Calcium-containing graphite steel with excellent cutting performance and its manufacturing method

Abstract

The present invention provides a calcium-containing graphite steel having excellent cutting performance, which contains calcium (Ca) and can form Ca-Al oxides that act as nuclei for graphitization to promote graphitization, and can generate Ca sulfides to improve machinability, and a method for producing the same. [Solution] The steel sheet contains, by weight, 0.60-0.90% carbon (C), 2.0-2.5% silicon (Si), 0.7-1.3% manganese (Mn), 0.2-0.5% sulfur (S), 0.01-0.05% aluminum (Al), 0.005-0.020% titanium (Ti), 0.003-0.015% nitrogen (N), 0.0001-0.050% calcium (Ca), with the remainder being iron (Fe) and unavoidable impurities. The steel sheet has a fine structure, graphite grains distributed in a ferrite matrix, a graphitization rate of 95% or more, and contains a total of 5% by weight or less of MnS inclusions and pearlite.

Description

[Technical field]

[0001] The present invention relates to a graphite steel having excellent cutting performance and a manufacturing method thereof, and more particularly to a calcium-containing graphite steel having excellent cutting performance, which contains calcium (Ca) and can form Ca-Al-based oxides that act as nuclei for graphitization to promote graphitization, and can generate Ca-based sulfides to improve machinability, and a manufacturing method thereof. [Background technology]

[0002] Generally, free-cutting steels with machinability-imparting elements such as Pb and Bi are used as materials for machine parts and other parts that require machinability. To improve the machinability of steel, low-melting-point machinability-imparting elements such as Pb and Bi are added to the steel to utilize the liquid metal bromide phenomenon, or a large amount of MnS is formed in the steel. Such free-cutting steels have excellent machinability, such as surface roughness, chip treatment, and tool life, during cutting.

[0003] However, the commonly known Pb-added free-cutting steel, which has excellent machinability, emits toxic fumes and other harmful substances during cutting, which are extremely harmful to the human body and are also disadvantageous in terms of recycling the steel.As a result, the addition of S, Bi, Te, Sn, etc. has been proposed as an alternative, but it has been known to be problematic in that cracks are easily generated during steel manufacturing, making production very difficult, and they cause cracks during hot rolling.

[0004] Graphite steel is a free-cutting steel developed to solve the above problems. Graphite steel is a steel containing fine graphite grains inside a ferrite matrix or a ferrite and pearlite matrix, and the fine graphite grains inside act as a crack source and chip breaker during cutting, resulting in good machinability.

[0005] However, despite these advantages of graphite steel, it has not been commercialized yet because, when carbon is added to steel, graphite is precipitated as a metastable phase of cementite, even though graphite is a stable phase, and it is difficult to precipitate graphite without a separate long-term heat treatment, and decarburization occurs during such a long-term heat treatment process, which has a detrimental effect on the performance of the final product.

[0006] Furthermore, even if graphite grains are precipitated through graphitization heat treatment, if they are unevenly distributed in irregular shapes, the distribution of physical properties during cutting is uneven, resulting in very poor chip processing and surface roughness, and shortened tool life, making it difficult to obtain the advantages of graphite steel. Therefore, there is a need to provide a method for producing graphite free-cutting steel with excellent cutting performance by utilizing MnS inclusions while still using graphite grains. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2015-0057400 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention aims to provide a calcium-containing graphite steel having excellent cutting performance and a method for producing the same.

[0009] However, the problems to be solved by the present application are not limited to those mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below. [Means for solving the problem]

[0010] The graphite steel of the present invention for achieving the above-mentioned objects contains, by weight, 0.60-0.90% carbon (C), 2.0-2.5% silicon (Si), 0.7-1.3% manganese (Mn), 0.2-0.5% sulfur (S), 0.01-0.05% aluminum (Al), 0.005-0.020% titanium (Ti), 0.003-0.015% nitrogen (N), 0.0001-0.050% calcium (Ca), with the balance being iron (Fe) and unavoidable impurities, has a fine structure, graphite grains distributed in a ferrite matrix, a graphitization rate of 95% or more, and contains a total of 5% by weight or less of MnS inclusions and pearlite.

