WO2023113428A1 - Calcium-containing graphite steel having excellent machinability, and manufacturing method therefor - Google Patents

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 a machinability superior to that of normal free-cutting steel, and a manufacturing method therefor.

Description

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

The present invention relates to graphite steel with excellent cutting performance and a method for manufacturing the same, and more particularly, to calcium-containing graphite steel with excellent cutting performance compared to general free cutting steel and a method for manufacturing the same.

In general, as a material for machine parts or the like requiring machinability, free-cutting steel to which machinability imparting elements such as Pb and Bi are added is used. In order to improve the machinability of steel materials, liquid metal embrittlement is used by adding low melting point machinability imparting elements such as Pb and Bi to steel, or a large amount of MnS is formed in steel. It has excellent cutting properties such as durability and tool life.

However, in the case of generally known Pb-added free-cutting steel having excellent machinability, it emits harmful substances such as toxic fumes during cutting, which is very harmful to the human body and has a very disadvantageous problem in recycling steel materials. Therefore, the addition of S, Bi, Te, Sn, etc. has been proposed to replace this, but there are many problems in that production is very difficult due to easy cracking during steel manufacturing or cracking occurs during hot rolling. this has been known

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 ferrite and pearlite matrix, and the fine graphite grains inside act as a crack source during cutting and serve as a chip breaker, so it has good machinability.

However, despite these advantages of graphite steel, graphite steel is currently not commercialized. This is because when carbon is added to steel, although graphite is a stable phase, it is precipitated as cementite, which is a metastable phase, and it is difficult to precipitate graphite without a separate long-term heat treatment. This is because there are adverse effects that adversely affect performance.

In addition, even if graphite grains are precipitated through graphitization heat treatment, if they are unevenly distributed in an irregular shape, the distribution of physical properties during cutting is non-uniform, resulting in very poor chip control and surface roughness, and shortening tool life, which is an advantage of graphite steel is difficult to obtain Therefore, it is necessary to provide a method for manufacturing graphite free-cutting steel having excellent cutting performance by utilizing MnS inclusions while using graphite grains.

(Prior Art Document) Patent Document 1: Korean Patent Publication No. 10-2015-0057400

The present invention is to provide a calcium-containing graphite steel having excellent cutting performance and a manufacturing method thereof.

However, the problem to be solved by the present application is not limited to the above-mentioned problem, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

Graphite steel according to an embodiment of the present invention for achieving the above object is carbon (C): 0.60 ~ 0.90%, silicon (Si): 2.0 ~ 2.5%, manganese (Mn): 0.7 ~ 1.3%, Sulfur (S): 0.2~0.5%, Aluminum (Al): 0.01~0.05%, Titanium (Ti): 0.005~0.020%, Nitrogen (N): 0.003~0.015%, Calcium (Ca): 0.0001~ 0.050%, the balance including iron (Fe) and unavoidable impurities; As a microstructure, graphite grains are distributed in the ferrite matrix, the graphitization rate is 95% or more, and a total of 5% by weight or less of MnS inclusions and pearlite may be included.

In the manufacturing method of graphite steel according to another embodiment of the present invention, carbon (C): 0.60 ~ 0.90%, silicon (Si): 2.0 ~ 2.5%, manganese (Mn): 0.7 ~ 1.3%, sulfur (in weight%) S): 0.2 to 0.5%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.020%, nitrogen (N): 0.003 to 0.015%, calcium (Ca): 0.0001 to 0.050%, balance Preparing a billet containing iron (Fe) and unavoidable impurities; hot-rolling the billet to manufacture a wire rod; and subjecting the manufactured wire rod to graphitization heat treatment.

By containing calcium (Ca), the graphite steel according to the present invention can promote graphitization by forming Ca-Al-based oxides that act as graphitization nuclei, and can improve machinability by generating Ca-based emulsions, The present invention can provide graphite steel and a method for manufacturing the same, which can replace existing free-cutting steel materials due to their excellent cutting performance.

The graphite steel according to the present invention has excellent cutting performance and can replace existing free-cutting steel materials, and can provide eco-friendly graphite free-cutting steel that replaces harmful elements such as Pb and Bi.

Graphite steel according to an embodiment of the present invention, in weight%, carbon (C): 0.60 ~ 0.90%, silicon (Si): 2.0 ~ 2.5%, manganese (Mn): 0.7 ~ 1.3%, sulfur (S): 0.2 to 0.5%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.0030 to 0.0150%, calcium (Ca): 0.0001 to 0.050%, balance of iron (Fe ) and unavoidable impurities; As a microstructure, graphite grains are distributed in a ferrite matrix, the graphitization rate is 95% or more, and a total of 5% by weight or less of MnS inclusions and pearlite are included.

