A steel for cold forming and the method of manufacturing of a cold formed steel part thereof

Abstract

A steel for cold forming comprising of the following elements, expressed in percentage by weight:0.03% ≦ C ≦ 0.18%;1.2 % ≦ Mn ≦ 2.5%;0.05% ≦ Si ≦ 1.5 %;0.01% ≦ Cr ≦ 2%;0.001% ≦ Al ≦ 0.1%;0.01% ≦ Nb ≦ 0.12%;0.001% ≦ Ti ≦ 0.1%;0.003% ≦ Mo ≦ 1%;S ≦ 0.015%;P ≦ 0.015%;N ≦ 0.01%; 0% ≦ Ni ≦ 1%;0 ≦ B ≦ 0.01%; 0% ≦ V ≦ 0.5%;0% ≦ Mg ≦ 0.010%;0% ≦ Zr ≦ 0.010%;0% ≦ Ce ≦ 0.1%;0% ≦ Ca ≦ 0.005%;0% ≦ Cu ≦ 1%;the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel having microstructure comprising in area fraction, 40% to 90% of Granular Bainite, 10% to 40% of Martensite- Austenite islands and the amount of niobium present as carbides, nitrides and / or carbo-nitrides is equal to or more than 50% of the nominal niobium content present in the steel.

Description

[0001] A steel for cold forming and the method of manufacturing of a cold formed steel part thereof

[0002]

[0001] The present invention relates to a steel for cold forming, in particular a steel for manufacturing of mechanical part by plastic deformation via cold heading such as the assembly parts of wheel spindle, ball joints, screws, bolts or any other fasteners that the automotive industry commonly uses for chassis, engine or wheel hub components of vehicles.

[0003]

[0002] As is known, the automotive industry continually aims to decrease the vehicle weight; which can be done by modifying their safety assembly. The weight reduction requires an increasingly reduced size of the parts. These parts, however, remain subject to the same mechanical stresses, and must therefore have increasingly high mechanical properties, in particular tensile strength.

[0004]

[0003] To produce these parts, it is necessary to ensure the deformability to perform a first heat treatment for softening the steel and, at the end of the production another heat treatment to reach the high mechanical properties such as to have a tensile strength of more than or equal to 1000MPa. These treatments lead to CO2emissions. This is the reason which necessitates to have a steel that would enable automobile manufacturers to obtain parts with adequate in-use properties such as a tensile strength of more than or equal to 1000MPa, which can be used for the intended application without having to undergo heat treatment to modify their metallurgical structure after the plastic deformation operation.

[0005]

[0004] WO2016 / 158470 is Age hardening steel excellent in machinability before aging treatment and excellent in fatigue characteristics, toughness, and low cycle fatigue characteristics after aging treatment, that is, age hardening steel containing predetermined amounts of C, Si, Mn, S, Cr, Al, V, Nb, Ca, and REM, limiting contents of P, Ti, and N to predetermined amounts or less, having a balance of Fe and impurities, having an area ratio of bainite structures of 70% or more. But the steel of WO2016 / 158470 does not demonstrate yield strength of 900 MPa or more.

[0005] WO2011 / 124851 is a mechanical steel part in steel with high characteristics, characterized in that its composition, comprising in weight percentages, is 0.05%^C^0.25%; 1.2%^Mn^2%; 1%^Cr^2.5%; wherein the contents of C, Mn and Cr are such that (830-270C %-90 Mn %-70Cr %)^560; 0< Si^1.55; 0< Ni^1%; 0< Mo^0.5%; 0< Cu^1%; 0< V^0.3%; 0< AI^0.1%; 0< B^0.005%; 0< Ti^0.03%; 0< Nb^0.06%; 0< S^0.1%; 0< Ca^0.006%; 0< Te^0.03%; 0< Se^0.05%; 0< Bi^0.05%; 0< Pb^0.1%; the remainder of the steel part being iron and impurities resulting from processing, and wherein the in that its structure of the steel is bainitic and contains no more than a total of 20% of martensite and / or pro-eutectoid ferrite and / or pearlite. But the steel of WO2011 / 124851 does not demonstrate yield strength of 900 MPa or more.

