WO2023113442A1 - Methods for manufacturing wire rod for cold forging and screw part, having excellent drilling characteristics - Google Patents

Abstract

The present invention relates to methods for manufacturing a wire rod for cold forging and a screw part, which can manufacture with only quenching heat treatment a part with excellent drilling characteristics, and which can specifically manufacture a part to be used for mechanical coupling of automotive parts manufactured with different materials.

Description

Manufacturing method of wire rod and screw parts for cold forging with excellent drilling characteristics

The present invention relates to a method of manufacturing a wire rod for cold forging and a screw part capable of manufacturing parts having excellent drilling characteristics only by quenching heat treatment.

When manufacturing parts such as flow drill screws used for mechanical fastening of automobile parts made of different materials, a high hardness of 500 HV or higher is required to withstand the wear caused by drilling for mechanical fastening. do. To this end, conventionally, a method of increasing the carbon content compared to the material for bolts and manufacturing to have a lower bainite structure through austempering heat treatment, which is an expensive constant temperature heat treatment, has been considered (see FIG. 2). This is what is happening.

On the other hand, in cold forging, the surface texture and dimensional accuracy of parts after forging are excellent, and parts manufactured by cold forging have lower production costs and better yield than parts manufactured by hot forging. For this reason, cold forging is widely applied to the manufacture of various industrial machines including automobiles, such as gears, shafts and bolts, and parts for building structures.

(Prior art literature)

Patent Document 1: Japanese Patent Registration No. 3966493 (registration announcement date: 2007.06.08)

The present invention provides a manufacturing method for securing hardness for reducing wear during drilling by applying only quenching heat treatment instead of conventionally used austempering heat treatment in manufacturing a high-strength cold-rolling wire rod having excellent drilling characteristics. In addition, the component produced by the method according to the present invention is characterized by having a hardness of 500 HV or more.

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.

In order to achieve the above object, an embodiment of the present invention, in weight percent, carbon (C): 0.30 ~ 0.50%, silicon (Si): 0.30 ~ 0.50%, manganese (Mn): 0.35 ~ 0.75% , Chromium (Cr): 0.40~0.70%, Titanium (Ti): 0.010~0.050%, Aluminum (Al): 0.01~0.05%, Boron (B): 0.0010~0.0050%, Nitrogen (N): 0.002~0.020% , providing a billet containing the balance of iron (Fe) and other impurities; heating the billet to a temperature of 900 to 1200° C., and performing finish hot rolling at a temperature of 800 to 1000° C. to manufacture a wire rod; Winding the wire rod at 700 to 900° C. to control the average austenite grain size to be 30 μm or less; And it provides a method for manufacturing a wire rod for cold forging with excellent drilling characteristics, comprising the step of cooling the coiled wire rod at a rate of 0.4 ~ 1.0 ° C / s.

In addition, the microstructure of the manufactured wire rod may include 45 to 60% of ferrite and 40 to 55% of pearlite in area fraction.

In order to achieve the above object, another embodiment of the present invention includes the steps of softening heat treatment of the prepared wire rod for cold forging at a temperature of 600 to 800 ° C; forming a screw part shape having a body diameter of 3 to 6 mm by cold forging the wire rod subjected to the softening heat treatment; heating for 500 to 4,000 seconds at a temperature of 850 to 950° C. so that the average austenite grain size is 15 μm or less after the cold forging; And after the heating step of quenching in a refrigerant of 20 ~ 100 ℃; containing, provides a method for manufacturing a screw part.

In addition, in the quenching step, the area fraction is controlled to have a microstructure of 70% or more of auto-tempered martensite, 0.1 to 5.0% of bainite, 1 to 28% of fresh martensite, and 0.1 to 1.0% of retained austenite. can include There is an advantage that brittleness of parts can be alleviated compared to parts made only of fresh martensite by controlling to have the above micro-refining.

