Steel for coil springs of automobiles and a method of manufacturing of a coil thereof

Abstract

A steel for coil 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%; 0.4% ≦ Cr ≦ 1.2%; 0.001% ≦ Nb ≦ 0.1%; 0 ≦ P ≦ 0.09%; 0 ≦ S ≦ 0.09%; 0% ≦ N ≦ 0.09%; 0% ≦ Ni ≦ 1%; 0% ≦ Mo ≦ 0.5%; 0% ≦ V≦ 0.2%; 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, 92% to 100% of Martensite, 0% to 8% of Residual Austenite, with a cumulative optional presence of bainite and ferrite from 0% to 5% wherein all the microconstituent have a prior austenite grain size from 8 to16.

Description

[0001] STEEL FOR COIL SPRINGS OF AUTOMOBILES AND A METHOD OF MANUFACTURING OF A COIL THEREOF

[0001] The present invention relates to a steel suitable for manufacturing of a coil spring for automotive industry.

[0002] Coil springs for the automotive industry are generally manufactured as a part of suspension unit, a valve spring used in an engine or a clutch torsion spring used in a clutch for pick-up, trucks cars and other vehicles. Material for such manufacturing inherently faces the problem of inability to meet the dual requirement of good fatigue performance and having high level of tensile strength at same time to meet the requirements of the automotive industry for its structural parts. Further one another compulsory requirement for these materials is that they must have good formability and good fatigue performance so that they can be used to manufacture mechanical parts for automotive industry such as coil springs and chassis members.

[0003] Therefore, intense research and development endeavors are put in to develop a material that is good in formability while having high yield strength that is above 1750 MPa along with a total elongation of 8%.

[0004] Earlier research and developments in the field of steels for coil springs for automotive industry 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:

[0005] EP3222746 is a steel for a suspension spring including, as a chemical composition, by % by mass: C: more than 0.40% and 0.65% or less; Si: 1.00% to 3.50%; Mn: more than 2.00% and 3.00% or less; Cr: 0.01% to 2.00%; V: 0.02% to 0.50%; P: 0.020% or less; S: 0.020% or less: N: 0.0100% or less; and a remainder of Fe and impurities, in which Kf is 280 or more, in which a structure contains a tempered martensite of which an area ratio is 90% or more, in which a Fe-based carbide precipitated in the tempered martensite is a cementite, and in which an average of an area ratio of prior austenite grains which stretch in a longitudinal direction of the steel and of which an aspect ratio exceeds 3.0, is 80% or more in a case of depths of 0.1 mm, 0.2 mm, and 0.3 mm in a sheet thickness direction from a surface of the steel. However, the steel of EP3222746 does not demonstrate total elongation or reduction of in area.

[0006] Therefore, in the light of the publications mentioned above, the object of the invention is to provide a steel for coil spring, that makes it possible to obtain simultaneously having a tensile strength above 1900 MPa, a hardness above 480HV and a reduction of area of at least 30% or more.

[0007] Hence the purpose of the present invention is to solve these problems by making available a steel suitable for mechanical operations for manufacturing a coil spring that simultaneously have: − a tensile strength greater than or equal to 1900 MPa and preferably above 2050 MPa, − a yield strength greater than or equal to 1750 MPa and preferably above 1850 MPa, − a total elongation greater than or equal to 8.0 % and preferably a total elongation greater than or equal to 9%. − a reduction of area of at least 30% or more and preferably more than 40%.

[0008] Preferably, such steel has a hardness of 500 Hv or more and more preferably more than 510 Hv.

[0009] Preferably, such steel is suitable for manufacturing of a coil spring wherein the coil diameter a cross section from 5 mm to 50 mm and the steel is also suitable for other structural parts for automotive such as chassis members.

[0010] 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.

[0011] Carbon is present in the steel of present invention is from 0.4% to 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 is adversely impacted due to the formation of proeutectoid cementite during the cooling after hot rolling. Further excessive formation of proeutectoid cementite is also detrimental for mechanical operations on the coil spring such as punching, drilling, honing or grinding. The carbon content is advantageously in the range form 0.45% to 0.6% and more especially from 0.5% to 0.6%.

