Free cutting steel and method for manufacturing same

Abstract

The present invention relates to a free-cutting steel and a method for manufacturing same and, more specifically, to a steel wire and a method for manufacturing same, wherein the steel wire comprises, in weight%, C: 0.60%-0.85%, Si: 1.50%-1.99%, Mn: 0.10%-0.60%, S: 0.010%-0.150%, Cr: 0.005%-0.050%, Ti: 0.005%-0.020%, N: 0.003%-0.015%, and the balance of Fe and other impurities, and has a microstructure including ferrite, graphite particles, and cementite particles after graphitization heat treatment.

Description

Free-cutting steel and its manufacturing method

[0001] The present invention relates to a free-cutting steel and a method for manufacturing the same.

[0002] Free-cutting steels containing large amounts of machinability-enhancing elements such as Pb, Bi, and S are used as materials for machine parts requiring machinability. By adding low-melting-point machinability-enhancing elements such as Pb, Bi, and S to steel, the liquid metal embrittlement phenomenon is utilized, or a large amount of MnS is formed in the steel. These free-cutting steels have excellent machinability, including surface roughness, chip control, and tool life, during cutting.

[0003] However, lead-added free-cutting steels, which offer the best machinability, emit toxic fumes and other hazardous substances during cutting, posing a risk to human health and hindering the recycling of the steel. Furthermore, additives such as sulfur, bismuth, and tin have been proposed as alternatives. However, these additions can lead to quality degradation, such as cracking during hot rolling.

[0004] Graphite steel is a free-cutting steel developed to address the aforementioned issues. Graphite steel contains fine graphite particles within a ferrite matrix or within a ferrite and pearlite matrix. During cutting, these fine graphite particles act as a source of cracks, acting as a chipbreaker, thereby improving machinability.

[0005] However, despite these advantages of graphite steel, it is difficult to precipitate graphite without long-term heat treatment of more than 15 hours, and decarburization may occur during such long-term heat treatment, which may adversely affect the performance of the final product.

[0006] In addition, even if graphite particles are precipitated through graphitization heat treatment, if the precipitated graphite particles are distributed unevenly in an irregular shape, the distribution of physical properties during cutting may become uneven, resulting in poor chip handling and surface roughness, and a shortened tool life.

[0007] The purpose of the present invention to solve the above-described problem is to provide a free-cutting steel and a manufacturing method thereof, which improves machinability and suppresses wear of a cutting bite by controlling the uniform distribution of graphite particles and cementite particles within a matrix during heat treatment while significantly shortening the heat treatment time.

[0008] The technical problems to be achieved in this document are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0009] In order to achieve the above object, a steel wire according to one embodiment of the present invention contains, in wt%, C: 0.60% to 0.85%, Si: 1.50% to 1.99%, Mn: 0.10% to 0.60%, S: 0.010% to 0.150%, Cr: 0.005% to 0.050%, Ti: 0.005% to 0.020%, N: 0.003% to 0.015%, the remainder being Fe and other impurities, and after graphitization heat treatment, the microstructure may include ferrite, graphite particles, and cementite particles.

[0010] Additionally, according to one embodiment of the present invention, the graphite particle area fraction may be 1% to 3%, and the average diameter of the graphite particles may be 1 to 7 ㎛.

[0011] Additionally, according to one embodiment of the present invention, the area fraction of the cementite particles may be 0.1% to 2%, and the average diameter of the cementite particles may be 1 to 5 ㎛.

[0012] In addition, a method for manufacturing a steel wire according to an embodiment of the present invention may include the steps of: manufacturing a billet including, in wt%, C: 0.60% to 0.85%, Si: 1.50% to 1.99%, Mn: 0.10% to 0.60%, S: 0.010% to 0.150%, Cr: 0.005% to 0.050%, Ti: 0.005% to 0.020%, N: 0.003% to 0.015%, and the remainder of Fe and other impurities; reheating the billet; hot-rolling the reheated billet to manufacture a wire rod; cooling the wire rod; and performing a graphitization heat treatment on the cooled wire rod at an A1 temperature or lower for 5 to 10 hours.