[0011] A method for producing graphite steel of the present invention includes the steps of producing a billet containing, by weight, 0.60 to 0.90% carbon (C), 2.0 to 2.5% silicon (Si), 0.7 to 1.3% manganese (Mn), 0.2 to 0.5% sulfur (S), 0.01 to 0.05% aluminum (Al), 0.005 to 0.020% titanium (Ti), 0.003 to 0.015% nitrogen (N), 0.0001 to 0.050% calcium (Ca), and the balance being iron (Fe) and unavoidable impurities, hot rolling the billet to produce a wire rod, and graphitizing heat treatment of the produced wire rod. Effect of the Invention

[0012] According to the present invention, since the graphite steel contains calcium (Ca), it can form Ca-Al-based oxides that act as nuclei for graphitization to promote graphitization, and can generate Ca-based sulfides to improve machinability. Therefore, it is possible to provide a graphite steel with excellent cutting performance that can replace conventional free-cutting steel materials, and a manufacturing method thereof.

[0013] The graphite steel according to the present invention has excellent cutting performance and can replace conventional free-cutting steel materials. It is also possible to provide an environmentally friendly graphite free-cutting steel that replaces harmful elements such as Pb and Bi. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The graphite steel of the present invention contains, by weight, 0.60 to 0.90% carbon (C), 2.0 to 2.5% silicon (Si), 0.7 to 1.3% manganese (Mn), 0.2 to 0.5% sulfur (S), 0.01 to 0.05% aluminum (Al), 0.005 to 0.02% titanium (Ti), 0.003 to 0.0150% nitrogen (N), 0.0001 to 0.050% calcium (Ca), with the balance being iron (Fe) and unavoidable impurities. The graphite steel has a fine structure with graphite grains distributed in a ferrite matrix, a graphitization rate of 95% or more, and contains a total of 5% by weight or less of MnS inclusions and pearlite.

[0015] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0016] The terms used in this application are merely used to describe specific examples. For example, a singular expression includes a plural expression unless the context clearly requires a singular expression.

[0017] In the following, unless otherwise specified, the unit is % by weight. Furthermore, when a part is described as "comprising" a certain component, this does not mean that other components are excluded, but that other components may be further included, unless otherwise specified.

[0018] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as that commonly understood by a person having ordinary skill in the art to which the present invention belongs. Therefore, unless clearly defined herein, specific terms should not be interpreted as having an overly ideal or formal meaning. For example, in this specification, singular expressions include plural expressions unless there is a clear exception in the context.

[0019] In addition, in this specification, the terms "about," "substantially," and the like are used in the sense of a numerical value or close to a numerical value when the tolerances of manufacturing and materials inherent in the referred meaning are presented, and are used to prevent unconscionable infringers from unfairly exploiting the disclosure in which precise or absolute numerical values ​​are referred to in order to aid in the understanding of the present invention.

[0020] Hereinafter, a graphite steel having excellent cutting performance and a method for producing the same according to the present invention will be described in detail.

[0021] <Graphite steel>

[0022] The graphite steel of the present invention can contain, by weight, 0.60 to 0.90% carbon (C), 2.0 to 2.5% silicon (Si), 0.7 to 1.3% manganese (Mn), 0.2 to 0.5% sulfur (S), 0.01 to 0.05% aluminum (Al), 0.005 to 0.020% titanium (Ti), 0.003 to 0.015% nitrogen (N), and 0.0001 to 0.050% calcium (Ca), with the remainder being iron (Fe) and unavoidable impurities.

[0023] <Component range>

[0024] Carbon (C): 0.60~0.90% by weight

[0025] Carbon is an essential element for forming graphite grains. If the carbon content is less than 0.60 wt%, the effect of improving machinability is insufficient, and the distribution of graphite grains is non-uniform even after graphitization is completed. On the other hand, if the carbon content is excessively high, such as more than 0.90 wt%, the graphite grains are generated coarsely, and the aspect ratio increases, which may deteriorate machinability, especially surface roughness. Therefore, the upper limit of the carbon content is preferably 0.90 wt%.