In addition, the quenching step may include controlling the average thickness of carbides precipitated in the prior austenite crystal grains to be 20 nm or less.

In addition, the screw part may have a hardness of 500 HV or more at room temperature.

In order to achieve the above object, another embodiment of the present invention is a screw part manufactured by the above method, wherein the screw part has an area fraction of 70% or more of auto-tempered martensite, 0.1 to 5% of bainite, Provides a screw part with a microstructure of 1-28% fresh martensite and 0.1-1% retained austenite, an average thickness of carbides precipitated in old austenite grains of 20 nm or less, and a hardness of 500 HV or more at room temperature .

In the method for manufacturing a wire rod for cold forging according to the present invention, only quenching heat treatment is applied instead of the expensive austempering heat treatment that has been mainly used in the prior art, and auto-tempered martensite is 70% or more, bainite is 0.1 to 5%, and fresh martensite is 1 It has a microstructure composed of ~28% and retained austenite 0.1~1%, and in particular, the average thickness of carbides precipitated in the prior austenite crystal grains can be made to be 20 nm or less, and thus, parts with a hardness of 500 HV or more. can provide.

1 is a graph of state change in the case of quenching heat treatment performed on an inventive material.

Figure 2 is a state change graph in the case of conventional austempering heat treatment performed on existing products.

In the method for manufacturing a wire rod for cold forging having excellent drilling characteristics according to an embodiment of the present invention, in weight%, carbon (C): 0.30 to 0.50%, silicon (Si): 0.30 to 0.50%, manganese (Mn): 0.35~0.75%, Chromium (Cr): 0.40~0.70%, Titanium (Ti): 0.010~0.050%, Aluminum (Al): 0.01~0.05%, Boron (B): 0.0010~0.0050%, Nitrogen (N): Providing a billet containing 0.002-0.020%, the balance of iron (Fe) and other impurities; heating the billet to a temperature of 900 to 1200° C., and performing finish hot rolling at a temperature of 800 to 1000° C. to manufacture a wire rod; Winding the wire rod at 700 to 900° C. to control the average austenite grain size to be 30 μm or less; and cooling the wound wire at a rate of 0.4 to 1.0° C./s.

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 addition, the terms "include" or "have" used in this application are used to clearly indicate that the features, steps, functions, components, or combinations thereof described in the specification exist, but other features It should be noted that it is not intended to be used to preliminarily exclude the presence of any steps, functions, components, or combinations thereof.

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.

The present invention is characterized in that it has excellent hardness while applying only quenching heat treatment instead of expensive austempering heat treatment that has been mainly used in the prior art to secure hardness for reducing wear during drilling.

One embodiment of the present invention, in weight percent, carbon (C): 0.30 ~ 0.50%, silicon (Si): 0.30 ~ 0.50%, manganese (Mn): 0.35 ~ 0.75%, chromium (Cr): 0.40 ~ 0.70 %, titanium (Ti): 0.010 to 0.050%, aluminum (Al): 0.01 to 0.05%, boron (B): 0.0010 to 0.0050%, nitrogen (N): 0.002 to 0.020%, balance iron (Fe) and others providing a billet comprising impurities; heating the billet to a temperature of 900 to 1200° C., and performing finish hot rolling at a temperature of 800 to 1000° C. to manufacture a wire rod; Winding the wire rod at 700 to 900° C. to control the average austenite grain size to be 30 μm or less; And it provides a method for manufacturing a wire rod for cold forging with excellent drilling characteristics, comprising the step of cooling the coiled wire rod at a rate of 0.4 ~ 1.0 ° C / s.

When the average austenite grain size is controlled to be 30 μm or less, cracking can be prevented during quenching heat treatment for manufacturing parts described later.

In addition, the microstructure of the manufactured wire rod includes 45 to 60% of ferrite and 40 to 55% of pearlite in area fraction, so that when the strength of the wire rod is 800 MPa or less, wire drawing is possible without additional softening heat treatment. Conventional materials for parts required two softening heat treatments, but the wire rod according to the present invention can be cold forged with one softening heat treatment.

