WO2022064249A1

WO2022064249A1 - Steel for leaf springs of automobiles and a method of manufacturing of a leaf thereof

Info

Publication number: WO2022064249A1
Application number: PCT/IB2020/058883
Authority: WO
Inventors: Jean-Michel JACHMICH; Bertrand MICHAUT; Laurent LORICH
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2020-09-23
Filing date: 2020-09-23
Publication date: 2022-03-31
Anticipated expiration: 2023-03-23
Also published as: MX2023003309A; CA3192265A1; US20230340631A1; CN116113720A; EP4217518A1; ZA202301864B; JP2023542952A; KR20230051544A; BR112023003724A2

Abstract

A steel for leaf spring comprising of the following elements 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%; 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%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel comprising, by area percentage, 75% to 98% of Martensite, 2% to 20% of Residual Austenite, with a cumulative optional presence of bainite and ferrite between 0% to 5%.

Description

STEEL FOR LEAF SPRINGS OF AUTOMOBILES AND A METHOD OF MANUFACTURING OF A LEAF THEREOF The present invention relates to a steel suitable for manufacturing of leaf of a leaf spring for automobiles. Leaf springs for the automobiles are generally manufactured for pick-up, trucks and other vehicles. Material for such manufacturing inherently faces the problem of inability to meet the dual requirement of good fatigue and having high level of tensile strength at same time to meet the requirements of the automobile industry for its structural parts. Further one another compulsory requirement for these materials is that they must have good formability and fatigue resistance so that they can be used to manufacture mechanical parts for automobiles such as leaf springs and chassis members. Therefore, intense Research and development endeavors are put in to develop a material that is good in machinability while having high yield strength that is above 1750 MPa with adequate impact toughness. Earlier research and developments in the field of steels for leaf springs for automobiles have resulted in several methods for producing high strength and good formability some of which are enumerated herein for conclusive appreciation of the present invention: EP2514846. is a suspension leaf spring obtained by using a steel for a leaf spring with high fatigue strength, the steel consisting of, C: 0.40 to 0.54%, Si: 0.40 to 0.90%, Mn: 0.40 to 1.20%, Cr: 0.70 to 1.50%, Ti: 0.070 to 0.150%, B: 0.0005 to 0.0050%, and N: 0.0100% or less, optionally at least one of Cu: 0.20 to 0.50%, Ni: 0.20 to 1.00%, V: 0.05 to 0.30%, and Nb: 0.01 to 0.30%, the balance being Fe and unavoidable impurities, wherein a Ti content and a N content satisfy a relation of Ti/N≥10, and wherein the suspension leaf spring has a Vickers hardness of at least 510 and a tempered martensite structure. EP2514846. suspension leaf spring have a bending stress of 650 to 1900 MPa being applied to the leaf spring. But the steel of EP2512846 does not have adequate striciton while having a good fatigue resistance. Therefore, in the light of the publications mentioned above, the object of the invention is to provide a steel for a leaf of a leaf spring, that makes it possible to obtain simultaneously having a tensile strength above 1750MPa, a hardness above 480HV and an striction of at least 25% or more. Hence the purpose of the present invention is to solve these problems by making available a steel suitable for mechanical operations for manufacturing a leaf of a leaf spring that simultaneously have: - a tensile strength greater than or equal to 1650 MPa and preferably above 1750 MPa, - a fatigue endurance of at least 120000 cycles at minimum stress of 1100 MPa and preferably more than fatigue endurance of at least 125000 cycles at minimum stress of 1100 MPa. - a striction of at least 25% or more and preferably more than 30%. - A hardness of 500Hv or more and more preferably more than 510 Hv . Preferably, such steel is suitable for manufacturing of a leaf spring wherein each leaf can have a cross section up to 60mm *100 mm and the steel is also suitable for other structural parts of a automobiles such as chassis members. Another object of the present invention is also to make available a method for the manufacturing of these mechanical parts that is compatible with conventional industrial applications while being robust towards manufacturing parameters shifts. Carbon is present in the steel of present invention is between 0.4% and 0.7%. Carbon is an element necessary for increasing the strength of the Steel of present invention by producing a low-temperature transformation phases such as Martensite, But Carbon content less than 0.4% will not be able to impart the tensile strength to the steel of present invention. On the other hand, at a Carbon content exceeding 0.7%,the toughness are adversely impacted due to the excessive formation of proeutectoid cementite during the cooling after hot rolling. Further excessive formation of proeutectoid cementite is also for mechanical operations on the leaf of the leaf spring such as punching, drilling, honing or grinding. The carbon content is advantageously in the range 0.45% to 0.6% and more especially 0.5% to 0.6%. Manganese is added in the present steel between 0.5% and 1.5%.This element is gammagenous. Manganese provides solid solution strengthening and suppresses