CALCIUM-CONTAINING GRAPHITE STEEL HAVING EXCELLENT MACHINABILITY, AND MANUFACTURING METHOD THEREFOR

Abstract

The present invention relates to a graphite steel having excellent machinability, and a manufacturing method therefor, and more specifically, to a calcium-containing graphite steel having machinability superior to that of normal free-cutting steel and a manufacturing method therefor.

Claims

1. A graphite steel comprising, in percent by weight (wt %), 0.60 to 0.90% of carbon (C), 2.0 to 2.5% of silicon (Si), 0.7 to 1.3% of manganese (Mn), 0.2 to 0.5% of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.020% of titanium (Ti), 0.003 to 0.015% of nitrogen (N), 0.0001 to 0.050% of calcium (Ca), and the balance of iron (Fe) and inevitable impurities, wherein the graphite steel includes a microstructure in which graphite grains are distributed in a ferrite matrix, has a graphitization rate of 95% or more, and includes a total of 5 wt % of MnS inclusions and pearlite.

2. The graphite steel according to claim 1, wherein the graphite steel has a graphitization rate of 99% or more.

3. The graphite steel according to claim 1, wherein the graphite steel does not include at least one phosphorus (P) and oxygen (O).

4. A method for manufacturing a graphite steel, the method comprising: preparing a billet including, in percent by weight (wt %), 0.60 to 0.90% of carbon (C), 2.0 to 2.5% of silicon (Si), 0.7 to 1.3% of manganese (Mn), 0.2 to 0.5% of sulfur (S), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.020% of titanium (Ti), 0.003 to 0.015% of nitrogen (N), 0.0001 to 0.05% of calcium (Ca), and the balance of iron (Fe) and inevitable impurities; hot rolling the billet to prepare a wire rod; and performing graphitizing heat treatment on the prepared wire rod.

5. The method according to claim 4, wherein the hot rolling comprises hot rolling in a temperature range of 900 C. to 1150 C.

6. The method according to claim 4, wherein the graphitizing heat treatment comprises heat treatment in a temperature range of 700 C. to 800 C. for 5 hours or more.

7. The method according to claim 6, wherein the graphitizing heat treatment is performed for 5 hours to 20 hours.

Description

EXAMPLES

[0052] Billets having compositions shown Table 1 below were maintained at a heating temperature of 1050 C. for 90 minutes and rolled at a high speed to prepare wire rods having a diameter of 19 mm. In this regard, graphitizing heat treatment times and graphitization rates are shown in Table 2. In addition, graphitizing heat treatment was performed by constantly applying Al temperature 50 C. as a graphitizing heat treatment temperature.

[0053] In Tables 1 and 2 below, Examples 1 to 11 show graphite steel wire rods satisfying the composition range of alloying elements and manufacturing conditions of the present disclosure, and Comparative Examples 1 to 7 show wire rods not satisfying the composition range of alloying elements and/or manufacturing conditions of the present disclosure.

TABLE-US-00001 TABLE 1 Composition of alloying elements (wt %) Category C Si Mn S Al Ti N Ca Example 1 0.67 2.1 0.85 0.31 0.015 0.015 0.005 0.010 Example 2 0.85 2.15 0.96 0.24 0.023 0.017 0.008 0.020 Example 3 0.72 2.35 1.07 0.35 0.043 0.007 0.010 0.015 Example 4 0.68 2.18 1.16 0.45 0.035 0.009 0.009 0.030 Example 5 0.82 2.25 0.95 0.35 0.040 0.012 0.014 0.040 Example 6 0.62 2.48 0.80 0.25 0.042 0.006 0.012 0.045 Example 7 0.73 2.42 1.20 0.30 0.034 0.011 0.006 0.035 Example 8 0.78 2.32 1.25 0.43 0.019 0.016 0.008 0.030 Example 9 0.85 2.15 0.95 0.27 0.027 0.009 0.013 0.025 Example 10 0.75 2.26 1.15 0.26 0.030 0.018 0.007 0.015 Example 11 0.73 2.46 1.20 0.42 0.020 0.008 0.008 0.035 Comparative 0.63 2.00 1.35 0.05 0.005 0.016 0.001 0.060 Example 1 Comparative 0.68 2.20 1.41 0.13 0.006 0.020 0.002 0.065 Example 2 Comparative 0.73 2.25 1.50 0.56 0.075 0.009 0.018 0.080 Example 3 Comparative 0.95 2.15 0.40 0.11 0.060 0.018 0.020 0.070 Example 4 Comparative 0.55 2.60 0.56 0.60 0.003 0.025 0.025 0.100 Example 5 Comparative 0.80 2.75 0.65 0.15 0.065 0.030 0.021 0.085 Example 6 Comparative 0.75 2.80 0.60 0.10 0.060 0.002 0.022 0.070 Example 7

