WO2023234702A1 - Non-quenched and non-tempered steel wire rod for hot forging with excellent machinability and impact toughness and method for manufacturing same - Google Patents

Abstract

The present invention relates to a non-quenched and non-tempered steel wire rod having improved machinability and toughness and to a method for manufacturing same, the non-quenched and non-tempered wire rod, according to the present invention, comprising, in percentage by weight: C: 0.3-0.5%; Si: 0.4-0.9%; Mn: 0.5-1.2%; P: 0.02% or less; S: 0.01-0.05%; sol.Al: 0.01-0.05%; Cr: 0.1-0.3%; Ti: 0.01-0.02%; Ca: 0.0005-0.002%; N: 0.007-0.02%; and the balance consisting of Fe and inevitable impurities, and comprising ferrite and perlite as a microstructure, wherein following relational expression 1 is satisfied, and the area fraction of MnS satisfies 0.10-0.60%. [Relational expression 1] 20 ≤ [Mn]/[S] ≤ 70

Description

Non-tempered wire for hot forging with excellent machinability and impact toughness and manufacturing method thereof

The present invention relates to a non-quenched wire with improved machinability and impact toughness and a manufacturing method thereof. More specifically, it relates to a non-quenched wire suitable for use as a material for automobiles or machine parts and a manufacturing method thereof.

Unlike tempered steel that secures a certain level of strength and toughness through QT (Quenching and Tempering) heat treatment, non-quenched steel omits the QT heat treatment process. Therefore, non-quenched steel not only has economic advantages such as reduced heat treatment costs, shortened delivery time due to simplification of the process, and improved productivity, but is also an environmentally friendly steel that can be expected to reduce CO ₂ generated by operating the furnace during heat treatment. In the early stages of development, non-quenched steel had relatively inferior toughness compared to tempered steel, so it was applied only to parts that did not require significant toughness. However, recently, as environmental issues and consumer demand for cost reduction have increased, the demand for improving the toughness of non-quenched steel has been increasing. In addition, as cutting processing is often performed to secure the final shape of the part, machinability is also required. To improve machinability, a large amount of MnS is generally created by adding S, which causes the problem of deterioration of product toughness.

One aspect of the present invention is to overcome the inferior toughness compared to existing tempered steel and to provide a non-quenched wire rod and a manufacturing method thereof that can simultaneously secure machinability and impact toughness without additional heat treatment through the addition of high S and high N. .

The non-tempered wire material with improved machinability and impact toughness according to an embodiment of the present invention has a weight percentage of C: 0.3-0.5%, Si: 0.4-0.9%, Mn: 0.5-1.2%, P: 0.02% or less, S : 0.01~0.05%, sol.Al: 0.01~0.05%, Cr 0.1%~0.3%, Ti: 0.01%~0.02%, Ca: 0.0005%~0.002%, N: 0.007%~0.02%, remaining Fe and inevitable It contains impurities, contains ferrite and pearlite as a microstructure, satisfies the following relational equation 1, and has an area fraction of MnS in the range of 0.10 to 0.60%.

[Relationship 1] 20 ≤ [Mn]/[S] ≤ 70

According to one embodiment of the present invention, a non-tempered wire with improved machinability and impact toughness may have a number density of MnS of 70 pieces/mm ² or more and an aspect ratio of MnS of 40 or less.

According to one embodiment of the present invention, a non-tempered wire material with improved machinability and impact toughness may have a tensile strength of 700 MPa or more, a yield strength of 350 to 500 MPa, and a yield ratio of 0.45 to 0.65.

According to an embodiment of the present invention, a non-tempered wire material with improved machinability and impact toughness may have an impact toughness of 60 J/cm ² or more, and the product of tensile strength and impact toughness may be 45,000 MPa·J/cm ² or more.

The method of manufacturing a non-tempered wire with improved machinability and impact toughness according to an embodiment of the present invention is calculated by weight percentage, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02%. Below, S: 0.01 ~ 0.05%, sol.Al: 0.01 ~ 0.05%, Cr 0.1% ~ 0.3%, Ti: 0.01% ~ 0.02%, Ca: 0.0005% ~ 0.002%, N: 0.007% ~ 0.02%, the rest Reheating a steel piece containing Fe and inevitable impurities and containing ferrite and pearlite as a microstructure at a temperature range of 950 to 1120°C; Manufacturing a wire rod by finishing rolling the reheated steel piece at 750 to 850°C; and a step of cooling the wire after winding it, wherein the cooling step after winding includes cooling the wire to 400° C. at an average cooling rate of 0.1 to 5.0° C./s, and the wire has ferrite and pearlite as a microstructure. It contains, satisfies the above relational expression 1, and the area fraction of MnS is 0.10 to 0.60%.

