Steel for automobile leaf springs and method for manufacturing the springs thereof

Abstract

This invention provides a steel for leaf springs that enables simultaneous achievement of a tensile strength exceeding 1750 MPa, a hardness exceeding 480 HV, and a strain of at least 25%. [Solution] In weight percent, C: 0.4~0.7%, Mn: 0.5~1.5%, Si: 1~2.5%, Al: 0.001~0.1%, Ni: 0.1~1%, Cr: 0.2~1.5%, P: 0~0.09%, S: 0~0.09%, N: 0~0.09%, Mo: 0~0.5%, V: 0~0.2%, Nb: 0~0.1%, Ti: 0~0.1%, Cu: 0~1%, B: 0~0 A steel for leaf springs, comprising 0.008%, Sn: 0-0.1%, Ce: 0-0.1%, Mg: 0-0.10%, and Zr: 0-0.10%, with the remaining composition consisting of iron and unavoidable impurities, wherein the microstructure of the steel, by area%, contains 75-98% martensite and 2-20% retained austenite, with the optional presence of bainite and ferrite accumulation being between 0-5%.

Description

Technical Field

[0001] The present invention relates to steel suitable for manufacturing spring plates of leaf springs for automobiles.

Background Art

[0002] Automobile leaf springs are generally manufactured for pickup trucks, trucks and other vehicles. The materials for such manufacturing face the problem that they cannot satisfy two important conditions, namely, having good fatigue and at the same time having a high level of tensile strength to meet the requirements of the automotive industry for its structural parts. Furthermore, another essential requirement for these materials is that they must have good formability and fatigue resistance in order to be used for manufacturing automotive mechanical parts such as leaf springs and chassis members.

[0003] Therefore, intensive research and development have been carried out to develop a material with good machinability while having a high yield strength exceeding 1750 MPa together with sufficient impact toughness.

[0004] Initial research and development in the field of steel for automobile leaf springs has led to several methods for producing high strength and good formability, and some of them are listed here for the ultimate understanding of the present invention.

[0005] EP2514846 has the following composition: C: 0.40~0.54%, Si: 0.40~0.90%, Mn: 0.40~1.20%, Cr: 0.70~1.50%, Ti: 0.070~0.150%, B: 0.0005~0.0050%, and N: 0.0100% or less, arbitrarily below, i.e., Cu: 0.20~0.50%, Ni: 0.20~1.00%, V: 0.05~0.30%. The suspension leaf spring is obtained using a leaf spring steel with high fatigue strength, consisting of at least one element with 0.01-0.30% Nb, the remainder being Fe and unavoidable impurities, the Ti and N content satisfying the relationship Ti / N ≥ 10, the suspension leaf spring having a Vickers hardness of at least 510, and having a tempered martensitic structure. The suspension leaf spring of EP2514846 has a bending stress of 650-1900 MPa acting on the leaf spring. However, the steel of EP2512846 has good fatigue resistance but does not have appropriate strain (striciton). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] European Patent Application Publication No. 2514846 [Overview of the Initiative]

[0007] Therefore, in light of the above publications, the object of the present invention is to provide a spring plate steel for leaf springs that enables the simultaneous acquisition of a tensile strength exceeding 1750 MPa, a hardness exceeding 480 HV, and a strain of at least 25%.

[0008] Therefore, the object of the present invention is to solve these problems by making available steel suitable for mechanical operation for manufacturing spring plates of leaf springs that simultaneously have the following properties. - Tensile strength of 1650 MPa or more, preferably exceeding 1750 MPa, - Fatigue endurance of at least 120,000 cycles at a minimum stress of 1100 MPa, preferably exceeding fatigue endurance of at least 125,000 cycles at a minimum stress of 1100 MPa. - A strain of at least 25%, preferably more than 30%, - Hardness of 500 Hv or higher, more preferably exceeding 510 Hv.

[0009] Preferably, such steel is suitable for manufacturing leaf springs in which each spring plate can have a cross-section of up to 60 mm x 100 mm, and the steel is also suitable for other structural components of automobiles, such as chassis members.

