Method for manufacturing steel parts and steel parts

Abstract

A method for manufacturing steel parts with improved resistance to hydrogen embrittlement. [Solution] By weight, the composition includes 0.05%≦C≦0.15%, 0.01%≦Si≦1%, 1.2%≦Mn≦2%, 0.1%≦Cr≦2%, 0.001≦Al≦0.1%, 0.003%≦N≦0.01%, 0≦S≦0.015%, 0≦P≦0.015%, 0%≦Ni≦1%, 0%≦B≦0.01%, 0%≦Mo≦1.0%, 0%≦Ti≦0.04%, 0%≦Nb≦0.1%, 0≦V≦0.5%, with the remainder being iron and non-metallic. A method for manufacturing steel parts made of impurities, comprising annealing a semi-finished product at an annealing temperature strictly lower than the Ac1 temperature of the steel, cooling it to room temperature, cold forming the semi-finished product to obtain a cold-formed product, subjecting the cold-formed product to a heat treatment including heating to a heat treatment temperature of Ac3 or higher, the complete austenitization temperature of the steel, quenching to room temperature, and optionally reheating the product at a holding temperature of 180°C to 400°C for 15 minutes to 2 hours.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing assembly parts such as screws and bolts commonly used for vehicle chassis or wheel hub components in the automotive industry by cold forming, particularly through cold forging.

Background Art

[0002] As is known, the automotive industry is constantly aiming to reduce vehicle weight. This can be achieved by modifying its safety device assembly. Weight reduction increasingly requires a reduction in the size of these parts. However, these parts still receive the same mechanical stress and therefore must have increasingly high mechanical properties, particularly tensile strength.

[0003] WO2016 / 158470 is an age-hardening steel that is excellent in machinability before aging treatment and excellent in fatigue characteristics, toughness, and low-cycle fatigue characteristics after aging treatment. That is, it contains predetermined amounts of C, Si, Mn, S, Cr, Al, V, Nb, Ca, and REM, restricts the contents of P, Ti, and N to predetermined amounts or less, has Fe and the remainder of impurities, and has an area ratio of bainite structure of 70% or more. However, the steel of WO2016 / 158470 lacks hydrogen embrittlement.

[0004] WO 2011 / 124851 relates to a mechanical steel component in steel with high properties. Its composition, in weight percent, is 0.05% ≤ C ≤ 0.25%, 1.2% ≤ Mn ≤ 2%, 1% ≤ Cr ≤ 2.5% (where the contents of C, Mn, and Cr satisfy (830 - 270C% -90Mn% - 70Cr%) ≤ 560), 0 < Si ≤ 1.55%, < Ni ≤ 1%, 0 < Mo ≤ 0.5%, < Cu ≤ 1%, 0 < V ≤ 0.3%, 0 < Al ≤ 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 component is iron and impurities resulting from processing. The microstructure of the steel is bainite, and it contains a total of 20% or less of martensite and / or primary crystal ferrite and / or pearlite. However, the steel of WO 2011 / 124851 does not exhibit hydrogen embrittlement and a cross-sectional reduction rate of 58% or more.

[0005] However, it is desirable to further improve the resistance of the component to hydrogen embrittlement.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] Therefore, the object of the present invention is to be used as an assembled component of an automobile, to have improved resistance to hydrogen embrittlement, and at the same time, to have the following: - A tensile strength of 1100 MPa or more, preferably exceeding 1150 MPa, or even more preferably exceeding 1180 MPa, - A yield strength of 880 MPa or more, preferably 900 MPa or more. - Total elongation of 12% or more, preferably 13% or more. - Hydrogen embrittlement index less than 0.09, preferably 0.08 or less. - Cross-sectional reduction rate of more than 58%, preferably 60% or more, more preferably 62% or more. The objective is to provide a steel part having the following properties.

[0008] In a preferred embodiment, the steel component exhibits a hardness of 360 Hv to 405 Hv. [Means for solving the problem]

[0009] The present invention can be better understood by reading the following description, which is given merely as an example.

[0010] Throughout this patent application, the content is expressed in weight percent (wt%).