[0026] Silicon (Si): 2.0 to 2.5% by weight

[0027] Silicon is a necessary component as a deoxidizer during the production of molten steel, and is a graphitization-promoting element that destabilizes cementite in steel so that carbon precipitates as graphite, so it is preferable that silicon is essentially contained. In order to achieve such effects in the present invention, the content is preferably 2.0 wt %.

[0028] On the other hand, if the content is too high, not only will the effect saturate, but the hardness will increase due to the solid solution strengthening effect, accelerating tool wear during cutting, and the increase in nonmetallic inclusions may cause brittleness and excessive decarburization during hot rolling. Therefore, the upper limit of the silicon content is preferably 2.5 wt.%.

[0029] Manganese (Mn): 0.7 to 1.3% by weight

[0030] Manganese improves the strength and impact properties of steel, and combines with sulfur in the steel to form MnS inclusions, which contribute to improving machinability. In order to achieve these effects in the present invention, it is preferable that manganese be contained in an amount of 0.7 wt % or more.

[0031] On the other hand, if the content is too high, it may inhibit graphitization, delay the completion time of graphitization, and increase the strength and hardness, thereby decreasing machinability. Therefore, the upper limit of the manganese content is preferably 1.3 wt.%.

[0032] Sulfur (S): 0.2~0.5% by weight

[0033] Sulfur can combine with manganese to form MnS inclusions, and the formation of MnS can improve machinability. However, if sulfur is contained in excess, it can inhibit graphitization of carbon in steel, segregate at grain boundaries to reduce toughness, form low melting point sulfides to inhibit hot rolling properties, and MnS elongated by rolling can cause mechanical anisotropy. Therefore, in the present invention, the sulfur (S) content can be adjusted within a range that does not cause mechanical anisotropy and contributes to improving machinability, thereby inducing the formation of MnS inclusions.

[0034] Therefore, if the sulfur content is controlled to less than 0.2 wt%, it is not possible to create a fraction sufficient to improve the cutting performance of MnS inclusions, and if it exceeds 0.5 wt%, the anisotropy of the material increases, causing a phenomenon in which the material is broken during cutting, which may pose a danger during processing.

[0035] Aluminum (Al): 0.01 to 0.05% by weight

[0036] Aluminum is the element that promotes graphitization next to silicon. This is because aluminum makes cementite unstable when it exists as solid solution Al, and therefore it is necessary for aluminum to exist as solid solution Al. In order to exhibit such an effect in the present invention, it is preferable that aluminum is contained in an amount of 0.01 wt % or more.

[0037] On the other hand, if the content is too high, not only will the effect be saturated, but it may also induce nozzle clogging during continuous casting, and AlN will be generated at the austenite grain boundaries, causing graphite to be distributed unevenly at the grain boundaries with AlN as a nucleus. Therefore, the upper limit of the aluminum content is preferably 0.05 wt%.

[0038] Titanium (Ti): 0.005 to 0.020% by weight

[0039] Titanium, like aluminum, combines with nitrogen to form nitrides such as TiN and AlN, and these nitrides act as nuclei for the formation of graphite during isothermal heat treatment.

[0040] However, since AlN has a low formation temperature, it precipitates unevenly at grain boundaries after austenite is formed, whereas TiN has a higher formation temperature than AlN and crystallizes before the formation of austenite is complete, resulting in uniform distribution at the grain boundaries and within the austenite grains. Therefore, the graphite grains formed using TiN as a nucleation site are fine and distributed uniformly.

[0041] In order to achieve this effect, it is preferable that titanium is contained in an amount of 0.005% by weight or more, but if the content exceeds 0.02%, it may become coarse carbonitrides and consume carbon necessary for graphite formation, thereby inhibiting graphitization. Therefore, the upper limit of the titanium content is preferably 0.020% by weight.

