Hot worked and annealed graphitised steel for machining of components and method of manufacturing thereof

Abstract

This invention relates to graphitized steel products that have undergone heat treatment and annealing, comprising, by weight percentage: 0.70% ≤ C ≤ 1.1%, 0.35% ≤ Mn ≤ 0.80%, 1.0% ≤ Si ≤ 3.0%, 0.010% ≤ Ti ≤ 0.090%, 0.001% ≤ Al ≤ 0.09%, P ≤ 0.090%, S ≤ 0.2%, 0.0090 ≤ N ≤ 0.0150%, and optionally comprising one or more of the following elements by weight percentage: 0% ≤ B ≤ 0.006%, 0% ≤ Cr ≤ 1%. The composition comprises 0%≤Mo≤0.5%, 0%≤V≤0.2%, 0%≤Nb≤0.1%, 0%≤Ni≤3.0%, 0%≤Co≤3.0%, 0%≤Cu≤3.0%, 0%≤Sn≤0.1%, 0%≤Ce≤0.1%, 0%≤Mg≤0.1%, 0%≤Zr≤0.1%, with the remainder being iron and unavoidable impurities from smelting. The steel has a microstructure comprising, by area fraction, 1% to 10% free graphite particles, up to 5% optional cementite, and the balance being ferrite.

Description

Technical Field

[0001] This invention relates to graphitized steel that has been heat-treated and annealed and is suitable for manufacturing mechanical parts. Background Technology

[0002] Steel used for mechanical components can be used to manufacture many machine parts, such as tools, rollers, ring shafts, or gears. These parts can undergo manufacturing operations with small chip sizes, such as forming, shearing, cutting, molding, and other metal removal processes. Such parts can be used in automotive and electronic equipment where complex shapes and dimensional accuracy are required.

[0003] Free-cutting steels typically contain a large amount of various metals (Pb, Bi) and non-metallic inclusions (MnS) to improve machinability. However, the machining of such steels is difficult to manage because lead fumes can be harmful to operator health. Furthermore, additional improvements are needed to reduce tool wear and energy consumption during steel bar machining. Summary of the Invention

[0004] Therefore, there is a need to develop steels that meet these requirements without using lead in their composition.

[0005] Another object of the present invention is to provide a method for manufacturing steel components that is compatible with conventional industrial applications and is robust to changes in manufacturing parameters.

[0006] This objective is achieved by providing a steel product according to claim 1. The steel product may also include the features of claims 2 to 3, used alone or in combination. Another objective is achieved by providing a method according to claim 4. Yet another aspect is achieved by providing a mechanical component according to claim 5.

[0007] Other features and advantages of the invention will become apparent from the following detailed description of the invention. Detailed Implementation

[0008] Carbon is present in the steel of this invention at a weight percentage of 0.70% to 1.1%. Carbon is an essential element for the formation of graphite to improve tool life and ensure post-quenching strength. When the carbon content is below 0.70% by weight, the effect on improving machinability is insufficient, and even if graphitization is complete, the distribution of graphite grains is uneven. On the other hand, when the content exceeds 1.1% by weight, there is a concern that this chemical composition leads to excessively low solidification temperatures, thereby adversely affecting casting capacity and productivity using continuous casting technology.

[0009] Manganese is present in the steel of the present invention at a weight percentage of 0.35% to 0.8%. Manganese is an essential element because it exists either as MnS or by combining with S and oxygen to form complexes with oxides, and serves as a nucleation site for graphite formation. Manganese sulfides also contribute to improved machinability. However, if the amount of Mn exceeds 0.8%, graphitization is significantly inhibited because its completion is delayed, and strength and hardness may increase excessively, thereby degrading machinability. The preferred limit for the presence of manganese is 0.35% to 0.65%, and more preferably 0.40% to 0.55%. In a preferred embodiment, the Mn / S weight ratio of the steel is in the range of 2 to 5.

[0010] Silicon is present in the steel of the present invention at a weight percentage of 1% to 3%. Silicon is an essential component as a deoxidizer in steelmaking and is added because it is an element that promotes graphitization. Its bonding force with carbon atoms is actually very weak, which destabilizes the cementite, allowing carbon to precipitate as graphite. At least 1% silicon must be added to precipitate a sufficient amount of graphite during annealing (meaning a high degree of graphitization). However, when the amount of silicon exceeds 3%, this chemical composition results in excessively low solidification temperatures, adversely affecting casting capacity and productivity using continuous casting technology. Furthermore, the increased hardness due to the solid solution strengthening effect accelerates tool wear during cutting, induces embrittlement with the increase of non-metallic inclusions, and may cause excessive decarburization during hot working (e.g., rolling), and may also lead to deterioration of cold workability. The preferred limit for the presence of silicon is 1.4% to 2.8%, and more preferably 1.9% to 2.7%.