[0010] Another objective of the present invention is to make available a method for manufacturing these machine parts that is stable to shifts in manufacturing parameters while being compatible with conventional industrial applications. [Modes for carrying out the invention]

[0011] The steel of the present invention contains between 0.4% and 0.7% carbon. Carbon is an element necessary to increase the strength of the steel of the present invention by generating low-temperature transformation phases such as martensite, but a carbon content of less than 0.4% cannot impart tensile strength to the steel of the present invention. On the other hand, if the carbon content exceeds 0.7%, the toughness is adversely affected because protereminate cementite is excessively formed during cooling after hot rolling. Furthermore, the excessive formation of protereminate cementite is also due to mechanical operations on the spring plates of leaf springs, such as punching, drilling, honing, or grinding. The carbon content is advantageously in the range of 0.45% to 0.6%, and particularly in the range of 0.5% to 0.6%.

[0012] In this steel, manganese is added in amounts between 0.5% and 1.5%. This element is gamma-geneous. Manganese provides solid solution strengthening, suppresses the ferrite transformation temperature, reduces the ferrite transformation rate, and therefore aids in martensite formation. At least 0.5% is necessary to aid in martensite formation and as well as to impart strength. However, if the manganese content exceeds 1.5%, adverse effects occur, such as delaying the conversion of austenite to martensite during cooling after mechanical operations. If the manganese content exceeds 1.5%, excessive segregation in the steel during solidification can impair the homogeneity of the material and cause surface cracking during the hot working process. The preferred limit for the presence of manganese is between 0.6% and 1.4%, and more preferably between 0.7% and 1.3%.

[0013] The steel of the present invention contains silicon in an amount between 1% and 2.5%. Silicon imparts strength to the steel of the present invention through solid solution strengthening and also acts as a deoxidizing agent. Silicon is a component that can delay the precipitation of carbides during cooling after mechanical operations, and therefore promotes the formation of martensite. However, since silicon is a ferrite-forming agent and also raises the Ac3 transformation point, which pushes the austenite temperature to a higher temperature range, the silicon content is kept at a maximum of 2.5%. A silicon content exceeding 2.5% can also cause tempering embrittlement. The preferred limit for the presence of silicon is between 1.1% and 2.4%, and more preferably between 1.2% and 2.3%.

[0014] The aluminum content is between 0.001% and 0.1%. Aluminum removes oxygen present in molten steel and prevents oxygen from forming a gas phase during solidification. Aluminum also fixes nitrogen in the steel to form aluminum nitride, reducing the size of the crystal grains. However, the deoxidation effect saturates at aluminum content exceeding 0.1%. Aluminum also controls the grain size of the steel. High aluminum content exceeding 0.1% leads to the formation of coarse aluminum-enriched oxides, which degrade the fatigue limit and machinability. The preferred limit for the presence of aluminum is between 0.001% and 0.09%, and more preferably between 0.001% and 0.03%.

[0015] Adding nickel between 0.1% and 1% to the present invention increases the strength of the steel, particularly improving its toughness after quenching and tempering. Nickel is beneficial in improving its pitting corrosion resistance. A minimum of 0.1% is required to obtain such effects. Adding Ni to the steel composition reduces the diffusion coefficient of carbon in the austenite, thereby promoting the formation of martensite. However, the presence of nickel content exceeding 1% lowers the martensite initiation temperature, leading to excessive stabilization of retained austenite, which adversely affects tensile strength and yield strength. In the steel of the present invention, it is preferable to have nickel between 0.1% and 0.9%.

[0016] Chromium is present in the steel of the present invention in an amount between 0.2% and 1.5%. Chromium is an essential element that gives strength to steel through solid solution strengthening, and a minimum of 0.2% is required to impart strength. However, if used in amounts exceeding 1.5%, the hardenability exceeds the acceptable limit due to the formation of coarse cementite after cooling, thereby impairing the formability as well as the ductility of the steel. The addition of chromium, like nickel, also lowers the diffusion coefficient of carbon in austenite, and therefore promotes the formation of martensite. The preferred limit for the presence of chromium is between 0.3% and 1.4%, and more preferably between 0.4% and 1.2%.