the ferritic transformation temperature and reduces ferritic transformation rate hence assist in the formation of martensite. An amount of at least 0.5% is required to impart strength as well as to assist the formation of Martensite. But when Manganese content is more than 1.5% it produces adverse effects such as it retards transformation of Austenite to Martensite during cooling after mechanical operation. Manganese content of above 1.5% can get excessively segregated in the steel during solidification and homogeneity inside the material is impaired which can cause surface cracks during a 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%. Silicon is present in the steel of present invention between 1% and 2.5%. Silicon impart the steel of present invention with strength through solid solution strengthening and also acts as a deoxidizer. Silicon is a constituent that can retard the precipitation of carbides during cooling after mechanical operation, therefore, Silicon promotes formation of Martensite. But Silicon is also a ferrite former and also increases the Ac3 transformation point which will push the austenitic temperature to higher temperature ranges that is why the content of Silicon is kept at a maximum of 2.5%.Silicon content above 2.5% can also cause temper 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%. The content of the Aluminum is between 0.001% and 0.1%. Aluminum removes Oxygen existing in molten steel to prevent Oxygen from forming a gas phase during solidification process. Aluminum also fixes Nitrogen in the steel to form Aluminum nitride to reduce the size of the grains. But the deoxidizing effect saturates for aluminum content more than 0.1%.Aluminum also control the grain size of the present steel. Higher content of Aluminum above 0.1% lead to the occurrence of coarse aluminum-rich oxides that deteriorate fatigue limit and machinability.The preferred limit for the presence of Aluminium is between 0.001% and 0.09% and more preferably between 0.001 and 0.03% Nickel is added to the present invention between 0.1% and 1% to increase the strength of the steel present invention and to improve toughness specially after quenching and tempering. Nickel is beneficial in improving its pitting corrosion resistance. A minimum of 0.1% is required to get such effects. Nickel is added into the steel composition to decreases the diffusion coefficient of carbon in the austenite thereby promoting the formation of martensite. But the presence of nickel content above 1% lowers the martensite start temperature hence leading to the excessive stabilization of residual austenite thereby having a detrimental impact on tensile strength and yield strength. It is preferred to have nickel between 0.1% and 0.9% in the steel of present invention Chromium is present between 0.2% and 1.5% in the steel of present invention. Chromium is an essential element that provide strength to the steel by solid solution strengthening and a minimum of 0.2% is required to impart the strength but when used above 1.5% increase the hardenability is beyond an acceptable limit due the formation of coarse cementite after cooling thereby impairing the formability as well as the ductilty of the steel. Chromium addition also decreases the diffusion coefficient of carbon in the austenite same as nickel hence promote 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%. Phosphorus is content of the steel of present invention is between 0 % and 0.09%. Phosphorus tends to segregate at the grain boundaries or co-segregate with Manganese. For these reasons, it is recommended to use phosphorus as less as possible. Specifically, content over 0.05% can cause rupture by intergranular interface decohesion which may be detrimental for the fatigue limit. The preferred limit for Phosphorus content is between 0% and 0.05%. Sulphur is contained between 0 % and 0.09%. Sulphur forms MnS precipitates which improve the machinability and assists in obtaining a sufficient machinability. During metal forming processes such as rolling and forming, deformable manganese sulfide (MnS) inclusions become elongated. Such elongated MnS inclusions can have considerable adverse effects on mechanical properties such as striction and impact toughness if the inclusions are not aligned with the loading direction. Therefore sulfur content is limited to 0.09%. A preferable range the content of Sulphur is 0 % between 0.05% and more preferably between 0% and 0.02%to obtain the best balance between machinability and fatigue limit. Nitrogen is in an amount between 0% and 0.09% in steel of present invention. Nitrogen is limited to 0.09% to avoid ageing of material and to minimize the precipitation of Aluminum nitrides during solidification which are detrimental for mechanical properties of the steel. Nitrogen also forms nitrides and carbonitrides with vanadium titanium and niobium to impart strength to the steel of present invention. Molybdenum is an optional element and may be present between 0 % and 0.5% in the present invention. Molybdenum is added to impart hardenability and hardness to steel by forming Molybdenum based carbides and also delays the appearance of Bainite hence promote the formation of Martensite. However, the addition of Molybdenum excessively increases the cost of the addition of alloy elements, so that for economic reasons its content is limited to 0.5%. The