[0011] Titanium exists in steel at a weight percentage of 0.01% to 0.09%. Like boron and aluminum, titanium combines with nitrogen to form nitrides such as TiN. During isothermal annealing heat treatment, these nitrides act as nuclei for the formation of graphite grains. Because the formation temperature of TiN is higher than that of AlN and BN, TiN crystallizes before austenite formation is complete, resulting in a uniform distribution of TiN within the grain boundaries and grains of austenite. Consequently, the graphite grains formed using TiN as nuclei are also finely and uniformly distributed. To achieve this effect, the Ti content is preferably controlled at 0.01% by weight or higher. Conversely, when the Ti content exceeds 0.09% by weight, the carbon required for graphite formation is consumed due to the formation of coarse carbonitrides, potentially inhibiting graphitization. Therefore, in this invention, the Ti content is preferably controlled at 0.01% to 0.06% by weight.

[0012] Aluminum is present in the steel of the present invention at a weight percentage of 0.001% to 0.09%. Aluminum is the second most important element for promoting graphitization after silicon. This is because aluminum destabilizes cementite when present in solid solution form. In the present invention, it is included in an amount of 0.001% by weight or greater to exhibit this effect. On the other hand, if the content is too high, not only will the effect be saturated, but it may also cause nozzle blockage during casting and AlN formation at austenite grain boundaries, resulting in uneven distribution of graphite, which serves as the nucleus, at the grain boundaries. Therefore, the upper limit of the aluminum content is 0.09% by weight, and preferably 0.009% by weight.

[0013] Boron is an optional element that can be present in steel at a weight percentage of up to 0.006%. Boron combines with nitrogen contained in the steel to form BN. BN promotes graphitization by acting as nuclei for the formation of graphite grains. The B content is preferably controlled at 0.001 wt% or more to achieve this effect. Conversely, when the B content is higher than 0.006 wt%, the BN content in the austenite grain boundaries is too high, resulting in uneven distribution of graphite grains after graphitization annealing, weakened grain boundaries, and significantly reduced hot rollability. Therefore, in this invention, the B content is preferably controlled at 0.001 wt% to 0.006 wt%, and more preferably 0.003 wt% to 0.006 wt%.

[0014] The phosphorus content of the steel of the present invention is limited to 0.09% by weight. Phosphorus is an unavoidable impurity in steel. Although machinability can be improved to some extent by weakening the grain boundaries of steel, phosphorus increases the hardness of ferrite due to its significant solid solution strengthening effect, reduces the toughness and resistance to delayed fracture of the steel, and leads to surface defects. Therefore, it is preferable to control the P content as low as possible. Although it is theoretically advantageous to control the P content to 0%, P is inevitably included during the manufacturing process. Therefore, it is important to control the upper limit of the P content to 0.09% by weight in the present invention.

[0015] Sulfur is included in the content at 0% to 0.2% by weight and can contribute to improved machinability by forming MnS inclusions. When in free form and not bound to manganese, it also inhibits the graphitization of carbon in steel, reduces toughness, and induces mechanical anisotropy due to the elongation of MnS during rolling. In this invention, the S content is preferably controlled to an upper limit of 0.2% by weight. In a preferred embodiment, the sulfur content is in the range of 0.01% to 0.2%, or preferably 0.05% to 0.2%, or even better, 0.07% to 0.18%.

[0016] Nitrogen is present in the steel of the present invention in an amount of 0.0090% to 0.0150% by weight. Nitrogen combines with other elements to form nitrides such as TiN, which are mainly formed in the grain boundaries of austenite. During graphitization heat treatment, graphite grains are uniformly distributed by using such nitrides as nuclei. For this purpose, the N content in the present invention is preferably 0.010% by weight or greater. However, when the N content is higher than 0.0150% by weight, nitrides are formed in an amount that may result in uneven distribution of graphite grains. Alternatively, nitrogen may not combine with the elements that form nitrides, but may exist in the steel in a solid solution state to improve strength and stabilize cementite, thereby delaying graphitization. Therefore, in the present invention, the N content is preferably controlled to be from 0.0090% to 0.015% by weight.