[0017] The phosphorus content of the steel of this invention is between 0% and 0.09%. Phosphorus tends to segregate at grain boundaries or co-segregate with manganese. For these reasons, it is recommended to use as little phosphorus as possible. Specifically, a content exceeding 0.05% may cause fracture due to intergranular interface segregation, which can adversely affect the fatigue limit. The preferred limit for phosphorus content is between 0% and 0.05%.

[0018] Sulfur content is between 0% and 0.09%. Sulfur forms MnS precipitates that improve machinability, helping to achieve sufficient machinability. During metal forming processes such as rolling and forming, deformable manganese sulfide (MnS) inclusions take on an elongated shape. Such elongated MnS inclusions can have a significant adverse effect on mechanical properties such as strain and impact toughness if the inclusions are not aligned in the direction of the load. Therefore, the sulfur content is limited to 0.09%. The preferred range for sulfur content is between 0% and 0.05%, and it is more preferable to be between 0% and 0.02% to obtain the best balance between machinability and fatigue limit.

[0019] The amount of nitrogen in the steel of this invention is between 0% and 0.09%. To avoid material aging and minimize the precipitation of aluminum nitride during solidification, which adversely affects the mechanical properties of the steel, nitrogen is limited to 0.09%. Nitrogen also forms nitrides and carbonitrides with vanadium, titanium, and niobium to impart strength to the steel of this invention.

[0020] Molybdenum is an optional element and can be present in amounts between 0% and 0.5% in this invention. Molybdenum imparts hardenability and hardness to steel by forming molybdenum-based carbides, and also delays the appearance of bainite, promoting the formation of martensite. However, for economic reasons, the addition of molybdenum excessively increases the cost of adding alloying elements, so its content is limited to 0.5%. The preferred limit for the molybdenum content is between 0% and 0.4%, and more preferably between 0% and 0.2%.

[0021] Vanadium is an optional element of the present invention, and its content is between 0% and 0.2%. Vanadium is effective in increasing the strength of steel, especially by forming carbides or carbonitrides and precipitation strengthening. The upper limit is maintained at 0.2% for economic reasons.

[0022] Niobium is present in the steel of the present invention between 0% and 0.1% and is suitable for forming carbonitrides to impart strength to the steel of the present invention by precipitation hardening. Niobium also affects the size of the components of the microstructure through its precipitation as carbonitrides and by delaying recrystallization during the heating process. Thus, a finer microstructure formed at the end of the holding temperature and as a result after complete austenitization leads to hardening of the product. However, a niobium content exceeding 0.1% is not economically interesting and produces coarser precipitates that have an adverse effect on the fatigue properties of the steel. Also, when the niobium content is 0.1% or more, niobium has an adverse effect on hot ductility and causes difficulties during casting and rolling of the steel.

[0023] Titanium is an optional element and is present between 0% and 0.1%. Titanium forms titanium nitrides that impart strength to the steel, but these nitrides can be formed during the solidification process and thus have an adverse effect on the fatigue limit. Therefore, the preferred limit of titanium is between 0% and 0.05%.

[0024] Copper is a residual element and may be present up to 1% by steel processing. Up to 0.5%, copper does not affect any properties of the steel, but above 0.5%, hot workability significantly decreases.

[0025] Other elements such as tin, cerium, magnesium or zirconium can be added individually or in combination in the following weight ratios, namely tin ≤ 0.1%, cerium ≤ 0.1%, magnesium ≤ 0.10%, 0% ≤ boron ≤ 0.008% and zirconium ≤ 0.10%. These elements make it possible to refine the crystal grains during solidification up to the indicated maximum content levels. The remainder of the steel composition consists of iron and inevitable impurities resulting from processing.

[0026] The microstructure of the base steel includes the following.

[0027] Martensite constitutes between 75% and 98% of the microstructure in terms of area fraction. The martensite of the present invention can include both fresh martensite and tempered martensite. However, fresh martensite is an optional minor component and is preferably limited in the steel in an amount between 0% and 10%, more preferably between 0% and 8%, and even better if less than 5%. Fresh martensite can be generated during cooling after tempering. Tempered martensite is formed from martensite generated during cooling after annealing, particularly below the Ms temperature, more specifically between Ms - 10°C and 20°C. Such martensite is tempered during holding at the tempering temperature, particularly when tempered between 250 and 500°C. The martensite of the present invention imparts strength and fatigue durability to the steel. Preferably, the content of martensite is between 80% and 97%, more preferably between 85% and 95%.