[0006]

[0006] However, it is also desirable for these parts have adequate resistance to hydrogen embrittlement.

[0007]

[0007] Therefore, an aim of the invention is to provide a steel cold forming to obtain a cold formed steel part which may be used as an assembly part for a motor vehicle, and while simultaneously having properties according to ISO 898 class 8.8 to 10.9 simultaneously:

[0008] - A tensile strength greater than or equal to 1000 MPa and preferably above 1050 MPa,

[0009] - a yield strength greater than or equal to 900MPa and preferably greater than or equal to 950MPa

[0010] - a total elongation of 7% or more and preferably 9% or more.

[0011] - a reduction in area of more than 45% and preferably above 50% or more and more preferably 55% or more.

[0012]

[0008] In a preferred embodiment, the steel part shows a hardness from 360Hv to 405Hv.

[0013]

[0009] The invention will be better understood upon reading the description that follows, given solely by way of example. In the entire patent application, the elemental compostion is indicated in weight % (wt%).

[0014]

[0010] Carbon is present in the steel of present invention from 0.03% to 0.18%. Carbon imparts strength to the steel by solid solution strengtheningand carbon is gammagenous hence delays the formation of Ferrite. At this level, C is used to obtain granular bainite with the targeted mechanical properties, which gives good deformability during cold froming. A minimum of 0.03% of carbon is required to reach the tensile strength but if carbon is present above 0.18%, carbon deteriorates ductility as well as machinability of the final product due to the formation of carbides. The carbon content is advantageously in the range 0.04% to 0.16% to obtain simultaneously high strength and high ductility and more preferably from 0.06% to 0.14%.

[0015]

[0011] Silicon is present in the steel of present invention from 0.05% to 1.5%.

[0016] Silicon imparts the steel of present invention with strength through solid solution strengthening. In particular, at the above-mentioned contents, the silicon has the effect of hardening the bainite microstructure through solid solution hardening. Silicon reduces the formation of cementite nucleation as silicon hinders precipitation and diffusion-controlled growth of carbides by forming a Si-enriched layer around precipitate nuclei and delays carbide precipitation, in particular during bainite formation. Therefore, resulting the granular bainite. Silicon also acts as a deoxidizer. A minimum of 0.05% of silicon is required to impart strength to the steel of present invention. An amount of more than 1.5 % raises the activity of carbon in austenite promoting its transformation into pro-eutectoid ferrite, which can deteriorate the yield strength, and also resulting retardation for formation of bainite under continuous cooling thereby too much retained austenite at the end of cooling. The preferred limit for Silicon from 0.08 to 1.4% and more preferably from 0.1% to 1.3%

[0017]

[0012] Manganese is added in the present steel from 1.2% to 2.5%.

[0018] Manganese provides hardenability to the steel. As a strong austenite stabilizer, It allows to decrease the critical cooling rate for which a bainitic transformation can be obtained in continuous cooling without any prior transformation and the manganese lowers the bainite start temperature of the steel, and therefore results in a refinement of the bainitic grain size and thus increases the strength and ductility of the part. A minimum content of 1.2% by weight is necessary to obtain the desired bainitic microstructure. But above 2.5 %, manganese has a negative effect on the steel of present invention asretained austenite can transform into MA islands or fresh martensite and these phases are detrimental for the properties. In addition, manganese forms sulphides such as MnS. These sulphides can increase machinability if the shape and distribution are well controlled. If not, they might have a very detrimental effect on toughness. The preferred limit of manganese is from 1.3% to 2.2 % and more preferably from 1.4% to 2%.

[0019]

[0013] Chromium is present from 0.01% to 2% in the steel of present invention. Chromium is added for its hardening effect. Chromium is an indispensable element in order to obtain the targeted microstructure over a wide range of cooling rates. A minimum content of 0.01% of Chromium is required to produce the targeted bainitic microstructure and the formation of carbides that trap hydrogen. But the presence of Chromium content of 2% or more excessively increases the hardness of the steel, it makes it difficult to form it by cold forming, and in particular cold heading. It is advantageous to have Chromium from 0.03% to 1.5% and more preferably from 0.04% to 1.4%.