[0011] The steel component according to the present invention is by weight, 0.05% ≤ C ≤ 0.15% 0.01% ≤ Si ≤ 1% 1.2% ≤ Mn ≤ 2% 0.1% ≤ Cr ≤ 2% 0.001 ≤ Al ≤ 0.1% 0.003% ≤ N ≤ 0.01% 0 ≤ S ≤ 0.015% 0 ≤ P ≤ 0.015% Optional 0% ≤ Ni ≤ 1% 0% ≤ Mo ≤ 1.0% 0% ≤ Nb ≤ 0.1% 0% ≤ Ti ≤ 0.04% 0 ≤ V ≤ 0.5% 0% ≤ B ≤ 0.01% It has a composition that includes [a certain substance], with the remainder consisting of iron and unavoidable impurities. [Modes for carrying out the invention]

[0012] Carbon is present in the steel of the present invention at a concentration of 0.05% to 0.15%. Carbon imparts strength to the steel through solid solution strengthening, and carbon is gamma-genetic, thus delaying ferrite formation. Carbon is an element that influences the formation of cementite-free lath-like bainite. A minimum of 0.05% carbon is required to achieve a tensile strength of 1100 MPa, but if carbon is present at a concentration exceeding 0.15%, it reduces the ductility and machinability of the final product by forming cementite. The carbon content is advantageously in the range of 0.08% to 0.14%, more preferably 0.09% to 0.14%, to obtain both high strength and high ductility simultaneously.

[0013] Silicon is present in the steel of the present invention at a concentration of 0.01% to 1%. Silicon imparts strength to the steel of the present invention through solid solution strengthening. In particular, at the above content, silicon has the effect of hardening the bainite microstructure through solid solution hardening. Silicon reduces the formation of cementite nuclei because it hinders the controlled precipitation and diffusion growth of carbides by forming a Si-enriched layer around the precipitation nuclei. Therefore, cementite-free lath-like bainite is obtained. Silicon also acts as a deoxidizing agent. A minimum of 0.01% silicon is required to impart strength to the steel of the present invention. At amounts exceeding 1%, the activity of carbon in the austenite increases, promoting the transformation to proterite ferrite, which can lead to decreased strength, slower bainite formation during continuous cooling, and excessive residual austenite at the end of cooling. The preferred limit for silicon is 0.01 to 0.9%, more preferably 0.01 to 0.5%.

[0014] Manganese is added to the base steel at 1.2% - 2%. Manganese imparts hardenability to the steel. Manganese enables the reduction of the critical cooling rate to obtain bainite transformation by continuous cooling without prior transformation. Manganese lowers the bainite start temperature of the steel, thus resulting in the refinement of the bainite structure, forming lath bainite, and accordingly enhancing the mechanical properties of the parts. A minimum content of 1.2 wt% is necessary to obtain the desired bainite fine structure. However, when it exceeds 2%, retained austenite can transform into MA islands or fresh martensite, and since these phases are harmful to this property, manganese has an adverse effect on the steel of the present invention. Furthermore, manganese forms sulfides such as MnS. These sulfides can enhance machinability when their shape and distribution are well controlled. Otherwise, the sulfides can have a very harmful effect on elongation. The preferred limit of manganese is 1.3% - 1.9%, more preferably 1.4% - 1.9%.

[0015] Chromium is present in the steel of the present invention at 0.1% - 2%. Chromium is an essential element for producing a bainite structure, especially lath bainite, and imparting elongation and ductility to the steel of the present invention. The addition of chromium promotes a homogeneous and finer bainite fine structure in the temperature range between Bs and room temperature. A minimum content of 0.1% of chromium is required to produce the targeted bainite fine structure. Chromium also slows down the softening during tempering, enabling a higher holding temperature that not only degasses but also favors the formation of carbides that trap hydrogen. However, the presence of a chromium content of 2% or more excessively increases the hardness of the steel, making it difficult to form the steel by cold forming, especially cold forging. It is advantageous to have 0.2% - 1.6%, more preferably 0.3% - 1.4% of chromium.