Preferred embodiments of the present invention are described below. However, the embodiments of the present invention can be modified in many different forms, and the technical spirit of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Terms used in this application are only used to describe specific examples. Therefore, for example, expressions in the singular number include plural expressions unless the context clearly requires them to be singular.

In the following, unless otherwise specified, units are % by weight. In addition, when a certain component is said to "include", it means that it may further include other components without excluding other components unless otherwise stated.

Meanwhile, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Accordingly, certain terms should not be interpreted in an overly idealistic or formal sense unless clearly defined herein. For example, in this specification, a singular expression includes a plurality of expressions unless there is a clear exception from the context.

In addition, "about", "substantially", etc. in this specification are used at or in the sense of or close to the value when manufacturing and material tolerances inherent in the stated meaning are presented, and are accurate to aid in understanding the present invention. or absolute numbers are used to prevent unfair use by unscrupulous infringers of the stated disclosure.

Hereinafter, graphite steel having excellent cutting performance and a manufacturing method thereof according to the present invention will be described in detail.

[graphite steel]

Graphite steel according to an embodiment of the present invention, in weight%, carbon (C): 0.60 ~ 0.90%, silicon (Si): 2.0 ~ 2.5%, manganese (Mn): 0.7 ~ 1.3%, sulfur (S): 0.2 to 0.5%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.020%, nitrogen (N): 0.003 to 0.015%, calcium (Ca): 0.0001 to 0.050%, the balance of iron (Fe ), and may further include unavoidable impurities.

[Ingredient range]

Carbon (C): 0.60 to 0.90% by weight

Carbon is an essential element for forming graphite grains. When the carbon content is less than 0.60% by weight, the effect of improving machinability is insufficient and the distribution of graphite grains is uneven even when graphitization is completed. There is a risk that cutting performance, especially surface roughness, may be reduced. Therefore, the upper limit of the carbon content is preferably 0.90% by weight.

Silicon (Si): 2.0 to 2.5% by weight

Silicon is a necessary component as a deoxidizer in the manufacture of molten steel, and since it is a graphitization-accelerating element that destabilizes cementite in steel so that carbon can be precipitated as graphite, it is preferable to include silicon. In order to exhibit these effects in the present invention, it is preferably 2.0% by weight.

On the other hand, if the content is excessive, the effect is saturated, and the hardness increases due to the solid solution hardening effect, accelerating tool wear during cutting, causing brittleness due to the increase of non-metallic inclusions, and causing excessive decarburization during hot rolling. There are concerns. Therefore, the upper limit of the silicon content is preferably 2.5% by weight.

Manganese (Mn): 0.7 to 1.3% by weight

Manganese improves strength and impact properties of steel, and contributes to improving machinability by forming MnS inclusions in combination with sulfur in steel. In order to exhibit these effects in the present invention, it is preferably included in an amount of 0.7% by weight or more.

On the other hand, if the content is excessive, graphitization may be inhibited, and the graphitization completion time may be delayed, and strength and hardness may be increased to decrease machinability. Therefore, the upper limit of the manganese content is preferably 1.3% by weight.

Sulfur (S): 0.2 to 0.5% by weight

Sulfur may combine with manganese to form MnS inclusions, and as the MnS is generated, machinability may be improved. However, when sulfur is excessively included, graphitization of carbon in steel may be inhibited, toughness may be reduced by being segregated at grain boundaries, and hot rolling properties may be inhibited by forming low melting point emulsifiers, and elongation by rolling may be inhibited. Mechanical anisotropy may appear due to MnS. Therefore, in the present invention, the generation of MnS inclusions can be induced by adjusting the content of sulfur (S) within a range that can contribute to improving machinability without causing mechanical anisotropy.

Therefore, if the sulfur content is controlled to less than 0.2% by weight, the MnS inclusions cannot be made in a fraction sufficient to improve cutting performance. In addition, when it exceeds 0.5% by weight, the anisotropy of the material increases, resulting in breakage during cutting, which can cause danger during processing.

Aluminum (Al): 0.01 to 0.05% by weight

Aluminum is an element that promotes graphitization next to silicon. This is because aluminum destabilizes cementite when it is present as solid solution Al and therefore it is necessary to be present as solid solution Al. In the present invention, in order to exhibit such an effect, it is preferably included in an amount of 0.01% by weight or more.