[0017] Chromium is an optional element that may be present in the steel of the present invention at a weight percentage of up to 1%. Chromium may be added to ensure hardenability. However, if added at more than 1%, graphitization is significantly suppressed, so this upper limit is set at 1%. The preferred limit for the presence of chromium is up to 0.8%, and more preferably up to 0.6%.

[0018] Molybdenum is an optional element and may be present in the invention at a maximum of 0.5% by weight. Molybdenum can be added to impart hardenability and hardness to steel by forming molybdenum-based carbides, and also to delay the formation of bainite, which promotes ferrite formation. However, the addition of molybdenum increases the cost of alloying element addition, and therefore its content is limited to 0.5% for economic reasons. A preferred limit for the molybdenum content is a maximum of 0.4%, and more preferably a maximum of 0.2%.

[0019] Vanadium is an optional element in this invention, and its content is up to 0.2% by weight. Vanadium is effective in improving the strength of steel by precipitation strengthening, particularly by forming carbides or carbonitrides. For economic reasons, the upper limit is kept at 0.2%.

[0020] Niobium is an optional element in the steel of this invention and can be added up to 0.1% by weight to form carbonitrides, thereby imparting strength through precipitation hardening. Niobium also affects the size of the microstructure components through its precipitation as carbonitrides and through recrystallization during the delayed heating process. Therefore, a finer microstructure formed at the end of the holding temperature and subsequently after complete austenitization leads to hardening of the product. However, niobium contents above 0.1% are not economically attractive and result in coarser precipitates, which are detrimental to the fatigue properties of the steel. Furthermore, when the niobium content is 0.1% or greater, it is also detrimental to the hot ductility of the steel, leading to difficulties during casting and rolling.

[0021] Other optional elements that can be added to the steel of this invention are nickel, cobalt, and copper, which have weak bonding forces with carbon atoms, destabilize cementite, promote graphitization, and are effective in enhancing hardenability and ensuring strength. Whenever added, the content of these elements is limited to 3.0%, and preferably to a minimum content of 0.05% by weight.

[0022] Other elements such as tin, cerium, magnesium, or zirconium may be added alone or in combination in the following proportions: tin ≤ 0.1%, cerium ≤ 0.1%, magnesium ≤ 0.1%, and zirconium ≤ 0.1% by weight. These elements, up to the maximum content levels shown, enable grain refinement during solidification. In a preferred embodiment, Mg is limited to less than 0.005% by weight.

[0023] The remaining portion of the steel composition consists of iron and unavoidable impurities resulting from processing. In particular, the steel of the present invention does not contain elements such as lead (Pb).

[0024] After graphitization, the microstructure of the steel consists of ferrite, free graphite particles, and optionally a small amount of cementite.

[0025] In particular, the newly developed steel within the framework of this invention contains graphite particles that have self-lubricating properties and can replace lead inclusions.

[0026] Ferrite is the matrix phase of the steel of the present invention and exists in the steel in an amount of 85% to 99% by area fraction. Such ferrite can include polygonal ferrite, lath ferrite, acicular ferrite, blocky ferrite, or epitaxial ferrite. The presence of ferrite in the present invention provides the steel of the present invention with relevant mechanical properties for machining purposes. In fact, the hardness of ferrite is lower than that of other types of microstructures such as pearlite, bainite, martensite, and cementite. During machining, ferrite causes less wear on the cutting tool, and thus reduces tool wear during machining. Furthermore, the lower mechanical properties achieved due to the predominantly ferrite microstructure ensure lower energy consumption during machining.

[0027] The steel of the present invention may contain up to 5% optional cementite by area fraction. This phase is obtained after the initial microstructure transformation of the steel, which is primarily composed of pearlite. During graphitization annealing, the cementite transforms into ferrite and graphite, but a portion of it may remain in the final microstructure of the component. This residual cementite is limited to 5% by area fraction because it is detrimental to the service properties of graphitized steel, such as machinability.