[0020]

[0014] Aluminum is present from 0.001% to 0.1%. Aluminum is a deoxidizer of the steel in the liquid state. It then contributes, in the form of nitrides, to controlling austenitic grain coarsening during hot rolling. On the other hand, present in too large an amount, it may lead to a coarsening of aluminate type inclusions in the steel which may prove damaging to the properties of the steel, especially its toughness. In particular, the aluminum content may be comprised at a content from 0.001 to 0.09%.

[0021]

[0015] The steel of present invention contains niobium from 0.01 to 0.12%.

[0022] Niobium improves the mechanical properties such as yield strength. In the present invention, niobium starts forming precipitates at temperature more than 840°C in the austenite region which limit the austenite grain size growth kinetics. It also forms nitrides and carbo-nitrides which enhance the steel strength of the steel of present invention as well as it can limit the formation of borocarbides Fe3(C,B) ; Fe23(C,B)26which consume “free” boron content available for segregation at the grain boundaries, therefore in a better distribution of harmful elements, such as phosphorous and sulfur, in lower concentration. It cannot be added to higher content than 0.12% to prevent the coarsening of niobium precipitates that can act as nuclei for ferritetransformation leading to the occurrence of ferrite in the microstructure and thus reducing the tensile strength and yield strength. In addition, content of 0.12% or more niobium is also detrimental for steel hot ductility resulting in difficulties during steel casting and rolling. The preferred limit for niobium is from 0.02% to 0.11% more preferably 0.04% and 0.1%.

[0023]

[0016] Titanium is an essential element and present in the steel from 0.001 to 0.1%. Titanium is added to the liquid steel in order to increase the hardness of the material. Here, within the ranges indicated, it also increases the delayed fracture resistance in several ways. It contributes to austenitic grain refinement and forms precipitates with nitrogen to increase the strength of the steel of present invention and also may act as trap for hydrogen. Titanium is also used to fix nitrogen and thus protect boron. Therefore, the titanium content is preferably greater than 3.5 *N, where N is the nitrogen content of the steel. The maximum titanium content is set here in order to avoid obtaining precipitates of too large a size which would then degrade the toughness of the steel. The preferred limit for titanium is from 0.001 % to 0.09% more preferably 0.001% to 0.06%.

[0024]

[0017] The molybdenum is an essential element and is comprised from 0.003% to 1%. Molybdenum interacts strongly with phosphorus and limits the damaging effect of the phosphorus by limiting its segregation at the prior austenite grain boundaries. Furthermore, molybdenum improves the hardenability of the steel and further facilitates the formation of lower bainite, lowering the temperature at which this structure appears, resulting in bainite resulting in good impact resistance. It is therefore an element that increases the resistance to delayed fracture. The preferred limit for molybdenum is from 0.03% to 0.5% more preferably 0.05% to 0.3%.

[0025]

[0018] In the steel according to the invention, the nitrogen content is from 0% to 0.01%. Nitrogen traps boron via the formation of boron nitrides, which makes the role of this element in the hardenability of the steel ineffective. Therefore, in the steel according to the invention, the nitrogen content is limited to 0.01 wt%. Nevertheless, added in small amounts, it makes it possible, via the formation in particular of titanium nitrides (TiN) and aluminum nitrides (AIN), to avoid excessive austenitic grain coarsening during hot rollingundergone by the steel. Therefore, in the steel according to the invention, the nitrogen content is preferably greater than or equal from 0.003% to 0.01%.

[0026]

[0019] In the steel according to the invention, the Phosphorus content is from 0% to 0.015%. In the steel according to the invention, the sulfur content is from 0% to 0.015%. The effect of phosphorus and sulfur are particularly harmful in the steels according to the invention, for several reasons. Indeed, since these elements are poisons for hydrogen recombination, they contribute to a higher concentration of atomic hydrogen capable of penetrating into the material, therefore to an increased risk of delayed fracture of the part in use. Moreover, by segregating at the grain boundaries, the phosphorus and the sulfur reduce the cohesion thereof. Their content must therefore be kept very low. For this purpose, measures must be taken to ensure that the steel is dephosphorization and desulfurized during its melting in the liquid state.