On the other hand, if the content is excessive, the effect is not only saturated, but also can cause nozzle clogging during playing, and AlN is generated at the austenite grain boundary, and graphite with it as a nucleus is non-uniformly distributed at the grain boundary. Therefore, the upper limit of the aluminum content is preferably 0.05% by weight.

Titanium (Ti): 0.005 to 0.020% by weight

Titanium, like aluminum, combines with nitrogen to produce nitrides such as TiN and AlN, and these nitrides act as nuclei for generating graphite during constant temperature heat treatment.

However, AlN has a low formation temperature and is non-uniformly deposited at grain boundaries after austenite is formed, whereas TiN has a higher formation temperature than AlN and crystallizes before austenite formation is completed, so that austenite grain boundaries and grains are uniformly distributed. Therefore, graphite grains generated by using TiN as a nucleation site are also finely and uniformly distributed.

In order to show these effects, it is preferable to include 0.005% by weight or more, but when the content is added in excess of 0.02%, it becomes coarse carbonitride and consumes carbon necessary for graphite formation, thereby inhibiting graphitization. . Therefore, the upper limit of the titanium content is preferably 0.020% by weight.

Nitrogen (N): 0.003 to 0.015% by weight

Nitrogen is combined with titanium and aluminum to produce TiN, AlN, etc. In particular, nitrides such as AlN are mainly formed at austenite grain boundaries. Since graphite is formed with these nitrides as nuclei during graphitization heat treatment, non-uniform distribution of graphite may be caused, so an appropriate amount of addition is required.

When the amount of nitrogen added is excessive, it cannot be combined with the nitride-forming element, and when it is present in steel as solid nitrogen, it has a detrimental effect of increasing strength and stabilizing cementite to delay graphitization.

Therefore, in the present invention, it is preferable to limit 0.003 wt% to an upper limit of 0.015 wt% in the present invention for the reason that it is consumed to form nitride that acts as a graphite nucleation site and does not remain as solid nitrogen.

Calcium (Ca): 0.0001 to 0.050% by weight

Calcium forms a Ca-Al-based oxide in the composition of the steel of the present invention, and the Ca-Al-based oxide acts as a graphitization nucleus to promote graphitization, and also generates a Ca-based emulsion to improve machinability. can do. Stress is concentrated during cutting at the interface between the Ca-based emulsion and the matrix structure, creating voids and growing and propagating as cracks, showing the effect of separating and cutting into chips in steel.

This effect is insufficient when the calcium content is less than 0.0001% by weight, and when it exceeds 0.050% by weight, a large amount of coarse oxide-based non-metallic inclusions are generated, which can reduce the fatigue strength of mechanical parts. Therefore, the content of calcium is preferably included in the range of 0.0001 to 0.050% by weight.

other ingredients

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, this cannot be excluded. However, the graphite steel according to the present invention may not contain phosphorus (P) or oxygen (O). Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

[Microstructure]

The graphite steel according to the present invention has a microstructure, graphite grains are distributed in a ferrite matrix, the graphitization rate is 95% or more, and a total of 5% by weight or less of MnS inclusions and pearlite are included.

The graphitization rate of the graphite steel according to an embodiment of the present invention may be preferably 98% or more, more preferably 99% or more, and most preferably 99.5% or more.

On the other hand, the graphitization rate means the ratio of the carbon content present in the graphite state to the carbon content added to the steel, and is defined by the following relational expression 1, and graphitization of 95% or more means that the added carbon mostly produces graphite. In the sense of being consumed (not considered because the amount of carbon dissolved in ferrite and fine carbides is extremely small), it means that there is no undecomposed pearlite, that is, it has a microstructure in which graphite grains are distributed in the ferrite matrix do.

[Relationship 1]

Graphitization rate (%) = (1-carbon content in undissolved pearlite/carbon content in steel) × 100

(Here, if there is no undecomposed pearlite, the graphitization rate is 100%)

In the manufacturing method of graphite steel to be described later, all of the contents described with respect to the graphite steel described above can be applied, and detailed descriptions of overlapping parts have been omitted, but even if the descriptions are omitted, the same can be applied.