TABLE-US-00002 TABLE 2 Graphitizing Heat Treatment Grgraphitization Category Hour (hr) rate (%) Machinability (%) Example 1 6.0 99 100 Example 2 9.5 99 100 Example 3 11.0 100 100 Example 4 6.5 100 100 Example 5 13.5 100 100 Example 6 5.5 99.5 100 Example 7 16.5 100 100 Example 8 8.2 98.5 100 Example 9 18.5 100 100 Example 10 5.7 98 100 Example 11 12.5 99 100 Comparative 3.5 86 88 Example 1 Comparative 2.0 85 80 Example 2 Comparative 3.0 86 89 Example 3 Comparative 4.5 84 92 Example 4 Comparative 2.5 91 84 Example 5 Comparative 3.0 92 93 Example 6 Comparative 4.5 90 90 Example 7

[0054] In Table 2, a (1000%-graphitization rate) structure consists of MnS inclusions, pearlite, and some common inclusions, and a graphitized structure consists of ferrite and graphite grains.

[0055] In Table 2, machinability is a value based on machinability of common free-cutting steels (100% refers to an equivalent level).

[0056] It may be confirmed that graphitization fraction and machinability may be achieved under the conditions for manufacturing graphite free-cutting steel as shown in Table 2.

[0057] Hereinafter, Examples and Comparative Examples will be evaluated with reference to Tables 1 and 2.

[0058] In Examples 1 to 11, it was confirmed that the graphitization rates were not less than 98.5% and machinability was 10000 of that of lead free-cutting steels because the compositions of alloying elements and manufacturing conditions of the present disclosure were satisfied.

[0059] On the contrary, according to Comparative Examples 1 to 7 in which Ca contents exceed 0.05 wt % in the compositions of alloying elements and graphitizing heat treatment was maintained for less than 5 hours, it was confirmed that the graphitization rates were not more than 92% and machinability was also not more than 95%.

[0060] Specifically, in the graphite steels according to Comparative Examples 1 and 2 including greater than 1.3 wt % of Mn, less than 0.2 wt % of S, and greater than 0.05 wt % of Ca, machinability was only 88% and 95% of that of lead free-cutting steels due to insufficient formation of MnS inclusions and the graphitization rates were not more than 86% because graphitizing heat treatment was maintained for 3.5 hours or less.

[0061] In addition, in the graphite steel of Comparative Example 3 including 1.50 wt % of Mn, 0.56 wt % of S, and 0.08 wt % of Ca, machinability was only 89% of that of lead free-cutting steel and the graphitization rate was only 86% because graphitizing heat treatment was maintained for 3.0 hours.

[0062] In addition, in the graphite steel of Comparative Example 4 including 0.95 wt % of C, 0.40 wt % of Mn, 0.011 wt % of S, and 0.07 wt % of Ca, machinability was only 92% of that of lead free-cutting steels and the graphitization rate was only 84% because graphitizing heat treatment was maintained for 4.5 hours.

[0063] In addition, in the graphite steel of Comparative Example 5 including 0.55 wt % of C, 2.6 wt % of Si, 0.56 wt % of Mn, 0.60 wt % of S, 0.025 wt % of TI, and 0.1 wt % of Ca, machinability was only 910% of that of lead free-cutting steels and the graphitization rate was only 91% because graphitizing heat treatment was maintained for 2.5 hours.

[0064] In addition, in the graphite steel of Comparative Example 6 including 2.7 wt % of Si, 0.65 wt % of Mn, 0.15 wt % of S, 0.03 wt % of Ti, and 0.085 wt % of Ca, machinability was only 93% of that of lead free-cutting steels and the graphitization rate was only 92% because graphitizing heat treatment was maintained for 3.0 hours.

[0065] In addition, in the graphite steel of Comparative Example 7 including 2.8 wt % of Si, 0.60 wt % of Mn, 0.10 wt % of S, 0.002 wt % of Ti, and 0.07 wt % of Ca, machinability was only 90% of that of lead free-cutting steels and the graphitization rate was only 90% because graphitizing heat treatment was maintained for 4.5 hours.

[0066] While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

[0067] According to the present disclosure, the graphite free-cutting steel according to the present disclosure having excellent machinability may replace conventional free-cutting steels and may be eco-friendly by replacing harmful substances such as Pb and Bi, and therefore the present disclosure has industrial applicability.