[0012] Manganese is added in the present steel from 0.5% to 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 from 0.6% to 1.4% and more preferably from 0.6% to 1.3%.

[0013] Silicon is present in the steel of present invention from 1% to 2.5%. Silicon imparts 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 from 1.1% to 2.4% and more preferably from 1.2% to 1.8%.

[0014] The content of Aluminum is from 0.001% to 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 controls 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 Aluminum is from 0.001% to 0.09% and more preferably from 0.001 to 0.03%.

[0015] Chromium is present from 0.4% to 1.2% 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 0.4% is required to impart the strength but when used above 1.2% increase the hardenability is beyond an acceptable limit due the formation of coarse cementite after cooling thereby impairing the formability as well as the ductility 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 from 0.5% to 1.1 % and more preferably from 0.6% to 1%.

[0016] Niobium is an essential element and present in the steel of present invention from 0.001% to 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 austenitization 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. The preferred limit for the presence of Niobium is from 0.001% to 0.09 % and more preferably from 0.001% to 0.07%.

[0017] Phosphorus is content of the steel of present invention is from 0 % to 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 0.09% can cause rupture by intergranular interface decohesion which may be detrimental for the fatigue limit for the coil spring when in operation. The preferred limit for Phosphorus content is from 0% to 0.05%.

[0018] Sulphur is contained from 0 % to 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 reduction in area and 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 from 0 % to 0.05% and more preferably from 0% to 0.02%to obtain the best balance between machinability and mechanical properties such as total elongation.

[0019] Nitrogen is in an amount from 0% to 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.

[0020] Nickel is added to the present invention from 0% to 1% to increase the strength of the steel present invention and to improve toughness and elongation 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 from 0.01% to 0.9% in the steel of present invention.

[0021] Molybdenum is an optional element and may be present from 0 % to 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 from 0% to 0.4% and more preferably from 0% to 0.2%.

[0022] Vanadium is an optional element for the present invention and is content is from 0% from 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.

[0023] Titanium is an optional element and present from 0% to 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 from 0% to 0.05%.

[0024] 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.

[0025] 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.

[0026] The microstructure of the steel comprises:

[0027] Martensite constitutes from 92% to 100% 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 from 0% to 10%, preferably from 0 to 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 from Ms-10°C to 20°C. Such martensite is then tempered during the holding at a tempering temperature especially when tempered from 380°C from 525°C. The martensite of the present invention imparts strength and fatigue endurance to steel. Preferably, the content of martensite is from 94% to 100% and more preferably from 96% to 100%.

[0028] Residual austenite is a microstructural constituent that is present from 0% to 8% in the steel. Residual austenite improves toughness and ductility to the steel of present invention. The preferable limit for the presence of austenite is from 0% to 6% and more preferably from 0 % to 4%.

[0029] The cumulated amount of ferrite and bainite represents from 0% to 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 from 0% to 4% and more preferably from 0% to 3%.

[0030] Bainite can form 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%.

[0031] The microstructure of the steel for coil spring has an average prior austenite grain size from 8 to 16 and preferably from 9 to 15 when measured by NF EN ISO 643 standard. Prior Austenite Grain Size from 8 to 16 is required to have adequate reduction in area and tensile strength simultaneously. The prior austenite grain size is ensured by the formation of precipitates of Niobium disperse at prior austenite grain boundaries. To obtain the balance between the tensile strength and reduction in area in the steel of present invention the preferred prior austenite grain size is from 9 to 15.

[0032] A coil spring according to the invention can be produced by any suitable manufacturing process, with the stipulated process parameters explained hereinafter.

[0033] 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.

[0034] 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 processed into a coil that can have a diameter cross section from 5mm to 50mm.

[0035] For example, the steel having the above-described chemical composition is casted in to a billet and then rolled in form of a bar or wire rod. For the purpose of illustration bar is demonstrated herein that can act as a semi-finished product for further mechanical operations. Multiple rolling steps may be performed to obtain the desired semi-finished product.

