CN111334708A

CN111334708A - High-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance and production method thereof

Info

Publication number: CN111334708A
Application number: CN202010147753.0A
Authority: CN
Inventors: 姜婷; 汪开忠; 于同仁; 孙凯; 牟祖茂; 何峰; 郝震宇; 葛晴晴; 丁新军; 许文喜
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2020-03-05
Filing date: 2020-03-05
Publication date: 2020-06-26
Anticipated expiration: 2040-03-05
Also published as: CN111334708B

Abstract

The invention discloses high-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance, which comprises the following chemical components in percentage by weight: 0.70 to 0.80 percent of C, 1.60 to 2.00 percent of Si, 0.40 to 0.60 percent of Mn, 0.80 to 1.00 percent of Cr, 0.10 to 0.20 percent of V, 0.03 to 0.05 percent of Nb, 0.0015 to 0.0035 percent of Mg, 0.005 to 0.008 percent of N, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.0015 percent of O, and the balance of Fe and other inevitable impurities, namely 30.5C +9.6Si +6.1Mn +11.5Cr +34.5V +19.8Nb +78.5, more than or equal to 135 percent; the high-strength spring steel with the tensile strength of more than or equal to 2250MPa is produced by the interaction of C, Si, Mn, Cr, V, Nb, Mg and N in a specific process at low cost, and has better toughness, fatigue strength and excellent anti-springing performance.

Description

High-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance and production method thereof

Technical Field

The invention belongs to the technical field of spring steel, and particularly relates to high-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance and a production method thereof.

Background

The light weight technology is a future development trend of automobiles, springs are one of safety parts of automobiles, and in order to reduce the self weight, the design stress of the springs is required to be improved continuously. The development trend of the spring to high stress, light weight and long service life is irreversible in the future, but the performance of the material for manufacturing the high-stress spring is limited at present, the use stress of the spring is further improved, and the research and development of a new material with high obdurability is urgent.

In recent years, the development of high-strength spring steel has been a hot issue. Chinese patent CN 103725984 a discloses a high-toughness high-strength spring steel, which is characterized in that the chemical components (weight percentage) of the material are: 0.35 to 0.50 percent of C, 1.50 to 2.50 percent of Si, 0.35 to 1.00 percent of Mn, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S, 0.50 to 1.20 percent of Cr, 0.15 to 0.50 percent of Ni, 0.10 to 0.30 percent of Cu0.10, 0.04 to 0.10 percent of V, 0.03 to 0.10 percent of Ti, and the balance of Fe and other inevitable impurities. The grain size of the treated material is 8.0 grade or above, when the tensile strength is more than 1920MPa, the reduction of area Z is more than or equal to 40 percent, and the elongation after fracture is more than or equal to 10 percent. When the method is insufficient, the tensile strength is not obviously improved compared with the strength of national standard 55SiCrV and other marks.

Chinese patent CN 103667983A discloses a high-strength spring steel and a preparation method thereof, which is characterized in that the chemical components (weight percentage) of the material are as follows: 1.0 to 1.3 portions of C, 0.6 to 0.9 portion of Si, 0.4 to 0.5 portion of Cu0.0 to 14.5 portions of Ni14.0 to 14.5 portions of Ni, 1.7 to 2.1 portions of Mn, 0.03 to 0.05 portion of Ce0.006 to 0.008 portion of Ti, 0.02 to 0.05 portion of Zn0.004 to 0.005 portion of Sn0.0 to 1.3 portions of Cr, less than or equal to 0.030 portion of P, less than or equal to 0.030 portion of S, and the balance of Fe. The formed alloy material has excellent comprehensive mechanical properties, especially ultrahigh strength, hardness, toughness, plasticity and fatigue resistance, and also has corrosion resistance. However, the method adds a large amount of noble metal element Ni, so the cost is extremely high, and the method is not suitable for mass production and application.