[0028] Retained austenite is a microstructure component present in the steel between 2% and 20%. The toughness and ductility of retained austenite in the steel of the present invention. The preferred limit for the presence of austenite is between 3% and 18%, more preferably between 4% and 16%.

[0029] The cumulative amount of ferrite and bainite corresponds to between 0% and 5% of the microstructure. The cumulative presence of bainite and ferrite does not adversely affect the present invention up to 5%, but mechanical properties may be adversely affected if it exceeds 5%. Therefore, the preferred limit for the cumulative presence of ferrite and bainite is maintained between 0% and 4%, more preferably between 0% and 3%.

[0030] Bainite forms during reheating before tempering. While bainite can impart formability to steel, excessive amounts can negatively affect its tensile strength. Ferrite may form during the first step of cooling after annealing, but it is not necessary as a component of the microstructure. Ferrite formation should be kept to a minimum, preferably less than 2%, or even less than 1%.

[0031] The leaf spring according to the present invention can be manufactured by any suitable manufacturing process using the specified process parameters described below.

[0032] Preferred exemplary methods are demonstrated herein, but these examples do not limit the scope of the disclosure or the embodiments on which these examples are based. Furthermore, none of the examples described herein are intended to be limiting, but merely describe some of the many possible ways in which various embodiments of this disclosure may be carried out.

[0033] A preferred method is to provide a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be carried out in any form such as an ingot, bloom, or billet, which can be manufactured or processed into a leaf spring having a spring plate that can have a cross-section of up to 60 mm × 100 mm.

[0034] For example, steel having the above chemical composition is cast into a billet and then rolled into a bar. This bar can further function as a semi-finished product for mechanical operations. Multiple rolling steps may be performed to obtain the desired semi-finished product.

[0035] To prepare steel for use as spring plates for leaf springs, the semi-finished product can be used directly at high temperatures after rolling, or it can be cooled to room temperature first and then reheated to manufacture the spring plates.

[0036] The semi-finished product is reheated to a temperature between Ac3 and Ac3+300°C, preferably between Ac3+30°C and Ac3+300°C, and held at that temperature for 5 to 1200 seconds to ensure a uniform temperature across the cross-section of the semi-finished product and to ensure 100% reliable austenite formation.

[0037] If the reheating temperature of the semi-finished product is lower than Ac3, excessive load may be applied to the tools used in mechanical operations, such as dies during forming or milling tools during taper processing. Furthermore, the temperature of the steel may drop below the ferrite transformation initiation temperature, which negatively impacts fatigue and mechanical properties, leading to ferrite formation in the final product. Moreover, metallurgical transformation under strain can result in significant changes to the microstructure obtained for a given cooling rate or chemical composition. As a result, the resulting microstructure will be completely different from the target, and therefore the mechanical properties will also be different. Therefore, it is preferable that the temperature of the semi-finished product be sufficiently high so that all mechanical operations are performed and completed 100% within the austenite temperature range. Reheating at temperatures above Ac3 + 300°C should be avoided as it is industrially costly and can lead to the creation of a liquid region that affects the forming and taper processing of the steel.

[0038] Next, the semi-finished product is subjected to at least one mechanical manufacturing operation between Ac3 and Ac3+300°C. The mechanical operation may include tapering, eye rolling, die forming, or any other suitable mechanical operation or manufacturing procedure required to form a high-temperature spring plate of a leaf spring from the semi-finished product. The preferred temperature for all mechanical operations is between Ac3+30°C and Ac3+300°C, and the more preferred temperature for all mechanical operations is between Ac3+50°C and Ac3+250°C.

[0039] The final mechanical operation temperature must be kept above Ac3, which is preferable for a microstructure favorable for recrystallization and mechanical manufacturing. It is preferable to carry out all mechanical operations, especially the final mechanical operation, at a temperature higher than Ac3 + 50°C, because below this temperature, the steel exhibits a significant decrease in mechanical manufacturability. Below the Ac3 temperature, the ductility of the steel deteriorates significantly. This can lead to problems not only with the final dimensions of the spring plate but also with the deterioration of the surface morphology. It can even lead to cracking or complete breakage of the spring plate in a leaf spring.