[Method for producing graphite steel]

In the manufacturing method of graphite steel according to an embodiment of the present invention, carbon (C): 0.60 ~ 0.90%, silicon (Si): 2.0 ~ 2.5%, manganese (Mn): 0.7 ~ 1.3%, sulfur (S) ): 0.2 to 0.5%, aluminum (Al): 0.01 to 0.05%, titanium (Ti): 0.005 to 0.02%, nitrogen (N): 0.0030 to 0.0150%, calcium (Ca): 0.0001 to 0.05%, balance iron Preparing a billet containing (Fe) and unavoidable impurities; manufacturing a wire rod by hot rolling the billet; and subjecting the manufactured wire rod to graphitization heat treatment.

rolling process

In addition, according to one embodiment of the present invention, the hot rolling step may include hot rolling in a temperature range of 900 ~ 1150 ℃. Specifically, the hot rolling step may be rolling after heat treatment for a predetermined time in a temperature range of 900 ~ 1150 ℃.

If the wire rolling temperature is in the range of 900 to 1150℃, surface defects may easily occur during hot rolling if the temperature is lower than 900℃, or rolling may be difficult due to an increase in the rolling load. This is because the graphitization heat treatment time after wire rod rolling may increase due to coarsening.

Graphitization heat treatment process

Further, according to one embodiment of the present invention, the graphitization heat treatment 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. For example, it may be preferable to perform the graphitization heat treatment temperature at A1 temperature - 50 °C.

When the wire rod is heat treated for 5 hours or more in the range of 700 to 800° C., a graphitization rate of 95% or more may be reached. However, below 700 ° C, the graphitization heat treatment time becomes longer and exceeds 20 hours or more, and above 800 ° C, the graphitization time becomes longer, and austenite is generated by reverse transformation of pearlite, and pearlite is generated again during cooling. undesirable because it can

Hereinafter, the present invention will be described in more detail through examples.

The following examples are presented to sufficiently convey the spirit of the present invention to those skilled in the art to which the present invention belongs, and the present invention is not limited to only the examples presented here, and may be embodied in other forms. there is.

[Example]

A billet having the components shown in Table 1 below was maintained at a heating temperature of 1050° C. for 90 minutes, and then subjected to high-speed wire rolling to produce a wire rod having a diameter of 19 mm. In addition, the graphitization heat treatment time and graphitization rate at this time are shown in Table 2. In addition, the graphitization heat treatment temperature was constantly applied as “temperature - 50 ° C”, and graphitization heat treatment was performed.

In Tables 1 and 2 below, Examples 1 to 11 correspond to graphite steel wires satisfying the alloy composition range and manufacturing conditions of the present invention, and Comparative Examples 1 to 7 satisfy the alloy composition range and/or manufacturing conditions of the present invention. It corresponds to an unsatisfied good.

division	Alloy composition (% by weight)
	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 Example 1	0.63	2.00	1.35	0.05	0.005	0.016	0.001	0.060
Comparative Example 2	0.68	2.20	1.41	0.13	0.006	0.020	0.002	0.065
Comparative Example 3	0.73	2.25	1.50	0.56	0.075	0.009	0.018	0.080
Comparative Example 4	0.95	2.15	0.40	0.11	0.060	0.018	0.020	0.070
Comparative Example 5	0.55	2.60	0.56	0.60	0.003	0.025	0.025	0.100
Comparative Example 6	0.80	2.75	0.65	0.15	0.065	0.030	0.021	0.085
Comparative Example 7	0.75	2.80	0.60	0.10	0.060	0.002	0.022	0.070

division	graphitization heat treatment		Machinability (%)
	time (hr)	Graphitization rate (%)
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 Example 1	3.5	86	88
Comparative Example 2	2.0	85	80
Comparative Example 3	3.0	86	89
Comparative Example 4	4.5	84	92
Comparative Example 5	2.5	91	84
Comparative Example 6	3.0	92	93
Comparative Example 7	4.5	90	90

In Table 2, the structure of (100%-graphitization rate) is composed of MnS inclusions, pearlite and some normally present inclusions, etc., and the graphitization structure is composed of ferrite + graphite grains.

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

As shown in Table 2, graphitization fraction and machinability were achieved under the manufacturing conditions of graphite free-cutting steel.

Hereinafter, Examples and Comparative Examples are evaluated with reference to Tables 1 and 2.

In Examples 1 to 11, it was confirmed that the graphitization rate was 98.5% or more and the cutting performance compared to lead free cutting steel was 100% by satisfying the alloy composition range and manufacturing conditions of the present invention.

On the other hand, Comparative Examples 1 to 7 in which calcium has an alloy composition exceeding 0.05% by weight and the graphitization heat treatment was maintained for less than 5 hours showed a graphitization rate of only 92% or less and a machinability of only 95% or less. there was.