Chinese patent CN 105648332a discloses a high-performance spring steel, which is characterized in that the chemical composition (weight percentage) of the material is: c: 1.9-2.2 parts of Cu: 5-6 parts of Si: 1-1.2 parts of Cr: 0.8-1.2 parts of Nb: 0.2-0.5 part, V: 0.2-0.3 part of Al: 0.5-0.8 part, and the balance of Fe and impurities which cannot be removed. The high-performance spring steel plate material is subjected to heating, forming, waste heat quenching, medium temperature tempering and final product shape forming. The invention has the beneficial effects that: the elasticity is good, is fit for normally working under the load environment, has higher yield strength to making the spring material to have longer life-span. However, the method adds 5% or more of copper element, which is very easy to cause copper brittle cracking of steel.

The tensile strength of the high-strength spring steel used at home and abroad is basically between 1800 and 2000MPa, the steel grade of 2000 and 2100 grade has been developed and tried on high-grade cars, but the steel grade of higher grade with ultrahigh obdurability has not been developed. And some spring enterprises improve the strength of the existing steel grade by reducing the tempering temperature, so that the plasticity and toughness are deteriorated, and the fatigue life is also reduced. New steel materials with higher strength and toughness for springs are urgently needed to be developed so as to improve the stress and the fatigue strength of the springs and meet the development requirements of the automobile industry.

Disclosure of Invention

The invention aims to provide high-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance and a production method thereof. The spring steel with high strength and high fatigue performance is produced by reasonably controlling the content range of each component in the spring steel and combining a proper production process.

The technical scheme adopted by the invention is as follows:

a high-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance comprises the following chemical components in percentage by weight: 0.70 to 0.80 percent of C, 1.60 to 2.00 percent of Si, 0.40 to 0.60 percent of Mn, 0.80 to 1.00 percent of Cr, 0.10 to 0.20 percent of V, 0.03 to 0.05 percent of Nb, 0.0015 to 0.0035 percent of Mg, 0.005 to 0.008 percent of N, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.0015 percent of O, and the balance of Fe and other inevitable impurities; in order to utilize the tempering softening resistance effect of the alloy element to the maximum extent, the A value is ensured to be more than or equal to 135%: a is 30.5C +9.6Si +6.1Mn +11.5Cr +34.5V +19.8Nb + 78.5.

The invention also provides a production method of the high-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance, and the production method comprises the following steps: electric furnace smelting, LF furnace, vacuum degassing, continuous casting, cogging, wire rolling and heat treatment.

Further, in the continuous casting step, a square billet of 250mm × 250mm is continuously cast.

In the cogging step, a large square billet of 250mm × 250mm is heated and rolled into a square billet of 150mm × 150mm, and then the billet is cooled in a heaped mode, a small square billet is rolled through the large square billet, the compression ratio of steel is improved, and therefore the internal quality of finished wire rods is improved.

The heating temperature is 1200-1250 ℃, the surface decarburization can be generated in the heating furnace when the temperature is higher than 1250 ℃, and the tapping temperature is insufficient when the temperature is lower than 1200 ℃, so that the rolling difficulty is caused. In addition, the total heating time is more than or equal to 230min, preferably 235-262 min.

The process of rolling the wire rod comprises the steps of peeling a 150mm × 150mm square billet → heating → high-speed wire rod controlled rolling → stelmor cooling by wire control → phi 6.5-16 mm wire rod finished products.

Controlling the stripping depth of the square billet to be more than 1.2mm, fully removing the surface defects of the small square billet, and improving the surface quality of the finished wire rod product;

controlling the heating temperature to be 1020-1060 ℃, wherein the temperature higher than the temperature range aggravates the surface decarburization of the blank, the temperature lower than the temperature range can not be fully austenitized and homogenized, and the temperature is preferably 1032-1060 ℃;

the finishing temperature is 790-830 ℃, the spinning temperature is 790-830 ℃, preferably 799-815 ℃, the temperature range higher than the temperature range can cause the structure of the subsequent cooling process to generate network carbides, the temperature range lower than the temperature range can cause the phase transition temperature of the subsequent cooling process to be too low, and the bainite abnormal structure is preferably 793-815 ℃;

the wire rod produced by the method has the structure of pearlite and a small amount of ferrite, and the austenite grain size is more than or equal to 10 grades.