[0016] The content of aluminum in the steel is 0.001 wt% to 0.1 wt%. Aluminum is a deoxidizer for molten steel. Aluminum contributes to the control of austenite grain coarsening during hot rolling in the form of nitride. On the other hand, if present in too large amounts, it may cause coarsening of aluminosilicate inclusions in the steel, which can be shown to impair the properties of the steel, especially its toughness. In particular, the aluminum content can be included at a content of 0.001 to 0.09 wt%.

[0017] In the steel of the present invention, the nitrogen content is 零% to 0.01 wt%. Nitrogen captures boron by forming boron nitride, which invalidates the role of this element in the hardenability of the steel. Therefore, in the steel according to the present invention, the nitrogen content is limited to 0.01 wt%. Nevertheless, when added in small amounts, it is possible to avoid excessive coarsening of austenite grains during the heat treatment received by the steel, especially by forming titanium nitride (TiN) and aluminum nitride (AlN). Similarly, nitrogen also enables the formation of carbonitride precipitates that contribute to the capture of hydrogen in this case. Therefore, in the steel according to the present invention, the nitrogen content is 0.003 wt% or more.

[0018] The steel according to the present invention contains a maximum of 0.015 wt% of phosphorus and a maximum of 0.015 wt% of sulfur. The effects of phosphorus and sulfur are particularly harmful in the steel according to the present invention for several reasons. In fact, since these elements are the drawbacks of hydrogen recombination, they cause a higher concentration of atomic hydrogen that can penetrate the material, and thus an increased risk of delayed fracture of the parts during use. Also, by segregating at grain boundaries, phosphorus and sulfur reduce its cohesion. Therefore, their content must be kept very low. For this purpose, measures must be taken to ensure that the steel is dephosphorized and desulfurized during melting in the liquid state.

[0019] The steel may optionally contain from 0.01 to 1 wt% of nickel. This element brings about an increase in the strength of the steel and has a beneficial effect on the resistance to brittle fracture. Nickel also improves the corrosion resistance of the steel by known methods.

[0020] Boron is an optional element and can be present in alloys at a content of 0.0003–0.01 wt%. By segregating at prior austenite grain boundaries, boron, even at very low concentrations, can strengthen grain boundaries and increase resistance to hydrogen-induced delayed fracture. Boron increases the cohesiveness of grain boundaries not only due to its inherent effect but also by making phosphorus segregation at these grain boundaries more difficult. Boron further enhances the hardening properties of steel, and therefore makes it possible to limit the carbon content required to obtain the desired bainite microstructure. Finally, boron acts synergistically with molybdenum and niobium, thereby increasing the effectiveness of these elements and their own influence, which is tolerated at their respective content levels. However, excess boron (above 0.01 wt%) will lead to the formation of brittle iron bocarbide.

[0021] Molybdenum is an optional element, present in amounts ranging from 0.003 to 1% by weight. Molybdenum strongly interacts with phosphorus, limiting its detrimental effects by restricting its segregation at prior austenite grain boundaries. Furthermore, molybdenum exhibits significant carbide formation behavior. Given mechanical properties, molybdenum allows for higher holding tempering temperatures, which is favorable for the formation of carbides that act as hydrogen traps. Therefore, molybdenum is an element that enhances resistance to delayed fracture.

[0022] Titanium is an optional element and is present in alloys at a content of 0.01–0.04 wt%. Titanium is added to molten steel to increase the hardness of the material. Within the range shown here, titanium also enhances delayed fracture resistance in several ways. Titanium contributes to the refinement of austenite grains and forms precipitates that trap hydrogen. Finally, the hardening effect of titanium allows tempering operations to be carried out at higher holding temperatures. Here, the maximum titanium content is set to avoid obtaining precipitates that are too large in size, which would reduce the steel's resistance to delayed fracture.