[0042] Nitrogen (N): 0.003 to 0.015% by weight

[0043] Nitrogen combines with titanium and aluminum to produce TiN, AlN, etc., and nitrides such as AlN are mainly formed at the austenite grain boundaries. During graphitization heat treatment, graphite is formed using these nitrides as nuclei, which can cause uneven distribution of graphite, so it is necessary to add an appropriate amount.

[0044] If the amount of nitrogen added is excessive and it is unable to combine with nitride-forming elements and exists in the steel as solid solution nitrogen, it will have the deleterious effect of increasing strength, stabilizing cementite, and retarding graphitization.

[0045] Therefore, in order to ensure that nitrogen is consumed in forming nitrides that act as nucleation sites for graphite and does not remain as solid solution nitrogen, it is preferable in the present invention to limit the nitrogen content to 0.003% by weight as the lower limit and 0.015% by weight as the upper limit.

[0046] Calcium (Ca): 0.0001 to 0.050% by weight

[0047] In the steel composition of the present invention, calcium forms Ca-Al oxides, which act as nuclei for graphitization to promote graphitization, and also form Ca sulfides to improve machinability. Stress is concentrated at the interface between the Ca sulfides and the matrix during cutting, generating voids, which grow and propagate as cracks and are cut into chips in the steel.

[0048] If the calcium content is less than 0.0001% by weight, the effect is insufficient, and if it exceeds 0.050% by weight, a large amount of coarse oxide-based nonmetallic inclusions may be generated, which may reduce the fatigue strength of the mechanical part. Therefore, the calcium content is preferably in the range of 0.0001 to 0.050% by weight.

[0049] Other Ingredients

[0050] The remaining component of the present invention is iron (Fe). However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to exclude them. However, the graphite steel according to the present invention may be free of phosphorus (P) and oxygen (O). These impurities are known to anyone skilled in normal manufacturing processes, and therefore all of them will not be specifically mentioned in this specification.

[0051] <Microstructure>

[0052] The graphite steel according to the present invention has a fine structure in which graphite grains are distributed in a ferrite matrix, has a graphitization rate of 95% or more, and contains MnS inclusions and pearlite in a total amount of 5% by weight or less.

[0053] The graphitization rate of the graphite steel of the present invention may be preferably 98% or more, more preferably 99% or more, and most preferably 99.5% or more.

[0054] Meanwhile, the graphitization rate means the ratio of the carbon content existing in the graphite state to the carbon content added to the steel, and is defined by the following Relational Formula 1. 95% or more graphitization means that most of the added carbon has been consumed to generate graphite (the amount of carbon dissolved in ferrite and in fine carbides is very small and is therefore not taken into consideration), and that there is no undecomposed pearlite, i.e., the steel has a fine structure in which graphite grains are distributed in the ferrite matrix.

[0055] [Relationship 1] Graphitization rate (%) = (1 - carbon content in undecomposed pearlite / carbon content in steel) x 100

[0056] (If there is no undecomposed pearlite, the graphitization rate will be 100%)

[0057] The method for producing graphite steel described below may be applied to all of the contents described above regarding the graphite steel, and although detailed descriptions of overlapping parts are omitted, the method may be applied in the same manner even if the descriptions are omitted.

[0058] <Graphite steel manufacturing method>

[0059] A method for producing graphite steel of the present invention includes the steps of producing a billet containing, by weight, 0.60 to 0.90% carbon (C), 2.0 to 2.5% silicon (Si), 0.7 to 1.3% manganese (Mn), 0.2 to 0.5% sulfur (S), 0.01 to 0.05% aluminum (Al), 0.005 to 0.02% titanium (Ti), 0.003 to 0.0150% nitrogen (N), 0.0001 to 0.05% calcium (Ca), and the balance being iron (Fe) and unavoidable impurities, hot rolling the billet to produce a wire rod, and graphitizing heat treatment of the produced wire rod.

[0060] Rolling process

[0061] Also, according to the present invention, the hot rolling step may include hot rolling at a temperature range of 900 to 1150° C. Specifically, the hot rolling step may include performing a heat treatment at a temperature range of 900 to 1150° C. for a predetermined time, followed by rolling.