In the non-roughened wire rod with improved machinability and impact toughness according to an embodiment of the present invention, Ti and Al combine with N to form nitrides of TiN and AlN, and these nitrides interfere with grain boundary growth to refine the grain size and improve toughness. . In addition, Ca-based oxides resulting from the addition of Ca act as MnS formation nuclei, suppressing MnS elongation during rolling and improving machinability and toughness. Therefore, even if heat treatment is omitted, it can be applied to automotive materials or mechanical parts materials that require both machinability and toughness.

The non-tempered wire material with improved machinability and impact toughness according to an embodiment of the present invention has a weight percentage of C: 0.3-0.5%, Si: 0.4-0.9%, Mn: 0.5-1.2%, P: 0.02% or less, S : 0.01~0.05%, sol.Al: 0.01~0.05%, Cr 0.1%~0.3%, Ti: 0.01%~0.02%, Ca: 0.0005%~0.002%, N: 0.007%~0.02%, remaining Fe and inevitable It contains impurities, contains ferrite and pearlite as a microstructure, satisfies the following relational equation 1, and has an area fraction of MnS in the range of 0.10 to 0.60%.

[Relationship 1] 20 ≤ [Mn]/[S] ≤ 70

This specification does not describe all elements of the embodiments, and general content or overlapping content between the embodiments in the technical field to which the present invention pertains is omitted. Additionally, when a part "includes" a certain component, this means that it may further include other components rather than excluding other components, unless specifically stated to the contrary. Singular expressions include plural expressions unless the context clearly makes an exception. Hereinafter, the present invention will be described in detail.

The present inventors examined various angles to provide a wire that can secure machinability and impact toughness, and as a result, it was found that machinability and toughness can be secured without separate heat treatment by appropriately controlling the alloy composition and microstructure of the wire. discovered and completed the present invention.

The non-tempered wire material with improved machinability and impact toughness according to an embodiment of the present invention contains, in weight percent, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S : 0.01 ~ 0.05%, sol.Al: 0.01 ~ 0.05%, Cr 0.1% ~ 0.3%, Ti: 0.01% ~ 0.02%, Ca: 0.0005% ~ 0.002%, N: 0.007% ~ 0.02%, the rest Fe and inevitable It contains impurities, contains ferrite and pearlite as a microstructure, satisfies the following relational equation 1, and has an area fraction of MnS of 0.10 to 0.60%.

[Relationship 1] 20 ≤ [Mn]/[S] ≤ 70

Hereinafter, the reason for limiting the numerical content of alloying elements in the embodiments of the present invention will be explained. Hereinafter, unless otherwise specified, the unit is weight%.

The C content is 0.3 to 0.5%.

C is an element that plays a role in improving the strength of wire rods. In order to achieve the above-mentioned effects, it is preferable to contain 0.3% or more of C. However, if the content is excessive, toughness and machinability may deteriorate, so it is desirable to limit the upper limit of the C content to 0.5%.

The Si content is 0.4 to 0.9%.

Si is a useful element as a deoxidizer and an element that plays a role in improving strength. If the Si content is less than 0.4%, the above-described effect cannot be achieved, and if it exceeds 0.9%, the deformation resistance of the steel rapidly increases due to solid solution strengthening, which may deteriorate cold workability. Therefore, in the present invention, the upper limit of the Si content is limited to 0.9%.

The content of Mn is 0.5 to 1.2%.

Mn is a useful element as a deoxidizing agent and desulfurizing agent. If the Mn content is less than 0.5%, the above-described effect cannot be achieved, and if the Mn content exceeds 1.2%, the strength of the steel itself becomes too high, the deformation resistance of the steel rapidly increases, and cold workability may deteriorate. Accordingly, the upper limit of the Mn content is limited to 1.2%.

The Cr content is 0.1 to 0.3% or less.

