot is heated to 1150°C with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1050°C, the deformation is 63%, the finishing rolling temperature is 860°C, the deformation is 25%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875°C, and air cooling is performed after rolling.

[0101] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 840°C, the heat preservation time was 40 min, the tempering temperature was 380°C, the medium-temperature tempering time was 58 min, and the heat preservation time was 1 hour before air cooling.

[0102] Example 8

[0103] A yttrium-based heavy rare earth spring steel is composed of the following raw materials in percentage by mass: rare earth raw materials: 0.008% Y, 0.006% Ce, 0.003% La, alloy raw materials: 0.53% C, 0.36% Si, 0.75% Mn, 1.0% Cr, 0.18% V, 0.018% P, 0.012% O, 0.022% S, and the balance is Fe, which is 100% in total.

[0104] The preparation method of the above yttrium-based heavy rare earth spring steel comprises the following steps:

[0105] Weigh each raw material according to the above mass percentage;

[0106] The alloy raw material is melted and then cast to obtain a cast alloy; the smelting includes 80t converter melting, refining outside the furnace, including LF furnace refining and VD furnace vacuum refining, and vacuum degassing; rare earth raw materials are added during the VD furnace vacuum refining process; the cast alloy is continuously hot rolled to obtain a rolled alloy; the rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

[0107] The converter smelting and refining outside the furnace are adopted: the converter smelting temperature is 1600℃, the time is 40min; the LF furnace refining temperature is 1700℃, the time is 9min; the vacuum degassing time is 15min; the rare earth raw materials are added after the VD furnace refining breaks the vacuum; the refining outside the furnace is subjected to soft argon blowing. Then continuous casting is carried out, the ingot is heated to 1150℃ with the furnace and kept warm for 1.5h, and then hot rolled on a short stress hot rolling mill, the rough rolling start temperature is 1010℃, the deformation is 61%, the finishing rolling temperature is 850℃, the deformation is 22%, a total of 19 rolling passes, the final product thickness is 25mm, the total reduction rate is 86%, the final rolling temperature is 875℃, and air cooling is carried out after rolling.

[0108] The hot-rolled plate was subjected to oil quenching and medium-temperature tempering treatment: the oil quenching temperature was 860°C, the heat preservation time was 40 min, the tempering temperature was 500°C, the medium-temperature tempering time was 40 min, and the heat preservation time was 1 hour before air cooling.

[0109] In order to further illustrate the technical effect of the present invention, the present invention also provides a comparative example, which is as follows:

[0110] Comparative Example 1

[0111] The conventional existing 51CrV4 spring steel is used.

[0112] The properties of the yttrium-based heavy rare earth spring steels prepared in Examples 1 to 8 and the 51CrV4 spring steel in Comparative Example 1 were tested, and the results are as follows:

[0113] Depend on Figure 1 to Figure 4 It can be seen that after adding rare earth, micron-sized rare earth composite inclusions can be formed, which can effectively purify the molten steel.

[0114] Depend on Figure 5-6 From the morphology of typical inclusions, it can be seen that after adding yttrium-based heavy rare earth, the refinement effect on inclusions is very obvious. The number of inclusions below 140μm electrolyzed increases significantly, the morphology is more regular, and most of the inclusions are rice-grained or spherical, which is more crystal-like and no longer has black inclusions compared to inclusions without rare earth and with a particle size of less than 140μm. For inclusions larger than 140μm, due to the addition of yttrium-based heavy rare earth, larger large inclusions are inherently less. According to the comparison of existing inclusions, there are fewer rhombus-shaped inclusions larger than 140μm in rare earth 51CrV4 spring steel, and more spherical inclusions.

[0115] Depend on Figure 7-Figure 8 It can be seen from the SEM image that rare earth addition can inhibit the formation of lath martensite and promote carbide spheroidization, which is very helpful for improving the toughness and fatigue properties of high-strength spring steel.

[0116] Depend on Figures 9 to 12 By comparing the strength and plasticity, it was found that the addition of rare earths has basically no effect on the strength and plasticity indicators.

[0117] Depend on Figure 13-14 It can be seen that the addition of rare earth can significantly improve the longitudinal impact energy of 51CrV4, which is almost twice that of the material without rare earth. This is partly because rare earth forms fine composite inclusions, which can inhibit crack propagation in the dimples at the impact fracture.