[0062] The wire rod rolling temperature is set in the range of 900 to 1150°C because, if the temperature is less than 900°C, surface grooves are likely to occur during hot rolling, the rolling load increases, and rolling becomes difficult, whereas, if the temperature exceeds 1150°C, the AGS (austenite grain size) becomes coarse, which may lengthen the graphitization heat treatment time after wire rod rolling.

[0063] Graphitization heat treatment process

[0064] Also, according to the present invention, the graphitization heat treatment step may include heat treatment at a temperature range of 700 to 800° C. for 5 hours or more, preferably 5 hours or more and less than 20 hours.

[0065] If the wire is heat-treated for 5 hours or more within the range of 700 to 800°C, a graphitization rate of 95% or more can be achieved. However, if the temperature is less than 700°C, the graphitization heat treatment time is extended to more than 20 hours, and if the temperature is more than 800°C, not only the graphitization time is extended, but also austenite is generated by reverse transformation of pearlite, which may cause pearlite to be generated again during cooling, which is not preferable.

[0066] The present invention will now be described in more detail with reference to examples.

[0067] The following examples are provided to fully convey the concept of the present invention to those skilled in the art to which the present invention pertains, and the present invention is not limited to the examples set forth herein and may be embodied in other forms.

[0068] <Example>

[0069] A billet having the composition shown in Table 1 below was maintained at a heating temperature of 1050°C for 90 minutes and was then rolled at high speed into a wire having a diameter of 19 mm. The graphitization heat treatment time and graphitization rate are shown in Table 2. The graphitization heat treatment temperature was constantly set to "A1 temperature - 50°C" for the graphitization heat treatment.

[0070] In the following Tables 1 and 2, Examples 1 to 11 correspond to graphite steel wire rods that satisfy the alloy composition range and manufacturing conditions of the present invention, and Comparative Examples 1 to 7 correspond to wire rods that do not satisfy the alloy composition range and / or manufacturing conditions of the present invention.

[0071] [Table 1]

[0072] [Table 2]

[0073] In Table 2, the (100%-graphitization rate) structure is composed of MnS inclusions, pearlite, and some other inclusions that are normally present, and the graphitized structure is composed of ferrite and graphite grains.

[0074] In Table 2, the machinability is a value based on the cutting performance of general free-cutting steel (100% means the same level).

[0075] It can be confirmed that the graphitized fraction and machinability are achieved under the manufacturing conditions of the graphite free-cutting steel, as shown in Table 2.

[0076] The Examples and Comparative Examples are evaluated with reference to Tables 1 and 2 below.

[0077] It was confirmed that Examples 1 to 11, which satisfied the alloy composition range and manufacturing conditions of the present invention, had a graphitization rate of 98.5% or more and 100% cutting performance compared to lead free-cutting steel.

[0078] On the other hand, in Comparative Examples 1 to 7, which had an alloy composition containing more than 0.05 wt. % calcium and in which the graphitization heat treatment was maintained for less than 5 hours, the graphitization rate was only 92% or less and the machinability was only 95% or less.

[0079] Specifically, the graphite steels of Comparative Examples 1 and 2, which had manganese contents exceeding 1.3 wt%, sulfur contents less than 0.2 wt%, and calcium contents exceeding 0.05 wt%, did not produce sufficient MnS inclusions, and their cutting performance was only 88% and 95%, respectively, compared to lead free-cutting steel. The graphitization heat treatment was maintained for 3.5 hours or less, and the graphitization rate was only 86% or less.

[0080] In addition, the cutting performance of the graphite steel of Comparative Example 3, which had a manganese content of 1.50 wt%, a sulfur content of 0.56 wt%, and a calcium content of 0.08 wt%, was only 89% of that of the lead free-cutting steel, and the graphitization rate was only 86% when the graphitization heat treatment was maintained for 3.0 hours.