[0023] The steel of the present invention may optionally contain niobium in a content of 0.01 to 0.1% by weight. Niobium can, on the one hand, limit the formation of borocarbide Fe3(C,B), thereby improving hydrogen resistance. 23 (C,B) 26 This consumes the "free" boron content available for segregation at grain boundaries, thus reducing it, while simultaneously limiting the growth of austenite grains by forming carbonitrides. Refinement of the grains increases the overall length of the grain boundaries, thus resulting in a better distribution of harmful elements such as phosphorus and sulfur at lower concentrations. Furthermore, the reduction in austenite grain size leads to an acceleration of the bainite transformation dynamics. The maximum niobium content is set to avoid obtaining precipitates that are too large in size, which would reduce the steel's resistance to delayed fracture. Moreover, if niobium is added in excessive amounts, it increases the risk of "crack" defects on the surface of continuously cast billets and blooms. If these defects cannot be completely eliminated, they can prove very detrimental to the integrity of the properties of the final part, particularly with respect to fatigue strength and hydrogen resistance. This is why the niobium content is maintained below 0.1% by weight.

[0024] Furthermore, optionally, the steel may contain vanadium at a content of 0.5% by weight or less. In the presence of vanadium, its hardening effect allows for tempering operations at higher temperatures. The maximum vanadium content is set to avoid obtaining precipitates that are too large, which can reduce the steel's resistance to delayed hydrogen decomposition. In particular, the vanadium content may be between 0.05% and 0.5% by weight.

[0025] The remainder of the composition consists of iron and unavoidable impurities, particularly those arising from refining.

[0026] More specifically, the composition of the steel parts consists of the elements listed above.

[0027] The steel parts according to the present invention are, more specifically, cold-formed steel parts, and more specifically, cold-headed steel parts.

[0028] The steel component has a microstructure that includes, in terms of surface fraction or area percentage, at least 80% bainite, and a cumulative presence of 1% to 25% retained austenite and martensite.

[0029] Bainite is present in the steel according to the present invention as a matrix phase, conferring strength to such steel. Bainite is present in the steel in an area fraction of at least 80 area %, preferably 80-95 area %, and more preferably 85-95 area %. Bainite is formed during quenching. Such bainite may include cementite-free lath-like bainite and lower bainite. Cementite-free lath-like bainite consists of bainite in the form of lath, and the number N of interlath carbides greater than 0.1 micrometers per unit surface area is 50,000 / mm 2 These laths contain carbides, as described below. This cementite-free lath-like bainite structure imparts good hydrogen resistance to the steel of the present invention. The lower bainite consists of bainite in the form of laths and contains fine iron carbide sticks precipitated within the laths. The lower bainite structure provides elongation and tensile strength to the steel of the present invention. Both the lower bainite and cementite-free lath-like bainite lath structures allow for a better distribution of hydrogen, which tends to sequester such an improved distribution of hydrogen that may be present within the bainite regions of the microstructure, and thus enhances hydrogen resistance.

[0030] Retained austenite and martensite are cumulatively present in the steel according to the present invention at an area fraction of 1% to 25%. Martensite is formed from unstable austenite formed during annealing, and then during cooling after soaking. Martensite is composed of fine, elongated laths in one direction within each grain that emerges from the primary austenite grains. <111> Fine iron carbide sticks, 50-200 nm in length, precipitate between the laths in the direction. Martensite imparts ductility and strength to the steel of the present invention. However, if the cumulative presence of martensite and retained austenite exceeds 25%, it imparts excessive strength because the martensite has the same carbon content as retained austenite, and therefore fresh martensite is brittle and hard, but it reduces the elongation beyond the allowable limit of the steel of the present invention. The preferred limit for the cumulative presence of retained austenite and martensite in the steel of the present invention is 4%-22%, more preferably 4%-20%.

[0031] The steel components according to the present invention can be advantageously used as components for chassis and wheel hub applications. In particular, these steel components can be used as bolts and screws for such applications, for example, as chassis bolts, hub bearing bolts, and rim hub bolts.

[0032] The diameter of the steel component is, for example, 22 mm or less, more specifically 20 mm or less, and even more specifically 16 mm or less. More specifically, the diameter of the steel component is, for example, 5.5 mm or more.