[0118] Depend on Fig.15 and Fig.16It can be seen from the comparison of the hardenability of the yttrium-based heavy rare earth spring steel of Example 5 and the 51CrV4 spring steel of Comparative Example 1 that the addition of RE reduces the hardenability of the 51CrV4 steel.

[0119] Depend on Fig.17 and Fig.18 It can be seen that by comparing the decarburized layer, RE can inhibit the diffusion of carbon.

[0120] Depend on Fig.19 and Fig. 20 It can be seen that after adding mixed rare earth, the cycle life under different stresses is more dispersed, and it is almost difficult to analyze the fatigue limit value. For the sample without adding rare earth, the curve is clearer. With the decrease of stress level, the cycle period gradually decreases, and basically reaches stability at 750MPa, and the maximum can reach 107. In terms of the stress borne by 107 cycles, the yttrium-based heavy rare earth spring steel is 800MPa, and its fatigue resistance is better than that of the 51CrV4 spring steel without rare earth elements in comparative example 1.

[0121] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A yttrium-based heavy rare earth spring steel, characterized in that: The invention is composed of the following raw materials in percentage by mass: 0.46% to 0.54% C, 0.17% to 0.39% Si, 0.50% to 0.90% Mn, 0.80% to 1.10% Cr, 0.10% to 0.20% V, P≤0.025%, O≤0.025%, S≤0.025%, 0.005% to 0.010% Y, 0.003% to 0.006% Ce, 0.002% to 0.005% La, and the balance is Fe, which is 100% in total.

2. The yttrium-based heavy rare earth spring steel according to claim 1, characterized in that: The invention is composed of the following raw materials in percentage by mass: 0.49% to 0.52% C, 0.25% to 0.37% Si, 0.60% to 0.85% Mn, 0.90% to 1.05% Cr, 0.13% to 0.17% V, P≤0.015%, O≤0.015%, S≤0.015%, Y+Ce+La≤0.015%, and the balance is Fe, which is 100% in total.

3. The yttrium-based heavy rare earth spring steel according to claim 1 or 2, characterized in that: In the yttrium-based heavy rare earth spring steel, Y, Ce and La exist in the form of rare earth composite inclusions.

4. The yttrium-based heavy rare earth spring steel according to claim 3, characterized in that: The rare earth composite inclusions are composite inclusions formed by combining Y, Ce and La with O and S respectively.

5. The yttrium-based heavy rare earth spring steel according to claim 4, characterized in that: The rare earth composite inclusions are dispersedly distributed, and the particle size of the rare earth composite inclusions is 1 μm to 10 μm.

6. A method for preparing the yttrium-based heavy rare earth spring steel according to claim 1, characterized in that: The following steps are involved: Weigh the raw materials according to mass percentage: rare earth raw materials: 0.005% to 0.010% Y, 0.003% to 0.006% Ce and 0.002% to 0.005% La; alloy Raw materials: 0.46%-0.54% C, 0.17%-0.39% Si, 0.50%-0.90% Mn, 0.80%-1.10% Cr, 0.10%-0.20% V, P≤0.025%, O≤0.025%, S≤0.025%, balance Fe; The alloy raw materials are melted and then cast to obtain a cast alloy; the melting includes converter melting, refining outside the furnace and vacuum degassing; rare earth raw materials are added during the refining outside the furnace; continuously hot-rolling the cast alloy to obtain a rolled alloy; The rolled alloy is heat treated to obtain yttrium-based heavy rare earth spring steel.

7. The preparation method according to claim 6, characterized in that: The out-of-furnace refining includes LF furnace refining and VD furnace vacuum refining, and rare earth raw materials are added during the VD furnace vacuum refining process.

8. The preparation method according to claim 6, characterized in that: During the vacuum degassing, the argon flow rate is 300 L / min, the initial vacuum pressure is 101 MPa, and the vacuum target pressure is ≤67 Pa.

9. The preparation method according to claim 6, characterized in that: The continuous hot rolling includes rough rolling and finish rolling; the starting temperature of the rough rolling is 1000°C to 1050°C, and the total deformation of the rough rolling is 60% to 66%; the starting temperature of the finish rolling is 850°C to 950°C, and the total deformation of the finish rolling is 20% to 22%.

10. The preparation method according to claim 6, characterized in that: The heat treatment includes quenching and medium-temperature tempering; the quenching temperature is 830°C to 880°C, the medium-temperature tempering temperature is 350°C to 500°C, the medium-temperature tempering time is 40min to 60min, and the medium-temperature tempering cooling method is air cooling.

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