In order to achieve the above object, another embodiment of the present invention includes the steps of softening heat treatment of the prepared wire rod for cold forging at a temperature of 600 to 800 ° C; forming a screw part shape having a trunk diameter of 3 to 6 mm by cold forging the wire rod subjected to the softening heat treatment; After the cold forging, heating for 500 to 4,000 seconds at a temperature of 850 to 950 ° C so that the average austenite grain size is 15 μm or less doing; And after the heating step of quenching in a refrigerant of 20 ~ 100 ℃; containing, provides a method for manufacturing a screw part.

By controlling the average austenite grain size to be 15 μm or less, it is possible to prevent cracks from occurring inside the part during quenching heat treatment.

In addition, in the quenching step, the area fraction is controlled to have a microstructure of 70% or more of auto-tempered martensite, 0.1 to 5.0% of bainite, 1 to 28% of fresh martensite, and 0.1 to 1.0% of retained austenite to obtain fresh martensite. It has the advantage of mitigating the brittleness of parts compared to parts consisting only of sites.

In addition, in the quenching step, the average thickness of carbides precipitated in the prior austenite crystal grains is controlled to be 20 nm or less, so that cracks can be prevented from occurring inside the part after the quenching heat treatment.

In addition, the screw part may have a hardness of 500 HV or more, preferably 500 to 700 HV at room temperature. When it has a hardness of 500 HV or more, it can withstand the wear caused by drilling work for mechanical fastening.

1 is a schematic diagram of a quenching heat treatment process performed on an inventive material, in which only quenching heat treatment is performed without austempering heat treatment for forming a separate specific microstructure, and quenching is performed using a refrigerant to obtain a desired microstructure.

Figure 2 is a schematic diagram of the case of conventional austempering heat treatment performed on existing products. Although manufactured to have a lower bainite structure through austempering heat treatment, such austempering heat treatment requires high-cost constant temperature heat treatment, resulting in low economic feasibility there is a problem

Hereinafter, the reason for limiting the composition range of each alloy element will be described. Hereinafter, unless otherwise specified, units are % by weight.

C: 0.30 to 0.50%

When the carbon content is less than 0.30%, it is not easy to obtain sufficient material hardness, and it is not easy to secure sufficient hardenability during final heat treatment. In addition, when the carbon content exceeds 0.50%, the hardenability is excessive, which is undesirable because it may cause delayed fracture when tempering is omitted.

Si: 0.30 to 0.50%

Silicon is not only useful for deoxidation of steel, but also effective in securing strength through solid solution strengthening, but is an element that deteriorates impact characteristics. If the content is less than 0.30%, strength is not sufficiently secured through deoxidation and solid solution strengthening of the steel, and if it exceeds 0.50%, it is undesirable because there is a concern about formability deterioration due to solid solution strengthening.

Mn: 0.35 to 0.75%

Mn is an alloying element that is advantageous for securing strength by improving hardenability of steel, and serves to increase rollability and reduce brittleness. If it is added less than 0.35%, it is difficult to secure sufficient hardness, and if it is added in excess of 0.75%, it is easy to generate a hard structure during cooling after hot rolling, and a large amount of MnS inclusions are generated, which can cause internal cracks during cold forging. Needs to be.

Cr: 0.40~0.70%

Cr (chrome) is an element that is effective in improving hardenability together with Mn to secure hardness, and can be added in an amount of 0.40% or more. However, if the content is excessive, there is a problem in that coarse carbides are formed, so the upper limit can be limited to 0.70%.