Cr is an element that promotes ferrite and pearlite transformation during hot rolling. In addition, without increasing the strength of the steel itself more than necessary, it reduces the amount of dissolved carbon by precipitating carbides in the steel and contributes to the reduction of dynamic strain aging caused by dissolved carbon. If the Cr content is less than 0.1%, the above-described effect cannot be achieved, and if it exceeds 0.3%, the strength of the steel itself becomes excessively high and the deformation resistance of the steel rapidly increases, which may result in deterioration of cold workability. Accordingly, the upper limit of Cr content is limited to 0.3%.

The P content is 0.02% or less.

P is an unavoidably contained impurity and is an element that segregates at grain boundaries, lowering the toughness of steel, and is the main cause of reduced delayed fracture resistance. Therefore, in the present invention, it is desirable to control the content as low as possible. In theory, it is advantageous to control the P content at 0%, but since it is inevitably contained during the manufacturing process, it is important to manage the upper limit, and in the present invention, the upper limit of the P content is managed at 0.02%.

The S content is 0.01 to 0.05%.

S is an element that segregates at grain boundaries, greatly reducing the ductility of steel, and forms emulsions in steel, deteriorating delayed fracture resistance and stress relaxation characteristics. It is an impurity that is inevitably contained during the manufacturing process. However, as in the present invention, S is actively used to improve cutting performance. S improves machinability by combining with Mn to form MnS. In the present invention, the content of S, which is effective in improving machinability without significantly reducing the toughness of the steel, is considered and is managed in the range of 0.01% to 0.05%.

The content of Sol.Al is 0.01~0.05%.

sol.Al is an element that acts usefully as a deoxidizing agent. In order to achieve the above-described effect, sol.Al may be included in an amount of 0.01% or more. However, if the Al content exceeds 0.05%, manufacturing difficulties may occur due to Al oxide generated during the casting process. Therefore, in the present invention, the upper limit of Al content is limited to 0.05%.

The content of Ti is 0.01 to 0.02%.

Titanium (Ti) is an element that plays a major role in improving the toughness of steel by forming TiN precipitates during the solidification process of steel, suppressing austenite grain growth during the heating and hot rolling process of slabs, and refining the grain size of the final structure. If the Ti content is less than 0.01%, TiN precipitates are insufficient and it is difficult to secure a sufficient amount to limit the movement of austenite grain boundaries. On the other hand, if it exceeds 0.02%, there is a problem that coarse titanium nitride is generated and the toughness is deteriorated, so in the present invention, the upper limit of the Ti content is limited to 0.02%.

The Ca content is 0.0005~0.002%.

Ca is an essential element to realize the effect of improving machinability and impact toughness by reducing the MnS aspect ratio. When Ca is added, oxide is formed, which acts as a nucleus for MnS generation and suppresses the elongation of MnS during wire rod rolling, thereby maintaining a low aspect ratio. The low aspect ratio of MnS not only improves machinability but also alleviates microstructure anisotropy to prevent deterioration of toughness. However, in order to achieve the above-mentioned effect, Ca must be included in an amount of 0.0005% or more, but if the Ca content exceeds 0.002%, there may be difficulties in manufacturing. Therefore, in the present invention, the upper limit of Ca content is limited to 0.002%.

The N content is 0.007 to 0.02%.

N is an essential element for improving impact toughness by forming nitrides with Ti and Al to refine the particle size. If the N content is less than 0.007%, it is difficult to secure sufficient nitride and the amount of precipitates such as Al and Ti is reduced, making it impossible to secure the toughness targeted by the present invention. If the N content exceeds 0.02%, nitride is used. As non-existent dissolved nitrogen increases, the toughness and ductility of the wire rod may decrease. Therefore, in the present invention, the upper limit of the N content is limited to 0.02%.

Other than the alloy composition, the remainder is Fe. The non-roughened wire rod of the present invention may contain other impurities that may be included during the industrial production process of ordinary steel. Since these impurities are known to anyone with ordinary knowledge in the technical field to which the present invention pertains, the type and content thereof are not particularly limited in the present invention.

The non-quenched wire rod according to an embodiment of the present invention may satisfy the following relational equation 1. In Equation 1, [S] and [Mn] each mean the content (% by weight) of the corresponding element.