[0013] Additionally, the reheating according to one embodiment of the present invention can be performed at 950 to 1150°C for 60 minutes or more.

[0014] Additionally, the hot rolling according to one embodiment of the present invention can be performed at 900 to 1150°C.

[0015] Additionally, the cooling according to one embodiment of the present invention can be performed up to 500°C at a cooling rate of 0.1 to 10°C / s in a temperature range of 750 to 900°C.

[0016] Additionally, after the cooling step according to one embodiment of the present invention, a natural cooling step may be further included.

[0017] In addition, the present invention may further include a step of cutting and processing the wire rod subjected to the graphitization heat treatment using a CNC lathe or a CAM automatic lathe.

[0018] According to one embodiment of the present invention, a free-cutting steel and a method for manufacturing the same can be provided, in which the heat treatment time is significantly shortened while the graphite particles and cementite particles are controlled to be uniformly distributed within the matrix during the heat treatment, thereby improving the machinability and suppressing the wear of the cutting bite.

[0019] Furthermore, the steel wire of the present invention has excellent machinability and can be applied to CNC lathes and CAM automatic lathes.

[0020] The effects that can be obtained from the present disclosure are not limited to the effects mentioned above, and other effects that are not mentioned can be clearly understood by a person having ordinary skill in the art to which the present disclosure belongs from the description below.

[0021] Figure 1 is a photograph of the microstructure measured using an optical microscope after graphitization heat treatment according to one embodiment.

[0022] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the present invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0023] Additionally, when a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise specifically stated.

[0024] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0025] A steel wire according to one embodiment of the present invention contains, in wt%, C: 0.60% to 0.85%, Si: 1.50% to 1.99%, Mn: 0.10% to 0.60%, S: 0.010% to 0.150%, Cr: 0.005% to 0.050%, Ti: 0.005% to 0.020%, N: 0.003% to 0.015%, and the remainder of Fe and other impurities.

[0026] The following explains in detail the reasons for limiting the composition of the above lecture. Unless otherwise specified, the composition below refers to weight percent.

[0027] The carbon (C) content may be 0.60% to 0.85%.

[0028] C is an essential element for forming graphite particles. C can improve machinability by forming graphite particles and can play a role in uniformly distributing graphite particles after graphitization heat treatment. If the C content is less than 0.60%, the effect of improving machinability is insufficient, the distribution of graphite particles is uneven even after graphitization is complete, and cementite formation may be difficult. In addition, if the C content exceeds 0.85%, graphite particles and cementite particles are formed coarsely, and the aspect ratio increases, which may deteriorate machinability and surface roughness. Considering this, the C content is preferably controlled to 0.60% to 0.85%, and more preferably, it is controlled to 0.62% to 0.83%.

[0029] The silicon (Si) content can be from 1.50% to 1.99%.

[0030] Silicon (Si) is a necessary component as a deoxidizer in the production of molten steel, and is a graphitization-promoting element that destabilizes cementite in the steel, allowing C to precipitate as graphite. Considering this, the Si content must be 1.50% or more to enable commercial graphitization heat treatment. However, if the Si content is excessive, not only will the above effect be saturated, but hardness will increase due to the solid solution strengthening effect, which may accelerate tool wear during cutting. In addition, it may cause embrittlement due to an increase in non-metallic inclusions, and excessive decarburization may occur during hot rolling. Furthermore, if the Si content is excessive, cementite particles may disappear and all be transformed into graphite particles. Therefore, in order to generate cementite particles for graphitization heat treatment and to improve machinability, the Si content is preferably controlled to 1.50% to 1.99%, and more preferably, it is controlled to 1.55% to 1.96%.

[0031] The manganese (Mn) content can be 0.10% to 0.60%.

[0032] Mn is an element that improves the strength and impact properties of steel, and contributes to improving machinability by combining with sulfur in the steel to form MnS inclusions. Considering this, Mn may be included in an amount of 0.10% or more. However, if the content of Mn is excessive, it may inhibit graphitization, delaying the completion time of graphitization, and may reduce machinability by increasing strength and hardness. Considering this, the content of Mn is preferably controlled to 0.10% to 0.60%, and more preferably, it is controlled to 0.15% to 0.57%.