[0027]

[0020] The steel may optionally contains from 0.01% to 1% of nickel. This element provides an increase in the strength of the steel and has beneficial effects on the resistance to brittle fracture. It also improves, in a known manner, the corrosion resistance of the steel.

[0028]

[0021] Boron is an optional element and can be present in the steel up to 0.01% and preferably from 0.0003% to 0.01% whenever added. By segregating at the prior austenitic grain boundaries, boron, even at very low contents, strengthens the grains boundaries, and makes it possible to increase the resistance to hydrogen-induced delayed fracture. The boron increases the cohesion of the grain boundary via its intrinsic effect, but also by making phosphorus segregation more difficult at these grain boundaries. The boron further strongly increases the hardenability of the steel and thus makes it possible to limit the carbon content needed to obtain the desired bainitic microstructure. Finally, boron acts in synergy with molybdenum and niobium, thus increasing the effectiveness of these elements and their own influence that their respective contents permit. An excess of boron that is above 0.01% would however lead to the formation of brittle iron boro-carbides.

[0029]

[0022] Further optionally, the steel may comprise vanadium at a content lower than or equal to 0.5%. When it is present, it increases the strength thanks to its hardening effect and contributes to grain refinement. The maximumvanadium content is set to avoid obtaining precipitates of too large size which might degrade the resistance of the steel to delayed hydrogen fracture. In particular, the vanadium content may be comprised at a content from 0.05% to 0.5%.

[0030]

[0023] The rest of the composition is iron and unavoidable impurities, in particular resulting from the elaboration. Other elements such as Cerium, Calcium, Copper, Boron, Magnesium or Zirconium can be added individually or in combination in the following proportions by weight: Cerium ^0.1%, Calcium ^0.005%, Copper^1% Magnesium

[0031]

[0032] 0.010% and Zirconium

[0033]

[0034] 0.010%. 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.

[0035]

[0024] More particularly, the composition of the steel part consists of the above-mentioned elements.

[0036]

[0025] The steel part according to the invention is more particularly a cold formed steel part, and more particularly a cold headed steel part.

[0037]

[0026] The steel part according to the invention is more particularly a cold formed steel part, and more particularly a cold headed steel part.

[0038]

[0027] The steel part has a microstructure comprising, in surface fractions or area%, of granular bainite from 60% to 90%, and martensite-austenite islands from 10% to 40%

[0039]

[0028] Granular Bainite is present in the steel according to the invention as a matrix phase and imparts strength to such steel. Granular Bainite is present in the steel from 60% to 90% by area fraction and preferably from 70% to 88% by area fraction and more preferably from 75% to 85%. Granular Bainite is formed during cooling after rolling due to a displacive transformation. Such granular bainite is composed of bainitic laths and sub-units of bainitic ferrite with islands of Residual austenite. The carbon remains in solid solution in the bainitic ferrite which assists in the stability the granular bainite during cold forming. The matrix demonstrates a density of boundaries with angles greater than 50°higher in comparison to lath-bainite. Granular bainite imparts the steel of present invention with yield strength and total elongation. The average grainsize for the granular bainite of the steel of present information is from 14micons to 30microns and preferably 15microns to 25microns and more preferably from 15microns to 20microns.

[0040]

[0029] Martensite Austenite islands (MA islands) are present in the steel of present invention from 10% to 40%. The MA islands consist of retained austenite at the periphery of the M / A island and of austenite partially transformed into martensite in the center of the M / A island. Hence the M / A islands are more ductile than the bainite areas of the microstructure and imparts striction and tensile strength properties simultaneously. MA islands of the present invention having preferably a diameter size above 0.5 pm have to obtain the required level of tensile strength for the present steel and more preferably above 0.5 pm or equal to 20 pm, and even more preferably from above 0.5 pm to 15 pm. In this context, "diameter" designates the largest dimension of the M / A island. The diameter of the M / A islands is in particular measured at a magnification of 500:1. The carbon content in the M / A islands is preferably greater than or equal to 1%. This particular carbon content is advantageous, since it stabilizes the retained austenite in the M / A islands against transformation into martensite specially during the cold forming. Preferred limit for the MA islands for the steel of present invention is from 12% to 35% and more preferably from 15% to 25%.