The invention also provides a heat treatment method of the high-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance, and the spring steel obtained by adopting the components and the production method is quenched at 870-920 ℃, oil-cooled, tempered at 420-450 ℃ and air-cooled. The tensile strength after heat treatment is more than or equal to 2250MPa, the elongation after fracture is more than or equal to 10 percent, the reduction of area is more than or equal to 40 percent, the fatigue strength is more than or equal to 880MPa, and the area of the reverse-return curve of the Bauschinger torsion test is more than or equal to 300mm²。

The high-strength spring steel with tensile strength of more than or equal to 2250MPa and excellent fatigue performance disclosed by the invention has the following effects of various elements in the components:

c: c is the most effective reinforcing element in steel, and is an important element for tempering hardness and ensuring wear resistance in spring steel, and is necessary for obtaining spring steel having high strength and hardness. The high carbon content, while beneficial to the strength, hardness, elasticity and spring-reducing properties of the steel, is detrimental to the plasticity and toughness of the steel. The content of C is controlled between 0.70 percent and 0.80 percent.

Si: si is an important element for strengthening in steel, and the strong hardness of the steel is improved through the solid solution effect, and meanwhile, the sag resistance of the spring steel is improved. However, the increase of the Si element increases the diffusion of carbon in the steel, and thus the decarburization of the steel is promoted. The Si content is controlled to be 1.60-2.00%.

Mn: mn and Fe form a solid solution, so that the hardness and strength of ferrite and austenite in the steel are improved, and meanwhile, Mn is used for improving the stability of an austenite structure and remarkably improving the hardenability of the steel. However, excessive Mn lowers the plasticity of the steel. The Mn content is controlled to be 0.40-0.60 percent.

Cr: cr and C can form a stable compound, prevent segregation of C or impurities, improve the stability of a matrix and obviously improve the antioxidation of steel. Cr dissolves in ferrite to cause solid solution strengthening, and can significantly increase hardenability and temper resistance of steel, but excessive Cr increases the temper brittleness tendency of steel. The Cr content is controlled to be 0.80-1.00 percent.

V: v is an excellent deoxidizer of steel, and vanadium is added into the steel to refine structure grains and improve the strength and the toughness. V forms fine carbonitrides, and can improve the hydrogen-induced delayed fracture resistance and the fatigue performance. However, too much VC diffuses into the crystal to cause the toughness of the steel to be reduced. The content of V is controlled between 0.10 percent and 0.20 percent.

Nb: nb is a micro-alloying element for refining grains very effectively, forms nitrides with N through the composite action of Nb and V, particularly can increase nucleation points through increasing a deformation zone, can generate finer ferrite in the phase transformation process, and improves the strength and the fatigue strength of steel. However, the strengthening effect of excess Nb is no longer significant and increases the crack sensitivity of the steel. The Nb content is controlled to be 0.03-0.05 percent.

N: n is mainly precipitated with vanadium and niobium in steel, so that the strength and toughness of the steel are improved. However, excessive N precipitates Fe in the steel₄N and the diffusion speed is slow, so that the steel has timeliness, and meanwhile, the cold processing performance of the steel is reduced by the N, and the content of the N is controlled to be 0.005-0.008%.