preferred limit for molybdenum content is between 0% and 0.4% and more preferably between 0% and 0.2%. Vanadium is an optional element for the present invention and is content is between 0% and 0.2%. Vanadium is effective in enhancing the strength of steel by precipitation strengthening especially by forming carbides or carbo-nitrides. Upper limit is kept at 0.2% due to the economic reasons. Niobium is present in the Steel of present invention between 0% and 0.1% and suitable for forming carbo-nitrides to impart strength of the Steel of present invention by precipitation hardening. Niobium will also impact the size of microstructural components through its precipitation as carbo-nitrides and by retarding the recrystallization during heating process. Thus, finer microstructure formed at the end of the holding temperature and as a consequence after the complete austenitzation lead to the hardening of the product. However, Niobium content above 0.1% is not economically interesting as well as forms coarser precipitates which are detrimental for the fatigue properties of the steel and also when the content of niobium is 0.1% or more niobium is also detrimental for steel hot ductility resulting in difficulties during steel casting and rolling. Titanium is an optional element and present between 0% and 0.1%. Titanium forms titanium nitrides which impart steel with strength, but these nitrides may form during solidification process, therefore have a detrimental effect fatigue limit. Hence the preferred limit for titanium is between between 0% and 0.05%. Copper is a residual element and may be present up to 1% due to processing of steel. Till 0.5% copper does not impact any of the properties of steel but over 0.5% the hot workability decreases significantly. Other elements such as Tin, Cerium, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Tin ≦0.1%, Cerium ≦0.1%, Magnesium ≦ 0.10%, 0% ≦ Boron ≦ 0.008%and Zirconium ≦ 0.10%. Up to the maximum content levels indicated, these elements make it possible to refine the grain during solidification. The remainder of the composition of the Steel consists of iron and inevitable impurities resulting from processing. The microstructure of the Steel comprises: Martensite constitutes between 75% and 98% of the microstructure by area fraction. The martensite of the present invention can comprise both fresh and tempered martensite. However, fresh martensite is an optional microconstituent which is preferably limited in the steel at an amount of between 0% and 10%, preferably between 0 and 8% and even better if less than 5%. Fresh martensite may form during cooling after tempering. Tempered martensite is formed from the martensite which forms during the cooling after annealing and particularly after below Ms temperature and more particularly between Ms-10°C and 20°C.Such martensite is then tempered during the holding at a tempering temperature especially when Tempered between 250°C and 500°C. The martensite of the present invention imparts strength and fatigue endurance to steel. Preferably, the content of martensite is between 80% and 97% and more preferably between 85% and 95%. Residual Austenite is a microstructural constituent that is present between 2% and 20% in the steel. Residual Austenite toughness and ductility to the steel of present invention. The preferable limit of for the presence of Austenite is between 3% and 18% and more preferably between 4 % and 16%. The cumulated amount of ferrite and bainite represents between 0% and 5% of the microstructure. The cumulative presence of bainite and ferrite does not affect adversely to the present invention till 5% but above 5% the mechanical properties may get impacted adversely. Hence the preferred limit for the cumulative presence ferrite and bainite is kept between 0% and 4% and more preferably between 0% and 3%. Bainite forms during the reheating before tempering. Bainite can impart formability to the steel but when present in a too big amount, it may adversely impact the tensile strength of the steel. Ferrite may form during the first step of cooling after annealing but is not required as a microstructural constituent. Ferrite formation must be kept as low as possible and preferably less than 2% or even less than 1%. A leaf spring according to the invention can be produced by any suitable manufacturing process, with the stipulated process parameters explained hereinafter. A preferred exemplary method is demonstrated herein but this example does not limit the scope of the disclosure and the aspects upon which the examples are based. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible ways in which the various aspects of the present disclosure may be put into practice. A preferred method consists in providing a semi-finished casting of steel with a chemical composition according to the invention. The casting can be done in any form such as ingots or blooms or billets which is capable of being manufactured or processd into a leaf spring having leafs that can have a cross section up to 60mm*100mm. For example, the steel having the above-described chemical composition is casted in to a billet and then rolled in form of a bar. This bar can act as a semi-finished product for further mechanical operations. Multiple rolling steps may be performed to obtain the desired semi-finished product. In order to prepare for the steel to be manufactured as a leaf for the leaf spring, the semi-finished product can be used directly at a high temperature after rolling or may be first cooled to room temperature and then reheated for manufacturing the leaf. The semi-finished product is reheated between temperature Ac3 and Ac3 +300° C, preferably between Ac3+30°C and Ac3 +300°C where it is held during 5 seconds to 1200 seconds to ensure homogenous temperature across the cross section of the semi-finished product as well as to ensure 100% austenite is formed. If the reheating temperature of the semi-finished product is lower than Ac3, excessive load is imposed on tools of mechanical operation such as dies during the forming operation or milling tool during tapering and, further, the temperature of the steel may also decrease below the Ferrite transformation start temperature that will lead to the ferrite formation in the final product which is detrimental for fatigue and mechanical properties. Additionally the metallurgical transformation under strain can lead to significant change in the obtained microstructure for a given cooling rate or a given chemical composition. As a result, the obtained microstructure will be completely different from the targeted one and so the mechanical properties. Therefore, the temperature of the semi-finished product is preferably sufficiently high so that all the mechanical operations are performed and completed in the 100% austenitic temperature range. Reheating at temperatures above Ac3 +300°C must be avoided because they are industrially expensive and can lead to the occurrence of liquid areas that will affect the forming and tapering of the steel. Then the semi-finished is subjected to at least one mechanical manufacturing operation between Ac3 and Ac3 +300° C. Mechanical operation may comprise tapering, eye rolling, die forming or any other suitable mechanical operation or manufacturing procedure that is required to form the hot leaf of a leaf spring from semi finished product. The preferred temperature for all the mechanical operations is between Ac3 +30° C and Ac3 +300° C and more preferable temperature for all the mechanical operations is between Ac3 +50° C and Ac3 +250° C. A final mechanical operation temperature must be kept above Ac3 and this is preferred a structure that is favorable to recrystallization and mechanical manufacturing. It is preferable to have all the mechanical operation especially the final mechanical operation to be performed at a temperature greater than Ac3 +50°C, because below this temperature the steel exhibits a significant drop in the mechanical manufacturability. Steel ductility below the Ac3 temperature will be strongly deteriorated. It can lead to issues regarding the final dimension of the leaf as well as a deterioration of the surface aspect. It can even provoke cracks or a full failure of the leaf of the leaf spring. The semi-finished product may be cooled to room temperature after any mechanical operation performed on the semi-finished product and then reheated to temperature between Ac3 and Ac3 +300° C for a subsequent mechanical operation. Multiple cooling and reheating between mechanical operations may be performed to obtain the desired hot leaf of the leaf spring. After the completion of the mechanical operations a hot leaf of the leaf spring is obtained and then hot leaf of the leaf spring is cooled. The cooling of the hot leaf of leaf springs is done down to a quenching temperature in a range fromMs-10° C to 20° C, herein also referred as QT, at an average cooling rate below 50°C/s and preferably below 40°C/s and more preferably below 38°C/s. The preferred QT temperature range is from Ms-50° C to 20° C. During this step the martensite form especially when the the hot leaf of the leaf spring is cooled after crossing the Ms temperature. Thereafter from temperature QT, the hot leaf of the leaf spring is heated up to a tempering temperature herein referred as TT in a range from 250°C to 500°C for tempering the hot leaf of the leaf spring,at an average heating rate between 0.5°C/s and 150°C/s and more preferably from 0.6°C/s to 100°C/s. The hot leaf of the leaf spring is held at TT temperature during 10seconds to 10000s. The preferred TT temperature range is from 300°C to 475°C. During this step, the martensite is tempered and will transform into tempered martensite. Thereafter the hot leaf of the leaf spring is brought to room temperature from TT, wherein the average cooling rate between TT and room temperature is kept below 5°C/s and preferably 4°C/s and more preferably below 2°C/s. These average cooling rates are chosen to perform homogenous cooling across the cross-section of the hot leaf of the leaf spring. Upon cooling to the room temperature, the leaf of the leaf spring is obtained. EXAMPLES The following tests, examples, figurative exemplification and tables which are presented herein are non-restricting in nature and must be considered for purposes of illustration only and will display the advantageous features of the present invention. Forged mechanical part made of steels with different compositions is gathered in Table 1, where the forged mechanical part is produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathers the microstructures of the forged mechanical part obtained during the trials and table 4 gathers the result of evaluations of obtained properties. Table 1