[0081] In addition, the cutting performance of the graphite steel of Comparative Example 4, which had a carbon content of 0.95 wt%, a manganese content of 0.40 wt%, a sulfur content of 0.011 wt%, and a calcium content of 0.07 wt%, was only 92% of that of the lead free-cutting steel, and the graphitization rate was only 84% after the graphitization heat treatment was maintained for 4.5 hours.

[0082] In addition, the cutting performance of the graphite steel of Comparative Example 5, which had a carbon content of 0.55 wt%, a silicon content of 2.6 wt%, a manganese content of 0.56 wt%, a sulfur content of 0.60 wt%, a titanium content of 0.025 wt%, and a calcium content of 0.1 wt%, was only 91% of that of the lead free-cutting steel, and the graphitization rate was only 91% when the graphitization heat treatment was maintained for 2.5 hours.

[0083] In addition, the cutting performance of the graphite steel of Comparative Example 6, which had a silicon content of 2.75 wt%, a manganese content of 0.65 wt%, a sulfur content of 0.15 wt%, a titanium content of 0.03 wt%, and a calcium content of 0.085 wt%, was only 93% of that of the lead free-cutting steel, and the graphitization rate was only 92% when the graphitization heat treatment was maintained for 3.0 hours.

[0084] In addition, the graphite steel of Comparative Example 7, which had a silicon content of 2.8 wt%, a manganese content of 0.60 wt%, a sulfur content of 0.10 wt%, a titanium content of 0.002 wt%, and a calcium content of 0.07 wt%, had a cutting performance that was only 90% of that of the lead free-cutting steel, and the graphitization rate was only 90% when the graphitization heat treatment was maintained for 4.5 hours.

[0085] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it should be understood that various changes and modifications can be made by those skilled in the art without departing from the concept and scope of the following claims. [Industrial Applicability]

[0086] According to the present invention, it is possible to provide an environmentally friendly graphite free-cutting steel which has excellent cutting performance and can replace conventional free-cutting steel materials by replacing harmful elements such as Pb and Bi, and thus the industrial applicability is recognized.

Claims

1. The alloy contains, by weight, 0.60-0.90% carbon (C), 2.0-2.5% silicon (Si), 0.7-1.3% manganese (Mn), 0.2-0.5% sulfur (S), 0.01-0.05% aluminum (Al), 0.005-0.020% titanium (Ti), 0.003-0.015% nitrogen (N), and 0.0001-0.050% calcium (Ca), with the remainder being iron (Fe) and unavoidable impurities; A calcium-containing graphite steel having excellent cutting performance, characterized in that it has a fine structure, graphite grains are distributed in a ferrite matrix, the graphitization rate is 95% or more, and it contains MnS inclusions and pearlite in a total amount of 5% by weight or less.

2. 2. The calcium-containing graphite steel with excellent cutting performance according to claim 1, wherein the graphite steel has a graphitization rate of 99% or more.

3. 2. The calcium-containing graphite steel having excellent cutting performance according to claim 1, which does not contain any one or more of phosphorus (P) and oxygen (O).

4. producing a billet containing, in weight percent, 0.60-0.90% carbon (C), 2.0-2.5% silicon (Si), 0.7-1.3% manganese (Mn), 0.2-0.5% sulfur (S), 0.01-0.05% aluminum (Al), 0.005-0.020% titanium (Ti), 0.003-0.015% nitrogen (N), 0.0001-0.05% calcium (Ca), with the balance being iron (Fe) and unavoidable impurities; hot rolling the billet to produce a wire rod; and The method for producing calcium-containing graphite steel having excellent cutting performance comprises the step of graphitizing the produced wire rod through a heat treatment.

5. 5. The method of claim 4, wherein the hot rolling is performed at a temperature in the range of 900 to 1150°C.

6. 5. The method of claim 4, wherein the graphitization heat treatment is performed at a temperature in the range of 700 to 800° C. for 5 hours or more.

7. 7. The method for producing calcium-containing graphite steel with excellent cutting performance according to claim 6, wherein the graphitization heat treatment is carried out for 5 to 20 hours.

Images