Mg: mg not only has excellent affinity with oxygen and sulfur, but also has strong control capability on the shape and size of the inclusion. For Al-killed steels, Mg treatment further reduces dissolution in the steelAt the same time of oxygen, Al in the steel can be removed₂O₃Is mixed with MgO and Al with high melting point₂O₃Because the oxide inclusions exist in the molten steel in a solid state and do not have a polymerization growth process, the oxide inclusions are very small in size and are dispersed in the steel, the obdurability of the steel is not negatively affected, the plasticity and the fatigue strength of the spring steel can be improved, and the excessive Mg can cause the inclusions to be large in size. The content of Mg is controlled between 0.0015 percent and 0.0035 percent.

S and P: the sulfur is easy to form MnS inclusion with manganese in the steel, and the fatigue property of the steel is deteriorated; p is an element with a strong segregation tendency and usually also causes co-segregation of sulphur and manganese, which is detrimental to the homogeneity of the product structure and properties. P is controlled to be less than or equal to 0.015 percent, and S is controlled to be less than or equal to 0.015 percent.

O: o forms oxide inclusions in the steel, the processing performance and the fatigue performance of the steel are damaged, and the content of O is controlled to be less than or equal to 0.0015 percent.

In order to utilize the tempering softening resistance effect of alloy elements to the maximum extent, the steel of the invention needs to ensure that the A value is more than or equal to 135%: the value a is an index for evaluating the degree of influence of C, Si, Mn, Cr, V, and Nb on the temper hardness of the steel core and the degree of influence of each element by weighting and adding them. C. Si, Mn, Cr, V and Nb are the main elements for improving the tempering softening resistance of the steel grade.

The following experiments were performed: the method is characterized in that various steel materials which are 0.70-0.80 percent of C, 1.60-2.00 percent of Si, 0.40-0.60 percent of Mn, 0.80-1.00 percent of Cr, 0.10-0.20 percent of V, 0.03-0.05 percent of Nb, 0.0015-0.0035 percent of Mg, 0.005-0.008 percent of N, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.0015 percent of O and the balance of Fe and other inevitable impurities and meet the A value of more than or equal to 135 percent are adopted, after the steel is produced according to the process method, the steel materials are quenched at 880 ℃ (oil cooling) and tempered at 430 ℃ (tempering), and then the Vickers Hardness (HV) on the.

The relationship between the temper hardness and the fatigue strength was evaluated. The fatigue test was a Zhongcun type rotational bending fatigue test, and after heat treatment, a sample was prepared by removing surface scale, and then the fatigue bending test was performed. 60% of 10 samples are addedProbability of occurrence of 10⁷The maximum load stress for the next or more life is taken as the fatigue strength, and the result is shown in fig. 2.

As can be seen from fig. 2: in order to ensure a fatigue strength of 880MPa or more, it is necessary to ensure a temper hardness of HV645 or more. On the other hand, as is clear from FIG. 1, in order to ensure the temper hardness of HV645 or more, it is necessary to maintain the A value at 135 or more.

According to the production method of the high-strength spring steel with the tensile strength of more than or equal to 2250MPa and excellent fatigue performance, the high-strength spring steel with the tensile strength of more than or equal to 2250MPa is produced at low cost under the interaction of C, Si, Mn, Cr, V, Nb, Mg and N in a specific process, and has good obdurability, excellent fatigue strength and excellent anti-springback performance.

Drawings

FIG. 1 is a graph showing the relationship between the Vickers hardness and the A value of a steel material having an A value of 135% or more after heat treatment;

FIG. 2 is a graph showing the relationship between Vickers hardness and fatigue strength after heat treatment of a steel material satisfying an A value of 135% or more.

Detailed Description

The present invention will be described in detail with reference to examples.