[0028] Graphite exists as particles in the steel of the present invention in an amount of 1% to 10% by area fraction. The graphitization rate of the steel according to the present invention is preferably 98% or higher, more preferably 99% or higher, and most preferably 99.5% or higher. Within the framework of the present invention, the graphitization rate is defined as the weight percentage of carbon converted into graphite particles. Most of the residual carbon not found in the graphite particles is found in the residual cementite. For machining purposes, cementite is more detrimental to cutting tools than ferrite. Therefore, it is important to minimize the amount of residual cementite as much as possible, thereby maximizing the graphitization rate. The graphite particles are finely and uniformly distributed in the ferrite matrix of the steel.

[0029] The steel used for mechanical parts according to the invention can be produced by any suitable manufacturing process, wherein the specified process parameters are described below.

[0030] A preferred method is described below, but this embodiment does not limit the scope of the invention or the aspects on which the embodiment is based. Furthermore, any embodiments set forth in this specification are not intended to be limiting, but merely illustrate some of the many possible ways in which various aspects of the invention can be practiced.

[0031] A preferred method includes providing a steel semi-finished casting having the chemical composition according to the invention. The casting can be made in any form, such as a steel ingot, a large billet, or a small billet, and can be manufactured or machined into mechanical parts, which may, for example, have a cross-section from 1 mm × 1 mm to 400 mm × 400 mm.

[0032] For example, steel with the above chemical composition can be cast into small billets, and then rolled into bars, coils, or wires. This steel product can then serve as a semi-finished product for further mechanical operations. Multiple rolling, bright drawing, or multi-die drawing steps can be performed to obtain the desired semi-finished product.

[0033] In preparation for manufacturing steel into machine parts, semi-finished products can be used directly at high temperatures after rolling, or they can be cooled to room temperature first and then heated to manufacture parts.

[0034] The semi-finished product is reheated to a temperature range of Ac3 to Ac3+400℃, preferably Ac3+30℃ to Ac3+400℃, and held at this temperature for 5 to 10800 seconds to ensure uniform temperature across the entire cross-section of the semi-finished product and to ensure the formation of 100% austenite.

[0035] If the reheating temperature of the semi-finished product is below Ac3, excessive loads will be applied to mechanically operated components such as dies during forming operations or to milling cutters during necking. Furthermore, the steel temperature may drop below the ferrite transformation initiation temperature, leading to the formation of ferrite in the final product, which is detrimental to fatigue and mechanical properties. Moreover, for a given cooling rate or chemical composition, metallurgical transformations under strain can cause significant changes in the obtained microstructure. Consequently, the obtained microstructure will be entirely different from the target microstructure, and so will the mechanical properties. Therefore, the temperature of the semi-finished product is preferably high enough that all mechanical operations are carried out and completed within the 100% austenitic temperature range. Reheating at temperatures above Ac3 + 400°C must be avoided, as it is industrially expensive and may lead to the formation of a liquidus region, which will affect the forming and necking of the steel.

[0036] The semi-finished product can then be subjected to at least one machining operation at Ac3 to Ac3+400°C. The machining operation may include rolling, necking, forming, cutting, or any other suitable machining operation or manufacturing process required to shape the semi-finished product into a machine part. The preferred temperature for all machining operations is Ac3+30°C to Ac3+400°C, and more preferably Ac3+50°C to Ac3+300°C.

[0037] The final mechanical operating temperature must be maintained above Ac3 to obtain a microstructure that is conducive to recrystallization and mechanical manufacturing.

[0038] After machining, the semi-finished product is cooled to room temperature at an average cooling rate of less than 50°C / second, and preferably less than 40°C / second.

[0039] Subsequently, the mechanical parts are subjected to graphitization annealing, wherein they are heated to a homogenization temperature of 600°C to 750°C at a heating rate of 10°C / hour to 200°C / hour.

[0040] The mechanical parts are then kept at a uniform temperature for 1 to 48 hours to ensure full graphitization, and cooled to room temperature at a cooling rate of less than 100°C / second, preferably less than 75°C / second.

[0041] In the second embodiment, the hot-worked steel is directly subjected to graphitization annealing, wherein it is heated to a homogenization temperature of 600°C to 750°C at a heating rate of 10°C / hour to 200°C / hour.

[0042] The steel is then held at a homogenization temperature for 1 to 48 hours to ensure full graphitization, and cooled to room temperature at a cooling rate of less than 100°C / second, and preferably less than 75°C / second.

[0043] Then the mechanical parts are manufactured using any suitable forming process, including cold forging, drawing, or machining.

[0044] In both implementations, the mechanical components may undergo final heat treatments such as quenching and tempering processes for strengthening and / or high-frequency induction hardening.