[Relationship 1] 20 ≤ [Mn]/[S] ≤ 70 (machinability)

Equation 1 is a formula related to machinability. In the present invention, MnS is formed by adding high S and Mn. MnS is an elongated inclusion that has an elongated shape and direction in the rolling direction and greatly improves the machinability of the medium-carbon, non-tempered wire rod according to the present invention. However, MnS acts as a crack initiation point and propagation path upon impact, thereby deteriorating impact toughness. If the ratio of [Mn]/[S] is less than 20, machinability may be satisfactory, but impact toughness may decrease, and if it exceeds 70, machinability may not be sufficient. Therefore, in the present invention, the ratio of [Mn]/[S] is limited to 20 to 70, and preferably 30 to 60.

In addition, the non-tempered steel according to an embodiment of the present invention may have an area fraction of MnS of 0.10 to 0.60%, preferably 0.15 to 0.50%, and more preferably 0.15 to 0.45%.

In addition, the non-tempered steel according to an embodiment of the present invention may include a number density of MnS of 70 pieces/mm ² or more, preferably 80 pieces/mm ² or more, and more preferably It may contain more than 90 pieces/mm ² . As the density of MnS increases, it acts as a stress concentration source during cutting, reducing cutting resistance and improving cutting performance. To achieve this, the density of MnS must be at least 70 pieces/mm ² or more.

In addition, the non-tempered steel according to an embodiment of the present invention may have an aspect ratio of MnS of 40 or less, preferably 30 or less, and more preferably 20 or less. At this time, if the aspect ratio of MnS exceeds 40, impact toughness can rapidly decrease.

Additionally, the non-quenched steel according to an embodiment of the present invention may have a tensile strength of 700 MPa or more.

Additionally, the non-tempered steel according to an embodiment of the present invention may have a yield strength of 350 to 500 MPa.

Additionally, the non-quenched steel according to an embodiment of the present invention may have a yield ratio of 0.45 to 0.65.

Additionally, the non-quenched steel according to an embodiment of the present invention may have an impact toughness of 60 J/cm ² or more.

In addition, the product of tensile strength and impact toughness of the non-quenched steel according to an embodiment of the present invention may be 45,000 MPa·J/cm ² or more.

Next, a method for manufacturing a non-tempered wire rod according to an embodiment of the present invention will be described.

The non-tempered wire material with improved machinability and impact toughness according to an embodiment of the present invention contains, in weight percent, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S : 0.01 ~ 0.05%, sol.Al: 0.01 ~ 0.05%, Cr 0.1% ~ 0.3%, Ti: 0.01% ~ 0.02%, Ca: 0.0005% ~ 0.002%, N: 0.007% ~ 0.02%, the rest Fe and inevitable Reheating a steel piece containing impurities and containing ferrite and pearlite as a microstructure at a temperature range of 950 to 1120°C; Manufacturing a wire rod by finishing rolling the reheated steel piece at 750 to 850°C; and a step of cooling the wire after winding it, wherein the cooling step after winding includes cooling the wire to 400°C at an average cooling rate of 0.1 to 5.0°C/s, and the wire has ferrite and pearlite as a microstructure. It satisfies the following relational expression 1, and the area fraction of MnS is 0.10 to 0.60%.

[Relationship 1] 20 ≤ [Mn]/[S] ≤ 70

Below, each manufacturing step will be described in more detail.

First, bloom that satisfies the above-mentioned composition is heated and then rolled into steel pieces to obtain a billet.

reheating step

The reheating step is a step of reheating the rolled billet and is a step to lower the rolling load during wire rod rolling. At this time, reheating may be performed at a temperature of 950 to 1120°C. If the reheating temperature of the steel piece is less than 950℃, the rolling load may increase, which may cause manufacturing difficulties. On the other hand, if it exceeds 1,120℃, the AlN finely generated in the steel piece will be re-dissolved during heating, significantly reducing the effect of grain size refinement. .

Wire rod rolling stage

In the wire rolling step, the reheated steel pieces are hot rolled to manufacture wire rods.