[0036] In order to prepare the steel to be manufactured in coil 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 coil spring.

[0037] The semi-finished product is reheated from temperature Ac3 to Ac3 +300° C, preferably from Ac3+30°C to Ac3 +300°C where it is preferably 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.

[0038] 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 or bending tools or tapering, spiral formation 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 mechanical properties such as tensile strength. 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, bending and any metal forming or removing process tapering of the steel.

[0039] Then the semi-finished is subjected to at least one mechanical manufacturing operation from Ac3 to Ac3 +300° C. Mechanical operation may comprise grinding forming, bending, coiling, tapering, eye rolling, die forming or any other suitable mechanical operation or manufacturing procedure that is required to form the hot coil spring from semi-finished product. The preferred temperature for all the mechanical operations is from Ac3 +30° C to Ac3 +300° C and more preferable temperature for all the mechanical operations is from Ac3 +50° C to Ac3 +250° C.

[0040] A final mechanical operation temperature must be kept at Ac3 or more 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 coil as well as a deterioration of the surface aspect. It can even provoke cracks or a full failure of the coil spring. The temperature from Ac3 to Ac3 +300° C and soaking for 100% austenite formation is selected to achieve the desired austenite grain size.

[0041] 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 from Ac3 to Ac3 +300° C for a subsequent mechanical operation. Multiple cooling and reheating between mechanical operations may be performed to obtain the desired hot coil spring. After the completion of the mechanical operations a hot coil spring is obtained and then hot coil spring is cooled.

[0042] The cooling of the hot coil springs is done down to a quenching temperature in a range from Ms-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 coil spring is cooled after crossing the Ms temperature.

[0043] Thereafter from temperature QT, the hot coil spring is heated up to a tempering temperature herein referred as TT in a range from 380°C to 525°C for tempering the hot coil spring, at an average heating rate from 0.5°C / s to 150°C / s and more preferably from 0.6°C / s to 100°C / s. The hot coil spring is held at TT temperature during 10seconds to 10000s. The preferred TT temperature range is from 400°C to 500°C. During this step, the martensite is tempered and will transform into tempered martensite.

[0044] Thereafter the hot coil spring is brought to room temperature from TT, wherein the average cooling rate from TT to 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 coil spring. Upon cooling to the room temperature, the coil spring is obtained.

[0045] EXAMPLES

[0046] 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.

[0047] Coil springs 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.

[0048] Table 1 Trials C Mn Si Al Cr Nb P S N Ac3Ms (°C)(°C) I1 0.579 0.677 1.600.0030.750.020.0120.008 0.005809 246I2 0.553 0.765 1.440.0030.680.020.012 0.01 0.0053 804 257 I3 0.540 0.667 1.510.0020.680.020.0130.0050.0063812 265I4 0.569 0.734 1.430.0020.680.010.0100.0110.0062 800 252 R1 0.553 0.765 1.440.0030.680.020.012 0.01 0.0053807 262R2 0.525 0.637 1.490.0020.670.020.012 0.01 0.0059 804 257 R3 0.540 0.667 1.510.0020.680.020.0130.0050.0063 813 265

[0049] Table 2

[0050] 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 coil spring according to the invention. This table also specifies the reference coil spring which are designated in table from R1 to R3. The table 2 is as follows: Holding at Mechanical Cooling TrialsReheating HeatingHolding CR T(°C)Reheating T operation rateQT (°C)rate (°C / s)TT (°C)at TT (s) (°C / s) (s) T (°C) (°C / s) I1 950 900 950 18 25 1.85 450 3600 0.2 I2 950 900 950 18 25 1.85 435 3600 0.2 I3 940 900 940 18 25 1.85 435 3600 0.2 I4 940 900 940 18 25 1.85 450 3600 0.2 R1 940 900 940 18 25 1.85 550 3600 0.2 R2 940 900 940 18 25 1.85 375 3600 0.2 R3 940 900 940 18 25 1.85 375 3600 0.2

[0051] I = according to the invention; R = reference; underlined values: not according to the invention.