[0033] The steel parts mentioned above include, for example: - Process for providing semi-finished steel products, - This semi-finished product is annealed at an annealing temperature strictly lower than the Ac1 temperature of steel. - Cold forming process to turn semi-finished products into cold-formed products. - A process of subjecting a cold-formed product to heat treatment to obtain a cold-formed steel part, wherein the heat treatment consists of the following steps: - A process of heating a cold-formed product to a heat treatment temperature above the complete austenitization temperature of steel (Ac3), - The process of hardening to room temperature, - Optional, - A step of holding the product at a holding temperature of 100℃ to 400℃ for 15 minutes to 2 hours. Heat treatment process including, It can be obtained using a method that includes [a specific method].

[0034] The semi-finished products provided during the supply process are as follows by weight: 0.05% ≤ C ≤ 0.15% 0.01% ≤ Si ≤ 1% 1.2% ≤ Mn ≤ 2% 0.1% ≤ Cr ≤ 2% 0.001 ≤ Al ≤ 0.1% 0.003% ≤ N ≤ 0.01% 0 ≤ S ≤ 0.015% 0 ≤ P ≤ 0.015% at will 0% ≤ Cu ≤ 1% 0% ≤ Ni ≤ 1% 0% ≤ Mo ≤ 1.0% 0% ≤ Nb ≤ 0.1% 0% ≤ Ti ≤ 0.04% 0 ≤ V ≤ 0.5% 0% ≤ B ≤ 0.01% It contains the following components, with the remainder consisting of iron and unavoidable impurities.

[0035] This composition corresponds to the composition described above for steel components.

[0036] Semi-finished products are, in particular, wires having diameters ranging from 5mm to 25mm.

[0037] As described above, the annealing process is carried out at an annealing temperature that is strictly lower than the steel's Ac1 temperature. Conventionally, the Ac1 temperature is the temperature at which austenite begins to form during heating.

[0038] The annealing process aims to prepare steel for cold forming by temporarily reducing its tensile strength. For example, at the end of the annealing process, the steel will have a tensile strength of 600 MPa or less. This type of annealing is called globuization (or spherodization) annealing.

[0039] More specifically, during the annealing process, the semi-finished product is heated to an annealing temperature of Ac1-20°C or higher.

[0040] During the annealing process, the semi-finished product is preferably held at the annealing temperature for a time selected as a function of the annealing temperature such that the tensile strength of the steel after annealing is 600 MPa or less. For example, the holding time at the annealing temperature is 5 to 9 hours.

[0041] According to a specific example, the annealing process is carried out at an annealing temperature equal to 720°C, and the holding time at the annealing temperature is equal to 5 hours.

[0042] The annealing process is preferably carried out in a neutral atmosphere, for example, in an atmosphere consisting of nitrogen gas.

[0043] After being held at the annealing temperature, the semi-finished product is cooled to room temperature.

[0044] Cooling is preferably carried out at a rate selected to avoid pearlite precipitation and bainite formation, and thus maintain a tensile strength of 600 MPa or less after cooling. This cooling rate can be easily determined using the CCT diagram of the steel.

[0045] According to a specific example, cooling from the annealing temperature is carried out in three stages. These are: a first cooling stage from the annealing temperature to approximately 670°C at a cooling rate of 25°C / hour or less; a second cooling stage from approximately 670°C to approximately 150°C at a cooling rate of 250°C / second or less; and a third cooling stage from approximately 150°C to ambient temperature at a cooling rate corresponding to cooling in ambient or natural air. These three cooling stages and corresponding temperatures and rates are given as examples only, and different temperatures and rates may be used depending particularly on the composition of the steel and the desired final tensile strength.

[0046] The cold forming process is, for example, a cold heading process, and a cold heading product is obtained at the end of the cold forming process, and a cold heading steel part is obtained at the end of the heat treatment.

[0047] This method optionally includes a step between the annealing step and the cold heading step in which the annealed semi-finished product is cold-drawn so that its diameter is reduced. This cold-drawing step is specifically a wire drawing step.

[0048] Preferably, a surface preparation step is performed to clean the surface of the semi-finished product before the cold stretching step, and then a step is performed to form a lubricating film on the surface of the semi-finished product.