At this time, the final rolling temperature of hot rolling may be 750 to 850°C. If the finish rolling temperature is less than 750°C, the rolling load may increase, and if it exceeds 850°C, the grains may become coarse, making it difficult to secure the high toughness targeted by the present invention.

winding step

A process of winding the wire manufactured according to the above into a coil shape may be performed, and at this time, the winding temperature may be 750 to 850°C. Since the temperature of the wire obtained by the finish rolling may increase due to transformation heat generation, the temperature of the wire immediately before winding may be higher than the temperature at which the finish rolling is performed. At this time, depending on the temperature raised by the heat generation, coiling can be performed after cooling to the above coiling temperature, or coiling can be performed without additional cooling. If the coiling temperature is less than 750°C, the surface layer martensite generated during cooling cannot be recovered by reheating, and tempered martensite is generated, which increases the possibility of causing surface defects during wire drawing. On the other hand, if the temperature exceeds 850°C, thick scale is formed on the surface of the wire, which not only makes it easy to cause surface defects during descaling, but also increases the cooling time during subsequent cooling, which may reduce productivity.

cooling step

The wound wire can be subjected to a cooling process, and at this time, the cooling rate is characterized by cooling to 400°C at an average cooling rate in the range of 0.1 to 5.0°C/s through air cooling or controlled cooling after hot forging. If the average cooling rate up to 400℃ after winding is less than 0.1℃/s, the target strength cannot be satisfied due to excessive production of proeutectoid ferrite, and if it exceeds 5℃/s, low-temperature structures such as martensite are generated. Toughness and machinability may be reduced.

Example

Bloom having the alloy composition shown in Table 1 below was heated at 1,200°C for 4 hours and then rolled into steel pieces at a finish rolling temperature of 1,100°C to obtain a billet. Thereafter, the billet was heated for 90 minutes under the temperature conditions shown in Table 2 below, then finish rolled at 800°C, coiled at 780°C, and then cooled under the temperature conditions shown in Table 2 below to produce a wire rod with a diameter of 26 mm. Wire rods having the composition of invention steels 1 to 7 and comparative steels 1 to 6 were manufactured (Table 1), and the machinability, tensile strength, and impact toughness of the collected wire specimens were measured.

Here, the room temperature tensile strength was measured by collecting from the center of the untreated steel specimen at 25℃, and the room temperature impact toughness was obtained by performing a Charpy impact test on a specimen with a U-notch (based on U-notch standard sample, 10x10x55mm) at 25℃. It was evaluated based on the impact energy value.

In addition, to evaluate machinability, a wire rod with a diameter of 26 mm was manufactured into a CD-Bar (Cold Drawn Bar) with a diameter of 24 mm by applying a reduction ratio of 14.8%. Machinability was evaluated using a CNC lathe, and a CD-Bar with a diameter of 24 mm was turned until its diameter was 15 mm, and then the degree of wear of the turning tool was evaluated. At this time, cutting conditions were performed using cutting oil under the conditions of a cutting speed of 100 mm/min, a feed rate of 0.1 mm/rev, and a cutting depth of 1.0 mm, and the cutting tool used was a cermet material tool with a chip breaker. . The wear depth of the tool was measured by measuring the depth of the flank wear surface after continuously machining 300 parts with the above-mentioned shape, and anything exceeding 0.2mm was judged to be defective, and anything less than 0.2mm was judged to be good.

In addition, the area fraction of MnS, the number density of MnS, and the aspect ratio of MnS were analyzed using image analysis software by taking 20 pictures of each L cross-section of the wire with an optical microscope at a magnification of 200 times.

division	Chemical composition (wt.%)
	C	Si	Mn	P	S	Al	Cr	Ti	Ca	N	Equation (1)
Invention Lecture 1	0.45	0.59	1.06	0.0055	0.026	0.047	0.22	0.0189	0.0013	0.0185	40.2
Invention Lecture 2	0.31	0.53	1.15	0.0156	0.032	0.046	0.16	0.0155	0.0017	0.0074	36.3
Invention Lecture 3	0.46	0.82	1.14	0.0104	0.036	0.014	0.15	0.0140	0.0008	0.0187	31.8
Invention Lecture 4	0.49	0.43	1.14	0.0200	0.030	0.044	0.30	0.0112	0.0009	0.0130	38.5
Invention Lecture 5	0.38	0.69	0.95	0.0178	0.018	0.017	0.18	0.0171	0.0014	0.0104	53.7
Invention Lecture 6	0.48	0.58	0.70	0.0137	0.021	0.011	0.13	0.0113	0.0007	0.0133	33.5
Invention Lecture 7	0.35	0.67	1.17	0.0160	0.027	0.050	0.27	0.0134	0.0013	0.0138	42.9
Comparison lecture 1	0.65	0.75	0.89	0.0092	0.016	0.027	0.14	0.0137	0.0014	0.0100	54.3
Comparison lecture 2	0.30	1.21	1.07	0.0123	0.049	0.050	0.26	0.0155	0.0014	0.0195	21.7
Comparison lecture 3	0.44	0.63	1.31	0.0069	0.033	0.032	0.15	0.0199	0.0014	0.0127	39.2
Comparison lecture 4	0.33	0.90	0.88	0.0058	0.041	0.038	0.24	0.0005	0.0017	0.0191	21.5
Comparison lecture 5	0.31	0.81	0.66	0.0184	0.017	0.027	0.25	0.0143	0.0002	0.0052	39.5
Comparison lecture 6	0.40	0.56	1.09	0.0177	0.0109	0.0465	0.22	0.0103	0.0013	0.0097	100.0