[0041]

[0030] In addition to this above-described microstructure of the steel, the steel of present invention, to achieve the targeted mechanical properties especially yield strength and tensile strength, must have a precipitation fraction of niobium equivalent to 50% or more, meaning that the weight amount of niobium present as carbides, nitrides and / or carbo-nitrides is equivalent to at least 50% of the nominal niobium content present in the steel. It is preferred the niobium equivalent above 60% and more preferably above 70%.

[0042]

[0031] Additionally, the steel of present invention in its preferred embodiments can have a titanium precipitation equivalent of at least 80% meaning that the amount of titanium present as carbides, nitrides and / or carbo-nitrides is equivalent to at least 80% of the nominal titanium content present in the steel. When such titanium equivalent level is reached, the mechanical properties, specifically tensile strength and yield strength, areimproved. It is preferred the titanium equivalent above 90% and more preferably above 95%.

[0043]

[0032] In addition to the above-mentioned microstructure, the microstructure of the steel is free from microstructural components such as ferrite, pearlite and cementite.

[0044]

[0033] The steel according to the invention may advantageously be used as parts for chassis, wheel hub applications. In particular, these steel parts may be used as bolts and screws or any other fastener for such applications, and for example chassis bolts, hub to bearing bolts, rim to hub bolts.

[0045]

[0034] The diameter of the steel part is lower than or equal to 25 mm, and more particularly lower than or equal to 22 mm, and even more particularly lower than or equal to 20 mm. More particularly, the diameter of the steel part is for example greater than or equal to 5 mm.

[0046]

[0035] A cold formed steel part according to the invention can be produced by any suitable method. 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 either into billet, bloom, ingots or continuously casted billet or blooms which are capable of being manufactured in a cold formed steel part having diameter from 5mm to 25mm in diameter.

[0047]

[0036] For example, the steel having the above-described chemical composition is casted into a bloom and then rolled in form of a bar which will act as a semi-finished product. Several operations of rolling can be achieved to obtain the desired semi-finished product from the bloom.

[0048]

[0037] The semi-finished product after the casting and rolling process can be used directly at a high temperature after the rolling or may be first cooled to room temperature and then reheated for wire hot rolling. Reheat the semifinished product from 1100° C to 1300° C.

[0038] The temperature of the semi-finished, which is subjected to wire hot rolling, is preferably at least 1100° C and must be below 1300°C because the temperature of the semi- finished product is lower than 1100° C, excessive load is imposed on wire rolling mill and, further, the temperature of the steel may decrease to a Ferrite transformation temperature during finishing wire rolling, whereby the steel will be rolling in a state in which transformed Ferrite contained in the structure. Therefore, the temperature of the semi-finished product is preferably sufficiently high so that complete wire hot rolling can be completed in the austenitic temperature range. Reheating at temperatures above 1300°C must be avoided because they are industrially expensive. The semi-finished product is subjected to wire hot rolling. The wire hot rolling may be done in single step rolling or multiple steps rolling.

[0049]

[0039] At the laying head exit of the finishing rolling mill of wire hot rolling temperature must be kept from 840°C to 1000°C and preferably from 850°C to 980°C and more preferably from 860°C to 960°C. At the laying head exit of wire hot rolling the niobium precipitates forms. Thereafter a hot wire rod is obtained having a diameter from 5mm to 25mm.