[0033] The content of S (sulfur) can be 0.010% to 0.150%.

[0034] Sulfur is a very important component that improves machinability by generating inclusions such as MnS, TiS, and Ti4C2S2 in steel. If sulfur is excessive, the machinability is improved by generating MnS inclusions, but mechanical anisotropy occurs due to MnS elongated by rolling. Therefore, in the present invention, the content of S is controlled within a range that can contribute to improving machinability without causing mechanical anisotropy, thereby inducing MnS generation, and improving machinability by generating Ti-based emulsions such as TiS and Ti4C2S2 through the combination of S and Ti. If the content of S is less than 0.010%, it may be difficult to generate emulsions such as MnS, TiS, and Ti4C2S2. In addition, if the content of S exceeds 0.150%, surface flaws may occur, which may cause difficulties in manufacturing parts. Considering this, it is preferable to control the content of S to 0.010% to 0.150%, and more preferably to 0.020% to 0.140%.

[0035] The content of chromium (Cr) can be from 0.005% to 0.050%.

[0036] Cr is an element that promotes the transformation of lamellar pearlite into spherical cementite during graphitization heat treatment. This is because Cr destabilizes cementite within pearlite, and to achieve this effect, it is preferable that Cr be included in an amount of 0.005% or more. On the other hand, if the Cr content is excessive, the formation of graphite particles may be delayed. Considering this, the Cr content is preferably controlled to 0.005% to 0.050%, and more preferably, it is controlled to 0.015% to 0.040%.

[0037] The content of titanium (Ti) can be 0.005% to 0.020%.

[0038] Ti combines with nitrogen to form TiN nitrides, which can act as nuclei for graphite formation during graphitization heat treatment. In particular, while other nitrides such as BN and AlN precipitate unevenly at grain boundaries, TiN crystallizes before austenite formation is complete due to its high formation temperature, so it is uniformly distributed at the austenite grain boundaries and within the grains. Therefore, graphite particles formed with TiN as nuclei can also be fine and uniformly distributed. In addition, Ti combines with sulfur to form sulfides such as TiS and Ti4C2S2. These sulfides improve chip breakage during cutting and, like sulfides such as MnS, have the effect of reducing cutting force. To achieve these effects, it is desirable to contain at least 0.005% Ti. However, if the Ti content exceeds 0.020%, coarse carbonitrides may form, consuming all the carbon needed for graphite formation, thereby inhibiting graphitization. Therefore, it is preferable to control the content of Ti to 0.005% to 0.020%, and more preferably to 0.007% to 0.018%.

[0039] The nitrogen (N) content can be 0.003% to 0.015%.

[0040] N combines with Ti, Al, etc. to form nitrides such as TiN and AlN, and in particular, AlN nitride is mainly formed at austenite grain boundaries. Since graphite is formed using these nitrides as nuclei, it can cause uneven distribution of graphite particles, and therefore, it is necessary to appropriately control the N content. If the N content is excessive, it cannot combine with the nitride-forming elements and exists as solid solution nitrogen in the steel. The presence of solid solution nitrogen in the steel increases the strength and stabilizes cementite, thereby delaying graphitization. Therefore, in order to prevent N from being consumed in forming nitrides that act as nuclei for graphite formation and from remaining as solid solution nitrogen, it is preferable to control the N content to 0.003% to 0.015%, and more preferably, N is controlled to 0.005% to 0.013%.

[0041] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0042] By controlling graphite particles through the alloy components and manufacturing method presented in the present invention, a steel wire with improved machinability can be provided.

[0043] According to one embodiment of the present invention, a steel wire includes ferrite, graphite particles and cementite particles as a microstructure after graphitization heat treatment, and the graphite particles and cementite particles are distributed in a ferrite matrix.