[0052] Table 3

[0053] 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. Prior Austenite Grains size for all the inventive and Reference steel are measured as per NF EN ISO 643 standards. The results are stipulated herein: Trials Martensite PAGS (Prior Austenite Grain Total of Residual Austenite (%) Size according to NF EN ISO Bainite and Ferrite 643) I1100 12 0I2100 9.5 0I3100 10.1 0I4100 9.8 0R1100 6.7 0R2100 6.4 0R3100 5.9 0

[0054] I = according to the invention; R = reference; underlined values: not according to the invention.

[0055] Table 4

[0056] Table 4 exemplifies the mechanical properties of both the inventive steel and reference steels. In order to determine the tensile strength, yield strength, total elongation and reduction of area tests are conducted in accordance of NF EN ISO 6892-1 standards for both inventive steel and reference steel. The results of the various mechanical tests conducted in accordance to the standards are gathered.

[0057] Table 4 YS Reduction Total Trials UTS(MPa) in Area Elongation (MPa)(%) (%) I1 20811910 50.1 9.5I2 20711904 49.1 11.3I3 21281956 46.7 10.6I4 21301965 45.7 10.3R1 21121910 25.4 7R2 21061900 25 7.9R3 21101893 10.7 3.8

[0058] I = according to the invention; R = reference; underlined values: not according to the invention.

Claims

CLAIMS 1. A steel for coil spring comprising of the following elements, expressed in percentage by weight: 0.4% ≦ C ≦ 0.7%;0% ≦ N ≦ 0.09%; and can contain one or more of the following optional elements0% ≦ 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, 92% to 100% of Martensite, 0% to 8% of Residual Austenite, with a cumulative optional presence of bainite and ferrite from 0% to 5% wherein all the microconstituent have a prior austenite grain size from 8 to 16 when measured as per NF EN ISO 643 standard.

2. Steel for coil spring according to claim 1, wherein the composition includes 1.1% to 2.4% of Silicon.

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

4. Steel for coil spring according to claim 1 to claim 3, wherein the composition includes 0.001 % to 0.09% of Aluminum. Steel for coil spring according to anyone of claim 1 to 4, wherein the composition includes 0.6% to 1.4% of Manganese.

6. Steel for coil spring according to anyone of claim 1 to 5, wherein the composition includes 0.5% to 1.1% of Chromium.

7. Steel for coil spring according to anyone of claims 1 to 6, wherein, the Martensite is from 94% and 100%.

8. Steel for coil spring according to anyone of claims 1 to 7, wherein, the Ultimate tensile strength is greater than 1900 MPa.

9. Steel for coil spring according to anyone of claims 1 to 8, wherein said steel has reduction of area of 30% or more.

10. A method of production a coil spring of steel comprising the following successive steps: - providing a steel composition according to anyone of claims 1 to 6 in form of semi-finished product. - reheating said semi-finished product to a temperature from Ac3 to 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 from Ac3 to Ac3 +300°C to obtain a hot coil spring - cooling said hot coil 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 coil spring at an average heating rate from 0.5°C / s to 150°C / s from QT to a temperature TT which is in a range from 380°C to 525°C - then said hot coil spring is held at a temperature TT for 10 seconds to10000 seconds, - then said hot coil spring is cooled, at an average cooling rate below 5°C / s, from TT to room temperature to obtain a coil spring.

11. A method according to claim 10, wherein the reheating temperature of the semi-finished product to is from Ac3+30°C to Ac3 +300°C.

12. A method according to anyone of claims 10 or 11, wherein the temperature TT is from 400°C to 500°C.

13. A method according to anyone of claims 10 to 12, wherein the temperature QT is from Ms-50°C to 20°C.

14. Use of a steel according to anyone of claims 1 to 9 or of a coil spring produced according to the method of claims 10 to 13, for the manufacture of automotive parts having coil spring.

15. Vehicle comprising a part obtained according to claim 14.

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