[0050]

[0040] Then, the hot wire rod is cooled down to a coiling temperature range T1 which is from 325°C to 20°C and preferably from 300°C to 25°C and more preferably from 275°C to 25°C while having a cooling rate from 0.1°C / s to 20°C / s and preferably the cooling rate is from 0.2°C / s to 15°C / s and more preferably the cooling rate is from 0.5°C / s to 10°C / s. The cooling is preferably performed at a cooling rate chosen so as to avoid the formation of pearlite and the formation of ferrite, and thus to maintain a tensile strength more than or equal to 750 MPa or more after cooling on the hot wire rod and to form the targeted granular bainite in steel of present invention are obtained and facilitate its subsequent cold forming. Thereafter the hotwire rod is coiled.

[0051]

[0041] Then the hot wire rod is uncoiled and then the uncoiled hot wire rod is subjected to a scale removal step to remove the scale formed during the wire hot rolling.

[0052]

[0042] Then the uncoiled hot wire rod is subjected to preparatory mechanical manufacturing operations. Preparatory mechanical operation may comprise of straightening, tapering, drawing, cutting, surface coating or any othersuitable mechanical operation or manufacturing procedure that is required to prepare the uncoiled hot wire rod for cold forming. The preferred temperature for all the mechanical operations is from 20° C and 225° C and more preferable temperature for all the mechanical operations is from 25° C and 200° C.

[0053]

[0043] Then subjecting the uncoiled hot wire to cold forming operation to obtain cold formed part. During the cold forming operation, the tensile strength of the present steel is increased due to work hardening above the targeted 1000MPa or more according to the present invention and formation of M / A islands in accordance of the present invention takes place due to the cold forming operation.

[0054]

[0044] Thereafter finishing mechanical operations are performed on the cold formed part to obtain a cold formed steel part, finishing mechanical operation may comprise of cleaning, coating wherein coating includes phosphate coating or any other chemical or electrochemical coating, tapering, deburring, cutting, honing, machining to nominal dimensions, burnishing, grinding polishing peening or any other finishing mechanical operation that may be required for obtaining the cold formed steel part ready to use in automotive industry.

[0055]

[0045] The thus obtained cold formed steel part has the microstructure described herein above.

[0056]

[0046] In addition, the steel of present invention or the cold formed steel part thus obtained does not require any heat treatment before or after cold forming step.

[0057]

[0047] EXAMPLES

[0058]

[0048] 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.

[0059]

[0049] The steel for cold forming with different compositions is gathered in Table 1, where the cold formed steel part is produced according to process parameters as stipulated in Table 2, respectively. Thereafter Table 3 gathersthe microstructures of the cold formed steel part obtained during the trials and

[0060] table 4 gathers the result of evaluations of obtained properties.

[0061]

[0050] Table 1: Chemical compositions of the castings

[0062] Steel

[0063] Sam C Mn Si Al Cr Ti Nb S P N Mo Cu Ni V B pies

[0064] 11 0.073 1.81 0.16 0.027 0.103 0.018 0.065 0.0067 0.006 0.0057 0.107 0.009 0.11 0.116 0.004 I2 0.074 1.82 0.159 0.026 0.105 0.026 0.06 0.0075 0.007 0.0047 0.109 0.009 0.22 0 0.0053 I3 0.127 1.72 0.157 0.027 0.051 0.018 0.063 0.0066 0.006 0.0048 0.2 0.014 0.1 0 0.0049 I4 0.128 1.7 0.159 0.026 0.051 0.0017 0.062 0.007 0.006 0.0046 0.108 0.009 0.11 0 0.0049 R1 0.215 1.32 0.33 0.036 0.248 0.0042 0 0.006 0.01 0.0009 0.016 0.095 0.049 0 0.0035 R2 0.083 1.6 0.98 0.02 0.028 0 0 0.004 0.005 0.0028 0.002 0.041 0.04 0 0

[0065]

[0066] R3 0.231 0.85 0.058 0.037 0.278 0.036 0 0.005 0.012 0.005 0.008 0.011 0.025 0.003 0.0031

[0067]

[0051] In the above Table 1, the compositions are indicated in wt% and the

[0068] 5 underlined values are not according to the invention.

[0069]

[0052] In all of the above compositions, the remainder of the composition consists of iron and unavoidable impurities.