[0049] The cleaning process includes, for example, degreasing and / or mechanical or chemical descaling or pickling, and optionally subsequent neutralization. In this context, neutralization is a cleaning process used to remove all foreign particles or substances from the steel surface to reduce the risk of corrosion.

[0050] The process of forming a lubricating film includes, for example, phosphate treatment and washing with soap.

[0051] After cold forming, the cold-formed steel parts are as follows: - A process of heating a cold-formed product to a heat treatment temperature of Ac3 or higher, the temperature at which steel is fully austenitized. - The process of hardening to room temperature, - Next, optionally, the product is held at a holding temperature within 100°C to 400°C for 15 minutes to 2 hours. It is subjected to heat treatment including [a specific process].

[0052] This optional heat treatment is tempering.

[0053] In one example, the product is held at a holding temperature in a furnace during the holding process. In another example, the product can be held at a holding temperature by immersion in a molten salt bath.

[0054] After the holding process is complete, the product is cooled to ambient temperature in ambient air or natural air.

[0055] The heating process is carried out so that the steel component has an overall austenite microstructure at the end of the heating process.

[0056] The average size of the austenite grains formed during this heating process is 20 μm or less, particularly within the 8-15 μm range. This size is measured, for example, at a magnification of 500:1.

[0057] This small grain size is due to the presence of non-heat-treated elements in the steel that form precipitates capable of fixing grain boundaries, thus avoiding grain growth during the austenitization process. This austenite grain size is the prior austenite grain size of the cold-formed, quenched, and tempered steel part according to the present invention.

[0058] The heat treatment temperature is, for example, at least 50°C higher than the complete austenitization temperature (Ac3) of steel.

[0059] More specifically, during the heating process, the steel parts are held at the heat treatment temperature for a time ranging from 5 to 120 minutes.

[0060] Preferably, the holding temperature during the holding process is within the range of 200 to 380°C.

[0061] At the end of the holding process, cold-formed, more specifically cold-headed, and quenched steel parts are obtained.

[0062] The steel component obtained in this manner has the microstructure described above for the steel component. [Examples]

[0063] Laboratory tests were conducted on castings having chemical compositions I1 to I6 according to the present invention. R1 to R4 are reference steel compositions not according to the present invention.

[0064] [Table 1]

[0065] In Table 1 above, the composition is shown in weight percent, and underlined values ​​are not according to the present invention.

[0066] In all of the above compositions, the remainder of the composition consists of iron and unavoidable impurities.

[0067] The steel of the present invention and the reference steel were reheated at 1150°C, and then hot-rolled in the form of 16 mm diameter wires at a finishing temperature exceeding 800°C. Subsequently, all wire rods (semi-finished products) of the steel of the present invention and the reference steel were held at a temperature of 720°C for 5 hours, and then annealed by cooling. Cooling was carried out in three stages, including cooling to 670°C at a cooling rate of 25°C / hour, followed by cooling to 150°C at 250°C / hour, and finally cooling to room temperature with natural air or ambient air. These cooling rates were obtained by appropriately adjusting the heating conditions in the annealing furnace, and heating was reduced or stopped as necessary, in a manner known to those skilled in the art. Ac1 and Ac3 for both the steel of the present invention (I1~I6) and the reference steel (R1~R3) were calculated by expansion measurement studies.

[0068] Subsequently, the cold-formed steel parts are subjected to heating and quenching heat treatment according to Table 2.

[0069] [Table 2]

[0070] In Table 2, the underlined values ​​are not in accordance with the present invention.

[0071] <Table 3: Mechanical properties> Tensile tests were performed directly on the wire. The tensile tests were conducted according to standard NF EN ISO 6892-1, i.e., at a crosshead speed of 8 mm / min. Each value is the average of three measurements.

[0072] A hardness profile was created along the cross-section of the sample. A Vickers hardness test was performed under a 30 kg load for 15 seconds. Hardness was measured according to the standard NF EN ISO 6507-1. Each value is the average of three measurements.

[0073] The results of these tests are summarized in Table 3 below.