[0044] In addition, the steel wire according to one embodiment of the present invention may have a graphite particle area fraction of 1% to 3%, an average graphite particle diameter of 1 to 7 µm, an area fraction of cementite of 0.1% to 2%, and an average cementite diameter of 1 to 5 µm.

[0045] Meanwhile, in the present invention, the average means the average value of values ​​measured at five arbitrary locations.

[0046] Next, a method for manufacturing a steel wire according to one embodiment of the present invention will be described.

[0047] A method for manufacturing a steel wire according to one embodiment of the present invention may include the steps of: manufacturing a billet including, in wt%, C: 0.60% to 0.85%, Si: 1.50% to 1.99%, Mn: 0.10% to 0.60%, S: 0.010% to 0.150%, Cr: 0.005% to 0.050%, Ti: 0.005% to 0.020%, N: 0.003% to 0.015%, and the remainder of Fe and other impurities; reheating the billet; hot-rolling the reheated billet to manufacture a wire rod; cooling the wire rod; and performing a graphitization heat treatment on the cooled wire rod at an A1 temperature or lower for 5 hours or longer.

[0048] The reason for the numerical limitation of the component range of each alloy composition is as described above, and each manufacturing step is described in more detail below.

[0049] After manufacturing a billet that satisfies the above alloy composition, it can undergo a series of reheating, rolling, cooling, and graphitization heat treatment processes.

[0050] First, the billet can be heat treated by maintaining it at 950 to 1150°C for 60 minutes or more.

[0051] If the billet reheating temperature is below 950℃, the load during rolling increases, potentially reducing rolling productivity. Furthermore, if the reheating temperature exceeds 1150℃, not only does it increase costs, but decarburization accelerates, thickening the decarburization layer and leaving it in the final product, which can negatively impact product performance. A reheating time of less than 60 minutes makes it difficult to ensure a uniform temperature distribution within and outside the billet for wire rod rolling. Therefore, a reheating time of 60 minutes or longer is preferred.

[0052] After the above reheating, it can be hot rolled at 900 to 1150°C to produce a wire rod.

[0053] If the above hot rolling temperature is less than 900℃, surface flaws may easily occur during hot rolling or the rolling load may increase, making rolling difficult. If it exceeds 1150℃, the austenite grain size may become coarser, making the subsequent graphitization heat treatment time longer.

[0054] Next, the rolled wire can be cooled to 500°C at a cooling rate of 0.1 to 10°C / s in a temperature range of 750 to 900°C.

[0055] The above cooling step is a step prior to graphitization heat treatment, and the cooling rate must be controlled so that the graphite particles can be uniformly distributed within the matrix. If the cooling rate exceeds 10°C / s, the hardness may increase excessively and the ductility may decrease, and if it is less than 0.1°C / s, the proeutectoid phase may be excessively formed, coarsenting the crystal grain size, and the graphite particles generated after the graphitization heat treatment may have an uneven distribution.

[0056] In addition, the present invention may further include a natural cooling step after the cooling step.

[0057] Next, the cooled billet can be subjected to graphitization heat treatment at a temperature lower than A1, i.e., lower than 700 to 800°C, for 5 to 10 hours.

[0058] Graphitization heat treatment is a process that shortens heat treatment time while maintaining a graphitization fraction of over 90%. Because spherical cementite is also formed during graphitization heat treatment, graphite and spherical cementite particles exist within the ferrite matrix after graphitization heat treatment.

[0059] The above graphitization heat treatment is preferably performed at 700 to 800°C, and more preferably at 700 to 780°C. If the graphitization heat treatment temperature is less than 700°C, the graphitization heat treatment time becomes longer and may exceed 10 hours, and if it exceeds 800°C, austenite transformation occurs and graphitization does not progress, and graphitization progresses unevenly during cooling, which is undesirable.

[0060] In addition, the present invention may further include a step of cutting the steel wire manufactured as described above using a CNC lathe or a CAM automatic lathe.

[0061] The above CNC composite lathe may have a cutting speed of 1500 rpm or more and a feed speed of 0.03 mm / rev or more, and the above CAM automatic lathe may also have a cutting speed of 1500 rpm or more and a feed speed of 0.03 mm / rev or more.