[0070]

[0053] Table 2 - Process parameters

[0071] 10

[0054] All the steels underwent hot wire rolling top obtain a diameter of 15.5mm and all the steels were uncoiled and underwent a cutting mechanical operation.

[0072] Cooling

[0073] laying head

[0074] Reheating rates Coiling

[0075] Steel exit

[0076] temperature after temperature

[0077] Sample temperature

[0078] (°C) exit (°C)

[0079] (°C)

[0080] (°C / s)

[0081] 11 1190 870 1 25

[0082] I2 1190 920 1 25

[0083] I3 1210 900 1 25

[0084] I4 1200 900 1 25

[0085] R1 1139 796 1 25

[0086] R2 1050 805 5 25

[0087]

[0088] R3 1150 900 1 25

[0089]

[0055] The underlined values are not according to the invention in Table 2. 15

[0056] Table 3: Microstructure

[0057] The results of these tests are summarized in Table 3 below.

[0090]

[0058] Furthermore, the microstructure of the obtained products was analyzed based on cross-sections of these products. More particularly, the structures present in the cross-sections were characterized by light optical microscopy (LOM) and by scanning electron microscopy (SEM). The LOM and SEM observations were performed after etching using a Nital containing solution.

[0091]

[0059] The presence of MA islands is determined with an image analysis done through a software named METALIA using the target method with an average taken from 10 images with 65targets / image after a LEPERA colour etching.

[0092]

[0060] The determination of micro alloying elements content (Nb and Ti) in precipitates is based on an electrolytic extraction followed by optical emission spectroscopy analysis (ICP-OES). The selective extraction of precipitates is carried out with an electrolyte made of lithium chloride and salicylic acid salts diluted in methanol. Methanol is preferred to other solvent (water or ethanol) to prevent oxidation and ensure an efficient filtration. Steel samples are submitted to a current density that allows only the matrix to dissolve. After this electrolytic operation, the obtained solution is filtered on 200 nm polycarbonate membrane. Strong acids (perchloric, nitric and hydrofluoric) mineralisation is performed on the filter and the solution is analysed with ICP- OES.Trials Ferrite+ Granular MA Nb Ti Granular Pearlite Bainite islands equivalent(%) equivalent(%) Average (%) (%) (%) grain size (microns) 11 0 83 17 75 100 16 I2 0 83 17 76 100 17 I3 0 83 17 84 100 15 I4 0 81 19 75 100 16 R1 97 0 3 0 Not 13

[0093] measured

[0094] R2 75 0 15 0 Not 14

[0095] measured

[0096] R3 100 0 0 0 Not 13

[0097] measured

[0098]

[0099]

[0061] the underlined values are not according to the invention.

[0100]

[0062] The titanium equivalent for the reference examples was not measured due to the presence of very high ferrite and pearlite.

[0101]

[0063] Table 4: Mechanical properties

[0102]

[0064] The final mechanical properties are obtained on the cold formed steel part and the properties of all the inventive steels that is 11 to I4 in accordance of ISO 898 class 8.8 to 10.9 without performing heat treatment.

[0103]

[0065] Tensile tests were performed directly on wire rods. The tensile testing was performed according to standard NF EN ISO 6892-1, i.e. with a cross head speed of 8 mm / min. Each value is the average of three measurements.

[0104]

[0066] A hardness profile along the cross section of the samples was performed. Vickers hardness tests were carried out under a load of 30 kg for 15 seconds durations. The hardness was measured according to standard NF EN ISO 6507-1. Each value is the average of three measurements.

[0105]

[0067] The results of these analyses are summarized in the following Table 4.

[0106]

[0068] In Tables 4, the following abbreviations are used:

[0069] TS (MPa) refers to the tensile strength measured by tensile test in the longitudinal direction relative to the rolling direction,

[0107]

[0070] YS (MPa) refers to the yield strength measured by tensile test in the longitudinal direction relative to the rolling direction,

[0108]

[0071] RA (%) refers to the percent reduction of area measured by tensile test in the longitudinal direction relative to the rolling direction,

[0109]

[0072] El (%) refers to the total elongation measured by tensile test in the longitudinal direction relative to the rolling direction.