division	steel grade	Heating temperature (℃)	Up to 400℃ average cooling rate (℃/s)	MnS area fraction (%)	MnS density (pcs/ mm2 )	MnS aspect ratio
Example 1	Invention Lecture 1	1090	0.5	0.23	117	28
Example 2	Invention Lecture 2	1090	0.5	0.36	141	19
Example 3	Invention Lecture 3	1090	0.5	0.41	160	37
Example 4	Invention Lecture 4	1090	0.5	0.27	132	33
Example 5	Invention Lecture 5	1090	0.5	0.18	82	22
Example 6	Invention Lecture 6	1090	0.5	0.20	93	36
Example 7	Invention Lecture 7	1090	0.5	0.29	121	28
Comparative Example 1	Comparison lecture 1	1090	0.5	0.12	73	27
Comparative example 2	Comparison lecture 2	1090	0.5	0.45	220	24
Comparative example 3	Comparison lecture 3	1090	0.5	0.30	148	23
Comparative example 4	Comparison lecture 4	1090	0.5	0.41	182	21
Comparative Example 5	Comparison lecture 5	1090	0.5	0.18	74	48
Comparative Example 6	Comparison lecture 6	1090	0.5	0.08	48	37
Comparative example 7	Invention Lecture 1	1150	0.5	0.26	117	33
Comparative example 8	Invention Lecture 2	1090	10.0	0.34	141	17
Comparative Example 9	Invention Lecture 3	1090	0.05	0.41	160	23

division	steel grade	tensile strength (MPa)	yield strength (MPa)	surrender fee	tenacity (J/ cm2 )	Tensile strength x Impact toughness (MPa·J/cm 2 )	Machinability (tool wear)
Example 1	Invention Lecture 1	865	418	0.48	75	62486	Good
Example 2	Invention Lecture 2	734	400	0.54	86	64969	Good
Example 3	Invention Lecture 3	886	403	0.45	85	54179	Good
Example 4	Invention Lecture 4	891	492	0.55	63	56033	Good
Example 5	Invention Lecture 5	814	460	0.57	72	62798	Good
Example 6	Invention Lecture 6	848	413	0.49	72	51875	Good
Example 7	Invention Lecture 7	797	410	0.51	89	60722	Good
Comparative Example 1	Comparison lecture 1	930	498	0.54	61	56730	error
Comparative example 2	Comparison lecture 2	723	394	0.55	58	41934	Good
Comparative example 3	Comparison lecture 3	861	459	0.53	55	47355	Good
Comparative example 4	Comparison lecture 4	710	386	0.54	58	41180	Good
Comparative Example 5	Comparison lecture 5	707	394	0.56	52	36764	Good
Comparative Example 6	Comparison lecture 6	820	450	0.55	65	53300	error
Comparative Example 7	Invention Lecture 1	738	424	0.57	57	42066	Good
Comparative example 8	Invention Lecture 2	870	432	0.50	48	41760	error
Comparative Example 9	Invention Lecture 3	688	390	0.57	85	58711	Good

As can be seen in Tables 1 to 3, in Examples 1 to 7, the chemical composition, relational formula, area fraction of MnS, number density, aspect ratio, and manufacturing conditions presented in the present invention were all satisfied, resulting in a tensile strength of 700 MPa or more. , a yield strength of 350 to 500 MPa, a yield ratio of 0.45 to 0.65, a value of the product of tensile strength and impact toughness of more than 45,000 MPa·J/cm ² , impact toughness of more than 60 J/cm ² , and good cutting properties can be secured.