Table 2 Table 2 gathers the process parameters implemented on semi-finished product made of steels of Table 1. The trials I1 to I4 serve for the manufacture of forged mechanical part according to the invention. This table also specifies the reference forged mechanical parts which are designated in table from R1 to R2. The table 2 is as follows:

I = according to the invention; R = reference; underlined values: not according to the invention. Table 3 Table 3 exemplifies the results of the tests conducted in accordance with the standards on different microscopes such as Scanning Electron Microscope for determining the microstructures of both the inventive and reference steels in terms of area fraction. The results are stipulated herein:

I = according to the invention; R = reference; underlined values: not according to the invention. Table 4 Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, tests are conducted in accordance of NF EN ISO 6892-1 standards. Tests to measure the toughness and fatigue for both inventive steel and reference steel are conducted in accordance of EN ISO 148-1 standard KCU specimen with U-notch at toom temperature. The results of the various mechanical tests conducted in accordance to the standards are gathered Table 4

I = according to the invention; R = reference; underlined values: not according to the invention.

Claims

CLAIMS 1. A steel for leaf spring comprising of the following elements, expressed in percentage by weight: 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%; and can contain one or more of the following optional elements 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%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel comprising, by area percentage, 75% to 98% of Martensite, 2% to 20% of Residual Austenite, with a cumulative optional presence of bainite and ferrite between 0% to 5%. 2. Steel for leaf spring according to claim 1, wherein the composition includes 1.

2% to 2.4% of Silicon.

3. Steel for leaf spring according to claim 1 or 2, wherein the composition includes 0.45% to 0.6% of Carbon.

4. Steel for leaf spring according to claim 1 to claim 3, wherein the composition includes 0.001 % to 0.09% of Aluminum.

5. Steel for leaf spring according to anyone of claim 1 to 4, wherein the composition includes 0.6% to 1.4% of Manganese.

6. Steel for leaf spring according to anyone of claim 1 to 5, wherein the composition includes 0.3% to 1.4% of Chromium.

7. Steel for leaf spring according to anyone of claim 1 to 6, wherein the composition includes 0.1% to 0.9% of Nickel.

8. Steel for leaf spring according to anyone of claims 1 to 7, wherein, the Martensite is between 80% and 97%.

9. Steel for leaf spring according to anyone of claims 1 to 8, wherein, the Residual Austenite is between 3% and 18%.

10. Steel for leaf spring according to anyone of claims 1 to 9, wherein, the cumulative optional presence of bainite and ferrite is between 0% and 4%.

11. Steel for leaf spring according to anyone of claims 1 to 10, wherein, the presence of ferrite is between 0% and 1%.

12. Steel for leaf spring according to anyone of claims 1 to 11, wherein, the Ultimate tensile strength is greater than 1650 MPa.

13. Steel for leaf spring according to anyone of claims 1 to 12, wherein said steel has hardness of 480Hv or more.

14. Steel for leaf spring according to anyone of claims 1 to 13, wherein said steel has a fatigue endurance of at least 120000 cycles when tested at minimum stress of 1100 MPa.

15. Steel for leaf spring according to anyone of claims 1 to 14, wherein said steel has striction equal to more than 25%.

16. A method of production a leaf of a leaf spring of steel comprising the following successive steps: - providing a steel composition according to anyone of claims 1 to 7 in form of semi-finished product; - reheating said semi-finished product to a temperature between Ac3 and Ac3 +300°C; - performing one or more mechanical operations on the said semi-finished product in the austenitic range wherein the mechanical operation finishing temperature shall be between Ac3 and Ac3 +300°C to obtain a hot leaf of a leaf spring; - cooling said hot leaf of a leaf spring down to a temperature QT which is in a range from Ms-10°C to 20°C at a cooling rate less than 50°C/s - thereafter heating the hot leaf of a leaf spring at an average heating rate between 0.5°C/s and 150°C/s from QT to a temperature TT which is in a range from 250°C to 500°C - then said hot leaf of a leaf spring is held at a temperature TT during 10 seconds to10000seconds, - then said hot leaf of a leaf spring is cooled, at an average cooling rate below 5°C/s, from TT to room temperature to obtain a leaf of a leaf spring.

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

18. A method according to anyone of claims 16 or 17,wherein the temperature TT is from 300°C to 475°C.

19. A method according to anyone of claims 16 to 18 , wherein the temperature QT is from Ms-50°C to 20°C.

20. Use of a steel according to anyone of claims 1 to 19 or of a mechanical part produced according to the method of claims 16 to 19, for the manufacture of structural or safety parts of a vehicle or an engine.

21. Vehicle comprising a part obtained according to claim 20.