[0110]

[0073] Table 3: Mechanical properties of the samples

[0111]

[0074]

[0112] TS (MPa) YS (MPa) El (%) RA (%) 11 1155 1010 11.1 55.6

[0113] I2 1076 960 11.6 61.5

[0114] I3 1150 1020 10 55.2

[0115] I4 1111 1010 10 55.7

[0116] R1 862 752 6.1 48

[0117] R2 770 741 48 72

[0118] R3 787 732 57 62

[0119]

[0120]

[0075] the underlined values are not according to the invention.

Claims

CLAIMS1. A steel for cold forming comprising of the following elements, expressed in percentage by weight:0.03% C 0.18%;1.2 % Mn 2.5%;0.05% Si 1.5 %;0.01% Cr ^ 2%;0.001% Al 0.1%;0.01% Nb 0.12%;0.001% Ti 0.1%;0.003% Mo 1%;S 0.015%;P 0.015%;N 0.01%;and can contain one or more of the following optional elements0% Ni 1%;0 B ^ 0.01%;0% g V I 0.5%;0% Mg 0.010%;0% Zr ^ 0.010%;0% Ce 0.1%;0% Ca 0.005%;0% Cu 1%;the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel having microstructure comprising in area fraction, 40% to 90% of Granular Bainite, 10% to 40% of Martensite-Austenite islands and the amount of niobium present as carbides, nitrides and / or carbo-nitrides is equal to or more than2. Steel for cold forming according to claim 1, wherein the composition includes 0.08% to 1.4% of Silicon.

3. Steel for cold forming according to claim 1 or 2, wherein the composition includes 0.04% to 0.16% of Carbon.

4. Steel for cold forming according to claim 1 to claim 3, wherein the composition includes 0.02 % to 0.11% of Niobium.

5. Steel for cold forming according to anyone of claim 1 to 4, wherein the composition includes 1.3% to 2.2% of Manganese.

6. Steel for cold forming according to anyone of claim 1 to 5, wherein the composition includes 0.03% to 1.5% of Chromium.

7. Steel for cold forming according to anyone of claims 1 to 6, wherein, the martensite-austenite islands content is from 12% to 35%.

8. Steel for cold forming according to anyone of claims 1 to 7, wherein the amount of niobium present as carbides, nitrides and / or carbo-nitrides is equal to or more than 60% of the nominal niobium content present in the steel.

9. Steel for cold forming according to anyone of claims 1 to 8, wherein said sheet has a tensile strength of 1000 MPa or more and an total elongation greater than or equal to 7%.

10. A method of production a cold formed steel part 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 1100°C to 1300°C;- hot wire rolling the said semi-finished product in the austenitic range wherein the laying head exit temperature of the finishing rolling mill is from 840°C to 1000°C to obtain a hot wire rod;- cooling the hot wire rod to a coiling temperature range T1 which is from 325°C to 20°C at a cooling rate from 0.1°C / s to 20°C / s- coiling the hot wire rod in a coiling temperature range T 1 which is from 325°C to 20°C- thereafter the hot wire rod is uncoiled and subjecting the uncoiled hot wire rod to a scale removal step- performing one or more preparatory mechanical manufacturing operations on the uncoiled hot wire rod in the temperature range from 20° C and 225° C- then subjecting the uncoiled hot wire to cold forming operation to obtain cold formed part- thereafter one or more finishing mechanical operations are performed on the cold formed part to obtain a cold formed steel part.

11. A method according to claim 10, wherein in the hot wire rod has a diameter from 5mm to 25mm.

12. A method according to claim 10 or 11, wherein the coiling temperature range T1 is from 300°C to 20°C.

13. A method according to anyone of claims 10 to 12, wherein the laying head exit of the finishing rolling mill is from 850°C to 980°C.

14. Use of a steel according to anyone of claims 1 to 9 or of a cold formed steel part produced according to the method of claims 10 to 13, to manufacture the assembly parts of wheel spindle, ball joints, screws, bolts or any other fasteners for chassis, engine or wheel hub components of vehicles.

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

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