On the other hand, in the case of Comparative Examples 1 to 9 that do not satisfy one or more of the conditions proposed in the present invention, it can be seen that at least one characteristic among tensile strength, impact toughness, tensile strength x impact toughness and machinability appears to be inferior.

Specifically, Comparative Example 1 had poor tool wear due to high strength outside the range of carbon content, and Comparative Examples 2 and 3 had insufficient impact toughness due to excessive Si and Mn contents. In addition, Comparative Example 4 had poor impact toughness because the Ti content was insufficient and the particle size refinement effect was not sufficiently achieved. In Comparative Example 5, impact toughness was reduced due to the large aspect ratio of MnS, and in Comparative Example 6, the value of Equation (1) was not satisfied, and the fraction and density of MnS were insufficient, resulting in poor cutting tool wear. Comparative Examples 7 to 9, although all chemical components were satisfied, had poor toughness or did not meet the target strength because the heating temperature and cooling rate were out of range.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art will recognize the present invention within the scope and spirit of the following claims. You will understand that various changes and modifications are possible.

According to the present invention, it is possible to provide a non-tempered wire rod that can simultaneously secure machinability and impact toughness without additional heat treatment and a manufacturing method thereof, and its industrial applicability is recognized.

Claims (12)

1. In weight percent, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.01 to 0.05%, Cr 0.1% ~ 0.3%, Ti: 0.01% ~ 0.02%, Ca: 0.0005% ~ 0.002%, N: 0.007% ~ 0.02%, including the remaining Fe and inevitable impurities, including ferrite and pearlite as a microstructure, and the following relational formula 1 A non-tempered wire rod with improved machinability and impact toughness that satisfies the requirements and has an area fraction of MnS of 0.10 to 0.60%.

   [Relationship 1] 20 ≤ [Mn]/[S] ≤ 70
2. According to clause 1,

   Non-tempered wire rod with improved machinability and impact toughness with MnS number density of 70 pieces/mm ² or more.
3. According to clause 1,

   A non-tempered wire rod with improved machinability and impact toughness with an MnS aspect ratio of 40 or less.
4. According to clause 1,

   Non-tempered wire rod with improved machinability and impact toughness with a tensile strength of over 700MPa.
5. According to clause 1,

   Non-tempered wire rod with improved machinability and impact toughness with a yield strength of 350 to 500 MPa.
6. According to clause 1,

   Non-tempered wire rod with improved machinability and impact toughness with a yield ratio of 0.45 to 0.65.
7. According to clause 1,

   A non-tempered wire rod with improved machinability and impact toughness with an impact toughness of 60 J/cm ² or higher.
8. According to clause 1,

   A non-tempered wire rod with improved machinability and impact toughness whose product of tensile strength and impact toughness is 45000 MPa·J/cm ² or more.
9. In weight percent, C: 0.3 to 0.5%, Si: 0.4 to 0.9%, Mn: 0.5 to 1.2%, P: 0.02% or less, S: 0.01 to 0.05%, sol.Al: 0.01 to 0.05%, Cr 0.1% ~ 0.3%, Ti: 0.01% ~ 0.02%, Ca: 0.0005% ~ 0.002%, N: 0.007% ~ 0.02%, the remaining Fe and inevitable impurities, and steel pieces containing ferrite and pearlite as microstructure are manufactured at 950 ~ Reheating at a temperature range of 1120°C;

   Manufacturing a wire rod by finishing rolling the reheated steel piece at 750 to 850°C; and

   It includes: winding the wire and then cooling it,

   The cooling step after winding includes cooling to 400°C at an average cooling rate of 0.1 to 5.0°C/s, the wire rod includes ferrite and pearlite as a microstructure, satisfies the following relational equation 1, and the area fraction of MnS Method for manufacturing non-tempered wire rod with improved machinability and impact toughness of 0.10 to 0.60%.

   [Relationship 1] 20 ≤ [Mn]/[S] ≤ 70
10. According to clause 9,

    Method for manufacturing non-tempered wire rod with improved machinability and impact toughness with MnS number density of 70 pieces/mm ² or more.
11. According to clause 9,

    Method for manufacturing non-tempered wire with improved machinability and impact toughness with an aspect ratio of MnS of 40 or less.
12. The method of claim 9, wherein the coiling temperature is 750 to 850°C.