[0074] Furthermore, the microstructure of the obtained products was analyzed based on cross-sections of these products. More specifically, the microstructure present in the cross-sections was characterized by optical microscopy (LOM) and scanning electron microscopy (SEM). LOM and SEM observations were performed after etching with a Nital-containing solution.

[0075] The results of these analyses are summarized in Table 4 below.

[0076] In Table 3, the following abbreviations are used. TS (MPa) refers to the tensile strength measured by a tensile test in the longitudinal direction relative to the rolling direction. YS (MPa) refers to the yield strength measured by a longitudinal tensile test in the direction of rolling. RA(%) refers to the percentage of the reduction in cross-sectional area measured by a longitudinal tensile test relative to the rolling direction. EI (%) refers to the total elongation measured by a longitudinal tensile test in the direction of rolling. HV30 refers to the result of the hardness measurement.

[0077] [Table 3]

[0078] <Table 4: Hydrogen embrittlement results> For each of experiments I1-I6 and R1-R3, the hydrogen resistance of the corresponding samples was determined according to the NF A-05-304 standard, first for the unfilled samples, and then for the hydrogen-filled samples, by 10 -5 s -1 This was determined by comparing the results of low-strain-rate tensile tests performed on smooth test specimens subjected to the specified strain rates.

[0079] More specifically, the inventors determined the ductility of the filled and unfilled samples (through the percentage decrease in area Ra) and compared the results using the embrittlement index.

[0080] The total H2 content inside the sample before filling was approximately 0.3 ppm.

[0081] Hydrogen charging is performed using an electrolyte consisting of 1N H2SO4, with the addition of 2.5 mg / L of thiourea, a hydrogen accelerator, and a current density I = 0.8 mA / cm². 2 Cathode charging was performed for 5 hours using [a specific method / tool].

[0082] For each pair of samples (filled and unfilled), the embrittlement index IRa with respect to the percentage of area reduction is calculated using the following formula. IRa = 1 - [RA(H2) / RA(H2=0)], where RA(H2) corresponds to the percentage reduction in area measured in the hydrogen-filled sample, and RA(H2=0) corresponds to the percentage reduction in area measured in the unfilled sample.

[0083] A brittleness index close to 1 indicates that the grade is highly sensitive to hydrogen embrittlement. A brittleness index IRa of less than 0.09 is considered satisfactory from the standpoint of the desired application, and a brittleness index IRa of 0.08 or less is favorable for the desired application.

[0084] The inventors further observed the fracture surface mode in all cases.

[0085] The results of these tests are summarized in Table 4.

[0086] [Table 4]

[0087] As can be seen from Table 4 above, the ductility of the steel of the present invention is not significantly affected by hydrogen.

[0088] Steels with compositions I1 to I6 exhibit higher hydrogen resistance than reference grades R1 to R4 after quenching.

[0089] Comparing samples I1-I6, which have a bainite content of 80% or more as shown in Table 5, with samples R1-R4, which have a martensitic microstructure, it can be seen that the bainite structure is less susceptible to hydrogen embrittlement than the martensitic structure.

[0090] Finally, it can be observed that the samples according to the present invention (I1-I6) absorb less hydrogen than the comparative samples (R1-R4) under the same packing conditions.

[0091] Therefore, these experiments demonstrate that the steel components according to the present invention are particularly well suited to the applications described above and that they possess very good mechanical properties, especially good tensile strength, related to improved resistance to hydrogen embrittlement compared to conventional steel components.

[0092] The method according to the present invention further has the advantage of allowing the use of conventional cold forming tools after annealing and enabling the acquisition of sufficiently low tensile strength to reduce wear, while simultaneously yielding a final part with high tensile strength (1100 MPa or more).

[0093] <Table 5: Microstructure> The microstructure of the steel was characterized using optical microscopy (LOM) and scanning electron microscopy (SEM) after 2% nital etching. Quantitative X-ray analysis was performed to determine the proportion of retained austenite.