[0062] The above-mentioned manufactured steel wire is then cut by a cutting company using a CNC lathe or CAM automatic lathe. Without cutting performance comparable to that of general free-cutting steel, market entry is impossible. The faster the cutting speed and feed rate during cutting, the better the cutting performance, and can achieve a level equivalent to that of general free-cutting steel. Considering this, the present invention improves cutting performance by controlling the cutting speed of a CNC lathe or CAM automatic lathe to 1500 rpm or higher and the feed rate to 0.03 mm / rev or higher.

[0063] As described above, the steel wire according to the present invention can be applied to a CNC lathe or a CAM automatic lathe by controlling the uniform distribution of fine graphite particles and cementite particles within the matrix while significantly reducing the heat treatment time, thereby improving machinability and suppressing wear of the cutting bite.

[0064] Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is intended only to illustrate the implementation of the present invention and is not intended to limit the present invention. This is because the scope of the present invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0065] Example

[0066] Billets were manufactured to satisfy various alloy compositions shown in Table 1 below.

[0067] The manufactured billet was maintained at 1050°C for more than 90 minutes and then rolled at 1000°C to produce a wire rod with a diameter of 16 mm. Next, the billet was cooled from 900°C to 500°C at a cooling rate of 5°C / s, and then graphitization heat treatment was performed under the graphitization heat treatment conditions shown in Table 2 to produce a specimen.

[0068] The units in Table 1 below are weight%.

[0069] CSiMnSCrTiN Example 10.621.550.150.0120.0050.0200.005 Example 20.691.650.250.0500.0100.0150.003 Example 30.811.750.350.1000.0200.0100.007 Example 40.711.850.450.1300.0300.0050.009 Example 50.781.600.550.0200.0400.0170.011 Example 60.651.700.190.0400.0500.0090.013 Example 70.831.800.270.0800.0150.0140.015Example 80.771.900.330.1100.0250.0160.012Example 90.661.580.490.1400.0350.0120.008Example 100.801.760.570.0700.0450.0180.005Example 110.701.960.400.1200.0470.0070.010Comparative Example 10.401.400.010.0010.0010.0220.001Comparative Example 20.452.100.030.0050.0030.0020.003Comparative Example 30.552.300.900.0600.0700.030.017Comparative Example 40.951.301.000.2100.0900.0350.019Comparative Example 50.901.200.800.1700.1100.0410.025Comparative Example 61.002.500.050.2500.0020.0040.002Comparative Example 70.751.200.700.1900.1500.0260.020

[0070] Table 2 below shows the graphite particle distribution (size and area fraction), cementite particle distribution (size and area fraction), and machinability evaluation results according to the heat treatment conditions for graphitization. The size and area fraction of graphite and cementite particles were measured based on photographs taken with a scanning electron microscope (SEM). In the case of area fraction, the area fraction of graphite or cementite particles within an area range of 100㎛Х100㎛ was calculated and displayed. In addition, the machinability evaluation results were measured using a CAM automatic lathe, and were evaluated as good if they were similar to the lead free-cutting steel, and as bad if they were not.

[0071] Graphitization heat treatment Graphite particle distribution Cementite particle distribution Machinability Time (hr) Temperature (℃) Size (㎛) Area fraction (%) Size (㎛) Area fraction (%) Example 15.57502.51.31.20.5 Good Example 27.17415.02.24.11.5 Good Example 37.87355.02.33.81.8 Good Example 48.17273.62.64.21.3 Good Example 58.57464.82.34.31.6 Good Example 65.77373.51.72.50.5 Good Example 76.87495.11.83.00.7 Good Example 87.17415.32.13.41.3 Good Example 98.37236.02.64.51.8 Good Example 107.67156.61.62.91.4 Good Example 119.57556.62.74.71.9 Good Comparative Example 11.28000.10.210.50.1 Poor Comparative Example 22.37900.060.3110.2 Poor Comparative Example 34.06000.50.47.54.0 Poor Comparative Example 43.36100.60.56.44.1 Poor Comparative Example 54.47700.20.76.20.3 Poor Comparative Example 64.18100.080.112.40.5 Defective Comparison Example 73.65500.150.45.51.2 Defective