[0040] Semi-finished products can be cooled to room temperature after mechanical operation and then reheated to a temperature between Ac3 and Ac3+300°C for subsequent mechanical operation. Multiple cooling and reheating cycles can be performed during mechanical operation to obtain a high-temperature spring plate of the desired leaf spring. After the completion of mechanical operation, a high-temperature spring plate of the leaf spring is obtained, and then the high-temperature spring plate of the leaf spring is cooled.

[0041] The cooling of the hot spring plates of the leaf spring is carried out at an average cooling rate of less than 50°C / second, preferably less than 40°C / second, and more preferably less than 38°C / second, down to a quenching temperature in the range of Ms-10°C to 20°C, also referred to herein as QT. The preferred QT temperature range is Ms-50°C to 20°C. During this process, particularly when the hot spring plates of the leaf spring are cooled after crossing the Ms temperature, martensite is formed.

[0042] Subsequently, from temperature QT, the hot spring plates of the leaf spring are heated to a tempering temperature, referred to as TT in this specification, in the range of 250°C to 500°C, at an average heating rate of 0.5°C / sec to 150°C / sec, more preferably 0.6°C / sec to 100°C / sec, to temper the hot spring plates of the leaf spring. The hot spring plates of the leaf spring are held at the TT temperature for 10 seconds to 10,000 seconds. A preferred TT temperature range is 300°C to 475°C. During this process, the martensite is tempered and transformed into tempered martensite.

[0043] Subsequently, the hot spring plate of the leaf spring is cooled from TT to room temperature, maintaining an average cooling rate of less than 5°C / second, preferably less than 4°C / second, and more preferably less than 2°C / second between TT and room temperature. These average cooling rates are selected to ensure uniform cooling across the cross-section of the hot spring plate of the leaf spring. Once cooled to room temperature, the spring plate of the leaf spring is obtained. [Examples]

[0044] The following tests, examples, illustrative examples and tables presented herein are not restrictive in nature and should be considered for illustrative purposes only, and illustrate advantageous features of the present invention.

[0045] Table 1 summarizes forged machine parts made of steel with different compositions. Each forged machine part is manufactured according to the process parameters specified in Table 2. Table 3 then summarizes the microstructure of the forged machine parts obtained during the trials, and Table 4 summarizes the evaluation results of the obtained properties.

[0046] [Table 1]

[0047] Table 2 summarizes the process parameters applied to the semi-finished products made from the steel shown in Table 1. Trials I1 to I4 are useful for manufacturing forging machine parts according to the present invention. This table also specifies the reference forging machine parts designated in the table, R1 to R2.

[0048] Table 2 is as follows:

[0049] [Table 2]

[0050] Table 3 illustrates the results of tests performed according to standards for different microscopes, such as scanning electron microscopy, to determine the microstructure of both the steel of the present invention and the reference steel in terms of area fraction. The results are specified herein.

[0051] [Table 3]

[0052] Table 4 illustrates the mechanical properties of both the steel of the present invention and the reference steel. To determine the tensile strength, tests are conducted according to the NF EN ISO 6892-1 standard. Toughness and fatigue tests are performed on both the steel of the present invention and the reference steel at room temperature according to the EN ISO 148-1 standard KCU specimen with a U-notch. The results of the various mechanical tests conducted according to the standards are summarized.

[0053] [Table 4]