Specifically, the graphite steels of Comparative Examples 1 and 2 having a manganese content of more than 1.3% by weight, a sulfur content of less than 0.2% by weight, and a calcium content of more than 0.05% by weight did not sufficiently generate MnS inclusions, resulting in poor cutting performance. It was only 88% and 95%, respectively, compared to leaded free-cutting steel, and the graphitization rate was only 86% or less because the graphitization heat treatment was maintained for less than 3.5 hours.

In addition, the cutting performance of the graphite steel of Comparative Example 3 having a manganese content of 1.50% by weight, a sulfur content of 0.56% by weight, and a calcium content of 0.08% by weight was only 89% compared to the lead free cutting steel, and the graphitization heat treatment was 3.0% by weight. The time was maintained and the graphitization rate was only 86%.

In addition, the cutting performance of the graphite steel of Comparative Example 4 having a carbon content of 0.95% by weight, a manganese content of 0.40% by weight, a sulfur content of 0.011% by weight, and a calcium content of 0.07% by weight is only 92% compared to lead free cutting steel. The graphitization heat treatment was maintained for 4.5 hours, and the graphitization rate was only 84%.

In addition, the carbon content is 0.55% by weight, the silicon content is 2.6% by weight, the manganese content is 0.56% by weight, the sulfur content is 0.60% by weight, the titanium content is 0.025% by weight, and the calcium content is 0.1% by weight. The cutting performance of the graphite steel of Comparative Example 5 was only 91% compared to the lead free cutting steel, and the graphitization rate was only 91% because the graphitization heat treatment was maintained for 2.5 hours.

In addition, the cutting performance of the graphite steel of Comparative Example 6 having 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% Silver was only 93% compared to leaded free cutting steel, and the graphitization rate was only 92% as the graphitization heat treatment was maintained for 3.0 hours.

In addition, the graphite steel of Comparative Example 7 having a silicon content of 2.8% by weight, a manganese content of 0.60% by weight, a sulfur content of 0.10% by weight, a titanium content of 0.002% by weight, and a calcium content of 0.07% by weight is cut The performance was only 90% compared to leaded free cutting steel, and the graphitization rate was only 90% as the graphitization heat treatment was maintained for 4.5 hours.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those skilled in the art within the scope that does not deviate from the concept and scope of the claims described below. It will be appreciated that many changes and modifications are possible.

According to the present invention, it is possible to substitute existing free-cutting steel materials due to its excellent cutting performance, and it is possible to provide eco-friendly graphite free-cutting steel that replaces harmful elements such as Pb and Bi, and its industrial applicability is recognized.

Claims (7)

1. By weight %, carbon (C): 0.60 to 0.90%, silicon (Si): 2.0 to 2.5%, manganese (Mn): 0.7 to 1.3%, sulfur (S): 0.2 to 0.5%, aluminum (Al): 0.01 ~ 0.05%, titanium (Ti): 0.005 ~ 0.020%, nitrogen (N): 0.003 ~ 0.015%, calcium (Ca): 0.0001 ~ 0.050%, the balance including iron (Fe) and unavoidable impurities;

   A graphite steel having a microstructure in which graphite grains are distributed in a ferrite matrix, a graphitization rate of 95% or more, and containing MnS inclusions and pearlite of a total of 5% by weight or less.
2. The method of claim 1,

   The graphite steel has a graphitization rate of 99% or more.
3. The method of claim 1,

   Graphite steel that does not contain any one or more of phosphorus (P) and oxygen (O).
4. Carbon (C): 0.60 to 0.90%, Silicon (Si): 2.0 to 2.5%, Manganese (Mn): 0.7 to 1.3%, Sulfur (S): 0.2 to 0.5%, Aluminum (Al): 0.01 to 0.01% by weight Preparing a billet containing 0.05%, titanium (Ti): 0.005-0.020%, nitrogen (N): 0.003-0.015%, calcium (Ca): 0.0001-0.05%, the balance of Fe and unavoidable impurities;

   manufacturing a wire rod by hot rolling the billet; and

   A method for producing graphite steel comprising the step of graphitizing the prepared wire rod.
5. The method of claim 4,

   The hot rolling step comprises hot rolling in a temperature range of 900 to 1150 ° C., a method for producing graphite steel.
6. The method of claim 4,

   The graphitization heat treatment step includes heat treatment for 5 hours or more in a temperature range of 700 to 800 ° C., a method for producing graphite steel.
7. The method of claim 6,

   The graphitization heat treatment is a method for producing graphite steel, which is performed for 5 to 20 hours.