[0072] As shown in Table 2, it was confirmed that the area fraction of graphite particles in the microstructure measured after graphitization heat treatment was 1 to 3%, and the average diameter was 1 to 7 μm, and the area fraction of cementite particles was 0.1 to 2%, and the average diameter was 1 to 5 μm. In addition, it was confirmed that the machinability was also good when the distribution of graphite particles and cementite particles was as above. On the other hand, in the case of Comparative Examples 1 to 7 that did not satisfy the alloy components and graphitization heat treatment conditions presented in the present invention, the distribution of graphite particles and cementite particles did not satisfy the present invention, and accordingly, it was confirmed that the machinability was also poor.

[0073] In addition, the microstructure of the steel wire of Example 7 was measured using an optical microscope after graphitization heat treatment, and the results are shown in Fig. 1.

[0074] As shown in Fig. 1, it was confirmed that the microstructure after the graphitization heat treatment of Example 7 was composed of ferrite and graphite particles, and spherical cementite particles.

[0075] Through these results, it was found that only when both the alloy composition and manufacturing method suggested in the present invention are satisfied can the heat treatment time be significantly shortened while controlling the uniform distribution of graphite particles and cementite particles within the matrix, thereby improving machinability and manufacturing a steel wire with suppressed wear of the cutting bite, and that such a steel wire can be applied to CNC lathes and CAM automatic lathes.

[0076] Although the embodiments of the invention disclosed above have been illustrated and described, the disclosed invention is not limited to the specific embodiments described above, and various modifications may be implemented by a person having ordinary skill in the art to which the disclosed invention pertains without departing from the gist claimed in the claims.

Claims

1. A steel wire containing, by weight%, C: 0.60% to 0.85%, Si: 1.50% to 1.99%, Mn: 0.10% to 0.60%, S: 0.010% to 0.150%, Cr: 0.005% to 0.050%, Ti: 0.005% to 0.020%, N: 0.003% to 0.015%, the remainder being Fe and other impurities, and having a microstructure after graphitization heat treatment including ferrite, graphite particles and cementite particles.

2. In paragraph 1, A steel wire having an area fraction of graphite particles of 1 to 3% and an average diameter of graphite particles of 1 to 7 ㎛.

3. In paragraph 1, A steel wire having an area fraction of the cementite particles of 0.1% to 2% and an average diameter of the cementite particles of 1 to 5 ㎛.

4. A step for manufacturing a billet containing, in wt%, C: 0.60% to 0.85%, Si: 1.50% to 1.99%, Mn: 0.10% to 0.60%, S: 0.010% to 0.150%, Cr: 0.005% to 0.050%, Ti: 0.005% to 0.020%, N: 0.003% to 0.015%, the remainder being Fe and other impurities; A step of reheating the above billet; A step of manufacturing a wire rod by hot rolling the above reheated billet; a step of cooling the above-mentioned pre-treatment material; and A method for manufacturing a steel wire, comprising: a step of performing graphitization heat treatment on the cooled wire at a temperature of A1 or lower for 5 to 10 hours.

5. In paragraph 4, The above reheating is a method for manufacturing a strong steel by heat treatment by maintaining it at 950 to 1150℃ for 60 minutes or more.

6. In paragraph 4, The above hot rolling is a method for manufacturing steel wire performed at 900 to 1150°C.

7. In paragraph 4, A method for manufacturing a steel wire, wherein the above cooling is performed at a cooling rate of 0.1 to 10°C / s in a temperature range of 750 to 900°C up to 500°C.

8. In paragraph 4, A method for manufacturing a steel wire, further comprising a natural cooling step after the above cooling step.

9. In paragraph 4, A method for manufacturing a steel wire, further comprising a step of cutting the wire that has undergone the above graphitization heat treatment using a CNC lathe or a CAM automatic lathe.

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