[0094] [Table 5]

Claims

1. A method for manufacturing steel parts, wherein the method is as follows: - A process for providing a semi-finished product made of steel, wherein the steel is by weight, 0.05% ≤ C ≤ 0.15% 0.01% ≤ Si ≤ 1% 1.2% ≤ Mn ≤ 2% 0.1% ≤ Cr ≤ 2% 0.001 ≤ Al ≤ 0.1% 0.003% ≤ N ≤ 0.01% 0 ≤ S ≤ 0.015% 0 ≤ P ≤ 0.015% Optional 0% ≤ Ni ≤ 1% 0% ≤ B ≤ 0.01% 0% ≤ Mo ≤ 1.0% 0% ≤ Ti ≤ 0.04% 0% ≤ Nb ≤ 0.1% 0 ≤ V ≤ 0.5% The supply process includes, with the remainder consisting of iron and unavoidable impurities. - A step of annealing this semi-finished product at an annealing temperature that is strictly lower than the Ac1 temperature of the steel, - The process of cooling it to room temperature, - A cold forming process to turn the aforementioned semi-finished product into a cold-formed product. - A step of subjecting the cold-formed product to heat treatment, wherein the heat treatment consists of the following steps: - A step of heating the cold-formed product to a heat treatment temperature of Ac3 or higher, the complete austenitization temperature of the steel. - The process of hardening to room temperature, - Optionally, a step of reheating the product at a holding temperature of 180°C to 400°C for 15 minutes to 2 hours. Heat treatment process including, Methods that include...

2. The method according to claim 1, wherein during the heating step of the heat treatment, the cold-formed product is heated to a heat treatment temperature at least 50°C higher than the complete austenitization temperature Ac3 of the steel.

3. The method according to claim 1 or claim 2, wherein the annealing temperature is Ac1 minus 20°C or higher.

4. The method according to any one of claims 1 to 3, wherein the semi-finished product is a wire having a diameter of 5 mm to 25 mm.

5. The method according to any one of claims 1 to 4, further comprising preparing the surface of the semi-finished product, which includes a step of cleaning the surface of the semi-finished product and a step of forming a lubricating film on the surface of the semi-finished product, prior to the cold forming step.

6. The method according to claim 5, wherein the step of forming a lubricating film on the surface of the semi-finished product includes the steps of phosphate treatment and washing with soap.

7. The method according to any one of claims 1 to 6, wherein the carbon content of the steel is included in 0.08 to 0.14% by weight.

8. The method according to any one of claims 1 to 7, wherein the manganese content of the steel is 1.3 to 1.9% by weight.

9. The method according to any one of claims 1 to 8, wherein the chromium content of the steel is 0.2 to 1.6% by weight.

10. The method according to any one of claims 1 to 9, wherein the cold forming step is a cold heading step.

11. The method according to any one of claims 1 to 10, wherein the product is maintained at the holding temperature by immersing it in a molten salt bath during the holding step.

12. A steel part made of an alloy, wherein the alloy is by weight, 0.05% ≤ C ≤ 0.15% 0.01% ≤ Si ≤ 1% 1.2% ≤ Mn ≤ 2% 0.1% ≤ Cr ≤ 2% 0.001 ≤ Al ≤ 0.1% 0.003% ≤ N ≤ 0.01% 0 ≤ S ≤ 0.015% 0 ≤ P ≤ 0.015% Optional 0% ≤ Ni ≤ 1% 0% ≤ B ≤ 0.01% 0% ≤ Mo ≤ 1.0% 0% ≤ Ti ≤ 0.04% 0% ≤ Nb ≤ 0.1% 0 ≤ V ≤ 0.5% It contains, and the remainder consists of iron and unavoidable impurities. A steel component having a microstructure containing at least 80 area percent of bainite and a cumulative presence of 1 to 25 area percent of martensite and retained austenite, and having a tensile strength of 1100 MPa or more.

13. The steel component according to claim 12, wherein the martensite of the steel has rod-shaped iron carbide, and the length of the rod is 50 to 200 nm.

14. The steel part according to any one of claims 12 or 13, wherein the steel part has a hardness of 360 HV to 405 HV.

15. A steel component according to any one of claims 12 to 14, having a hydrogen embrittlement index of less than 0.

09.

16. A steel part according to any one of claims 12 to 15, having a cross-sectional reduction ratio of more than 58%.

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