[0045] Embodiments

[0046] The tests, examples, illustrative illustrations, and tables presented herein are non-limiting in nature and should be considered for illustrative purposes only, and will show advantageous features of the invention.

[0047] Several steel samples with the compositions summarized in Table 1 were prepared.

[0048] Table 1

[0049]

[0050] Underlined values: do not conform to this invention.

[0051] After casting into 160×160 mm billet dimensions, the billets are cooled (naturally air-cooled) to room temperature. The billets are then reheated at 1150°C and rolled into 26.5 mm coils. For R1 and I1, the coils are then heated to 750°C at 100°C / hour and annealed for 16 hours for graphitization.

[0052] Finally, all the steel coils are cold-drawn into 25 mm bright bars.

[0053] The microstructures of I1 and I2 were observed, and the microstructures of I1 and I2 are consistent with the present invention. I2 does not contain residual cementite, while I1 contains some cementite at a surface fraction of less than 5%.

[0054] Tool wear tests were then conducted during bar face turning operations using high-speed steel cutting tools. The cutting operation studied was single-point turning using high-speed steel WKE45 cutting tools. The cutting tools were required to have a hardness of 65 HRC. The cutting tools were initially 8×8 mm square bars, 100 mm in length, ground according to international standard NF ISO 3002-1 with the following angles: rake angle: 0°, major clearance angle: 6°; minor clearance angle: 6°, approach angle: 90°, and inclination angle: -1°.

[0055] For the tool wear test, the face turning parameters were: feed rate = 0.1 mm / revolution, depth of cut = 3 mm. The cutting test was conducted using an overflow emulsion (8% oil and 92% water). Machining was periodically stopped to measure the flank wear. The tool wear test ended when the flank wear reached 1500 µm.

[0056] In addition, two reference free-cutting steel grades, 11SMn30 and 11SMnPb30, were characterized using the same scheme for comparison.

[0057] The results of the tool wear test are summarized in Table 2 below.

[0058] Table 2

[0059]

[0060] The results of the tool wear test show that the steel according to the invention (which does not contain lead) has better performance than the reference steel.

Claims

1. A graphitized steel product that has undergone heat treatment and annealing, comprising, by weight percentage: 0.70%≤C≤1.1% 0.35%≤Mn≤0.80% 1.0%≤Si≤3.0% 0.010%≤Ti≤0.090% 0.001%≤Al≤0.09% P≤0.090% S≤0.2 0.0090≤N≤0.0150 And optionally include one or more of the following elements: by weight percentage, 0%≤B≤0.006% 0%≤Cr≤1% 0%≤Mo≤0.5% 0%≤V≤0.2% 0%≤Nb≤0.1% 0%≤Ni≤3.0% 0%≤Co≤3.0% 0%≤Cu≤3.0% 0%≤Sn≤0.1% 0%≤Ce≤0.1% 0%≤Mg≤0.1% 0%≤Zr≤0.1% The remaining component consists of iron and unavoidable impurities produced during smelting. The steel has a microstructure comprising 1% to 10% free graphite particles, up to 5% optional cementite, and the balance being ferrite, by area fraction.

2. The graphitized steel product according to claim 1, comprising aluminum in the range of 0.001 to 0.03% by weight.

3. The graphitized steel product that has been heat-treated and annealed according to claim 1 or 2, comprising boron in the range of 0.001 to 0.002% by weight.

4. A method for manufacturing a graphitized steel product that has been heat-treated and annealed according to any one of claims 1 to 3, comprising the following steps: - Provide a semi-finished steel product having the composition according to any one of claims 1 to 3, - Reheat the product to a temperature in the range of Ac3 to Ac3+00°C, and hold at this temperature for 5 to 10800 seconds. - The reheated product is subjected to at least one machining operation at a temperature ranging from Ac3 to Ac3+400°C. - Cool the product to room temperature at an average cooling rate of less than 50°C / second. - The product is subjected to graphitization annealing, wherein it is heated to a homogenization temperature of 600°C to 750°C at a heating rate of 10°C / hour to 200°C / hour. - The product is kept at the homogenization temperature for 1 hour to 48 hours and then cooled to room temperature at a cooling rate of less than 100°C / second.

5. A mechanical component made of a graphitized steel product that has been heat-treated and annealed according to claims 1 to 3, or obtained by the method according to claim 4.

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