The wire rod with specific components is adopted in the invention, the components of the examples and the comparative examples are shown in the table 1, and the production process is as follows:

electric furnace smelting: oxygen is determined before tapping, and steel retaining operation is adopted in the tapping process, so that slag is prevented from being discharged;

and (4) LF furnace: C. adjusting elements such as Si, Cr, Mn, V, Nb, Mg and the like to target values;

vacuum degassing: the pure degassing time is more than or equal to 15 minutes, and the content of H after vacuum treatment is ensured;

continuous casting, namely controlling the target temperature of the tundish molten steel to be 10-40 ℃ above the liquidus temperature, and continuously casting a square billet with the thickness of 250mm and × 250 mm;

cogging, namely heating a large square billet with the thickness of 250mm × 250mm → rolling the large square billet with the thickness of 150mm × 150mm → stacking and cooling, controlling the soaking temperature of the heating furnace to be 1200-1250 ℃, and controlling the total heating time to be more than or equal to 230 min;

the wire rolling route comprises the steps of peeling a 150mm × 150mm square billet, heating → high-speed wire control rolling → stelmor cooling line control cooling → a wire rod finished product of phi 6.5-16 mm, wherein the peeling depth is controlled to be more than 1.2mm, zero decarburized layer after peeling is ensured, and the heating temperature is controlled to be 1020-1060 ℃, the finish rolling temperature is controlled to be 790-830 ℃, and the spinning temperature is controlled to be 790-830 ℃ during wire rolling.

The hot rolling state performance detection method comprises the following steps:

hot rolling structure: taking a cross section sample on a hot-rolled wire rod, wherein the height of the cross section sample is about 10mm, polishing the cross section sample, corroding the cross section sample by using 4% nitric acid alcohol, and observing a tissue under a metallographic microscope after corroding the cross section sample;

austenite grain size rating: the austenitizing heat treatment process comprises the following steps: quenching at 880 ℃, oil cooling, wherein the temperature of a quenching medium is 18-35 ℃, and metallographic sample preparation and austenite grain size grading are carried out after cooling;

TABLE 1 examples and comparative examples chemical composition (wt%)

TABLE 2 production parameter control, wire structure and grain size obtained for each example and comparative example

The performance detection method after quenching and tempering heat treatment comprises the following steps:

tensile property of quenching and tempering heat treatment: processing the wire rod into a blank sample of a standard tension sample (sampling on an intermediate blank in a small specification), and adopting the following quenching and tempering heat treatment process: quenching at 880 ℃, oil cooling, tempering at the temperature of a quenching medium of 18-35 ℃, naturally cooling along with air, performing finish machining on a standard tension sample, and performing a tensile test;

and (3) fatigue test: quenching and tempering treatment is carried out on the material (880 ℃ quenching, oil cooling, quenching medium temperature of 18-35 ℃, tempering at 430 ℃ and natural cooling along with air) And removing the surface scale to prepare a sample, and then performing a fatigue bending test. 10 samples were presented with a probability of 60% or more⁷The maximum load stress of the next or more life is taken as the fatigue strength;

and (3) determining a reverse curve of a Bauschinger torsion test: the material is roughly processed, then quenched and tempered, and then finely ground to a standard sample for Bauschinger torsion test, and the test is carried out on a standard torsion machine. The sample was unloaded by twisting it to 90 ℃ with a twisting speed of 15 °/min and unloaded again after reloading so that the sample was still twisted to 90 ℃ in the original direction. A closed torsion hysteresis loop is obtained on a torque-torsion angle curve chart, and the area of the loop is calculated. The larger its area, the greater the resistance to elastic reduction.

TABLE 3 tensile properties, fatigue strength and resistance to ballistic reduction after quenching and tempering heat treatment for the examples and comparative examples

The examples 1-5 are examples in which chemical components and production processes are reasonably controlled, the strength of the steel after heat treatment reaches more than 2250MPa, the elongation reaches more than 12%, and the surface shrinkage reaches more than 42%, which indicates that the examples have better strong plasticity, excellent fatigue strength and good anti-ballistic performance, can be used for manufacturing 2250 MPa-grade high-stress springs, and can be applied to high-end manufacturing industries such as automobiles, instruments and the like.