Claims

1. Steel for leaf springs, containing the following elements expressed as weight percentages, namely: 0.4% ≤ C ≤ 0.7%, 0.5% ≤ Mn ≤ 1.5%, 1% ≤ Si ≤ 2.5%, 0.001% ≤ Al ≤ 0.1%, 0.1% ≤ Ni ≤ 1%, 0.2% ≤ Cr ≤ 1.5%, 0 ≤ P ≤ 0.09%, 0 ≤ S ≤ 0.09%, 0% ≤ N ≤ 0.09%, It includes, and any of the following elements, namely, 0% ≤ Mo ≤ 0.5%, 0% ≤ V ≤ 0.2%, 0% ≤ Nb ≤ 0.1%, 0% ≤ Ti ≤ 0.1%, 0% ≤ Cu ≤ 1%, 0% ≤ B ≤ 0.008%, 0% ≤ Sn ≤ 0.1%, 0% ≤ Ce ≤ 0.1%, 0% ≤ Mg ≤ 0.10%, 0% ≤ Zr ≤ 0.10%, A steel for leaf springs, which may contain one or more of the following, the remaining composition consisting of iron and unavoidable impurities resulting from processing, the microstructure of the steel containing, in area percent, 75% to 98% martensite and 2% to 20% retained austenite, with the optional presence of bainite and ferrite accumulation being between 0% and 5%.

2. The steel for leaf springs according to claim 1, wherein the composition contains 1.2% to 2.4% silicon.

3. The leaf spring steel according to claim 1 or 2, wherein the composition contains 0.45% to 0.6% carbon.

4. The leaf spring steel according to any one of claims 1 to 3, wherein the composition comprises 0.001 to 0.09% aluminum.

5. The leaf spring steel according to any one of claims 1 to 4, wherein the composition contains 0.6% to 1.4% manganese.

6. The leaf spring steel according to any one of claims 1 to 5, wherein the composition contains 0.3% to 1.4% chromium.

7. The leaf spring steel according to any one of claims 1 to 6, wherein the composition contains 0.1% to 0.9% nickel.

8. A steel for leaf springs according to any one of claims 1 to 7, wherein the martensite content is between 80% and 97%.

9. A leaf spring steel according to any one of claims 1 to 8, wherein the retained austenite is between 3% and 18%.

10. The leaf spring steel according to any one of claims 1 to 9, wherein the cumulative presence of bainite and ferrite is between 0% and 4%.

11. The leaf spring steel according to any one of claims 1 to 10, wherein the presence of ferrite is between 0% and 1%.

12. A leaf spring steel according to any one of claims 1 to 11, wherein the ultimate tensile strength is greater than 1650 MPa.

13. A leaf spring steel according to any one of claims 1 to 12, having a hardness of 480 Hv or more.

14. The leaf spring steel according to any one of claims 1 to 13, having a fatigue durability of at least 120,000 cycles when tested at a minimum stress of 1,100 MPa.

15. A leaf spring steel according to any one of claims 1 to 14, having a strain equal to more than 25%.

16. A method for manufacturing a spring plate of a steel leaf spring, comprising the following sequential steps. - A step of providing the steel composition according to any one of claims 1 to 7 in the form of a semi-finished product, - A step of reheating the semi-finished product to a temperature between Ac3 and Ac3 + 300°C. - A process to obtain a high-temperature spring plate for a leaf spring by performing one or more mechanical operations on the semi-finished product in the austenite range where the mechanical operation finishing temperature is between Ac3 and Ac3 + 300°C. - A step of cooling the high-temperature spring plate of the aforementioned leaf spring to a temperature QT in the range of Ms -10°C to 20°C at a cooling rate of less than 50°C / second. - Subsequently, the high-temperature spring plate of the leaf spring is heated to a temperature TT in the range of 250°C to 500°C from QT at an average heating rate between 0.5°C / sec and 150°C / sec. - Next, the process involves holding the high-temperature spring plate of the leaf spring at a temperature TT for 10 to 10,000 seconds. - Next, the high-temperature spring plate of the leaf spring is cooled from TT to room temperature at an average cooling rate of less than 5°C / second to obtain the spring plate of the leaf spring.

17. The method according to claim 17, wherein the reheating temperature of the semi-finished product is between Ac3 + 30°C and Ac3 + 300°C.

18. The method according to claim 16 or 17, wherein the temperature TT is 300°C to 475°C.

19. The method according to any one of claims 16 to 18, wherein the temperature QT is Ms -50°C to 20°C.

20. Use of steel according to any one of claims 1 to 19, or mechanical parts manufactured according to the method described in claims 16 to 19, for the manufacture of structural components or safety components of a vehicle or internal combustion engine.

21. A vehicle comprising the part obtained according to claim 20.

Images