In comparative example 1, the inclusion modification was not performed by adding Mg element, and the oxygen content control level was low, resulting in more inclusions in steel grade, and in addition, since 380 round billets were used, the center composition segregation was severe, resulting in poor plasticity of steel and severe deterioration of fatigue properties.

Comparative example 2 is an example in which the compositions are all in the controlled range, but the A value is lower, resulting in lower core temper hardness and ultimately lower fatigue strength due to the A value < 135.

Comparative examples 3 and 4 are spring steels which are commonly used in the market, and are obtained by controlling the production process of the invention, but the chemical components are out of the range of the invention, the tensile strength is below 2000MPa, and the fatigue performance and the anti-bullet performance are poorer than those of the examples.

The above detailed description of a high strength spring steel having a tensile strength of 2250MPa or more and the method for manufacturing the same with reference to the embodiments is illustrative and not restrictive, and several embodiments may be enumerated within the limits of the embodiments, so that changes and modifications may be made without departing from the general concept of the present invention and fall within the scope of the present invention.

Claims

1. A high-strength spring steel with tensile strength not less than 2250MPa and excellent fatigue performance is characterized by comprising the following chemical components in percentage by weight: 0.70 to 0.80 percent of C, 1.60 to 2.00 percent of Si, 0.40 to 0.60 percent of Mn0.80 to 1.00 percent of Cr0.80 to 1.00 percent of V, 0.10 to 0.20 percent of Nb, 0.03 to 0.05 percent of Nb, 0.0015 to 0.0035 percent of Mg0.0015, 0.005 to 0.008 percent of N, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, less than or equal to 0.0015 percent of O, and the balance of Fe and other inevitable impurities; and 30.5C +9.6Si +6.1Mn +11.5Cr +34.5V +19.8Nb +78.5 is more than or equal to 135 percent.

2. The heat treatment method for a high-strength spring steel having a tensile strength of 2250MPa or more and excellent fatigue properties according to claim 1, characterized in that the spring obtained by rolling is quenched at 870 to 920 ℃, oil-cooled, tempered at 420 to 460 ℃, and air-cooled.

3. A method of producing a high-strength spring steel having a tensile strength of 2250MPa or more and excellent fatigue properties as set forth in claim 1, characterized by comprising the steps of: electric furnace smelting, LF furnace, vacuum degassing, continuous casting, cogging, wire rolling and heat treatment.

4. The production method according to claim 3, wherein in the continuous casting step, a 250mm × 250mm bloom is continuously cast.

5. The production method according to claim 3, wherein in the cogging step, a 250mm × 250mm bloom is heated, rolled into a 150mm × 150mm bloom, and then cooled in a pile.

6. The method according to claim 5, wherein the heating temperature is 1200-1250 ℃ and the total heating time is 230min or more.

7. The production method according to claim 3, wherein the wire rolling process comprises 150mm × 150mm square billet peeling → heating → high speed wire controlled rolling → stelmor cooling by wire control → phi 6.5-16 mm wire rod finished product.

8. The production method according to claim 7, wherein the depth of the billet peeling is controlled to be more than 1.2mm, and the heating temperature is controlled to be 1020-1060 ℃, the finishing temperature is controlled to be 790-830 ℃, and the spinning temperature is controlled to be 790-830 ℃.

9. The production method according to claim 7 or 8, characterized in that the peeling depth is controlled to be more than 1.2mm, the heating temperature is controlled to be 1032-1060 ℃, the finishing temperature is controlled to be 793-815 ℃, and the spinning temperature is controlled to be 799-815 ℃.

10. The production method according to claim 3, wherein the heat treatment step is carried out by the heat treatment method according to claim 2, the tensile strength of the spring steel after the heat treatment is not less than 2250MPa, the elongation after fracture is not less than 10%, the reduction of area is not less than 40%, the fatigue strength is not less than 880MPa, and the area of the reverse-return curve of the Bauschinger torsion test is not less than 300mm²。