Ti: 0.010 to 0.050%

Titanium combines with nitrogen introduced into steel to form titanium carbonitride, preventing boron from combining with nitrogen. If the content of titanium is less than 0.010%, it is difficult to utilize the effect of boron because it is not sufficient to form nitrogen introduced during the steelmaking process into titanium carbonitride, and if it exceeds 0.050%, coarse carbonitride is formed, It is undesirable because it causes the occurrence of microcracks. It makes delayed failure resistance inferior.

Al: 0.01 to 0.05%

Al is widely used as a deoxidizer in the steelmaking process, and is effective in refining austenite grains by AlN formed by reacting with N. If it is added less than 0.01%, the number of nitrogen compounds is insufficient and the effect of crystal grain refinement is lowered, and if it is added in excess of 0.05%, non-metallic inclusions such as alumina are excessively generated, which intensifies the occurrence of defects in steel, so it is necessary to limit it. there is.

B: 0.0010 to 0.0050%

Boron is a hardenability improving element. When the content of boron is less than 0.0010%, it is difficult to expect an effect of improving hardenability, and when it exceeds 0.0050%, Fe ₂₃ (CB) ₆ carbide is formed at grain boundaries to induce brittleness of austenite grain boundaries, which is not preferable.

N: 0.002 to 0.020%

Nitrogen is effective in refining austenite crystal grains by AlN formed by reacting with Al, and is an element that is widely used instead of inputting expensive alloy elements. If it is added less than 0.002%, the number of nitrogen compounds is insufficient and the austenite grain refinement effect is lowered, and if it exceeds 0.020%, the forging heat generated during cold forging causes the movement and proliferation of dislocations inside the material, and free nitrogen It adheres to the mold and increases the deformation strength, reducing the life of the mold.

other ingredients

The remaining component of the present invention is the remaining amount of 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. Since these impurities are known to anyone skilled in the ordinary manufacturing process, not all of them are specifically mentioned in this specification.

In addition, in order to achieve the above object, another embodiment of the present invention provides a screw component manufactured by the method according to the present invention.

Screw parts have a microstructure of 70% or more of auto-tempered martensite, 0.1-5% of bainite, 1-28% of fresh martensite, and 0.1-1% of retained austenite in area fraction, and precipitate within the prior austenite grains. It is characterized in that the average thickness of the carbide to be 20nm or less.

In addition, the screw component according to the present invention is characterized by having a hardness of 500 HV or more at room temperature.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

[Example]

Parts of Examples and Comparative Examples were manufactured using steel materials having component compositions as shown in Table 1 below.

Specifically, after heating the billet having the composition shown in Table 1 below at 900 to 1200 ° C, finishing wire rod rolling at 800 to 1000 ° C to a diameter of Φ 7 mm, followed by winding at 700 to 900 ° C and cooling at a cooling rate of 0.4 to 1.0 ° C / s Cooling, lowering the strength through softening heat treatment between 600 and 800 ° C, and cold forging to make a screw part shape with a body diameter of 4 mm, heating to a temperature between 850 and 950 ° C, and quenching in a refrigerant at 60 ° C. An experiment was conducted using the manufactured screw parts.

(weight%)	C	Si	Mn	Cr	Ti	Al	B	N
Example 1	0.38	0.40	0.46	0.53	0.023	0.04	0.0020	0.004
Example 2	0.45	0.41	0.45	0.54	0.022	0.03	0.0022	0.004
Example 3	0.40	0.40	0.36	0.52	0.024	0.03	0.0021	0.004
Example 4	0.41	0.44	0.49	0.52	0.024	0.03	0.0024	0.004
Example 5	0.39	0.41	0.44	0.46	0.026	0.03	0.0021	0.004
Example 6	0.42	0.41	0.43	0.59	0.021	0.02	0.0023	0.004
Example 7	0.43	0.31	0.42	0.52	0.023	0.04	0.0024	0.004
Example 8	0.41	0.48	0.44	0.53	0.027	0.03	0.0019	0.004
Comparative Example 1-1	0.27	0.41	0.46	0.52	0.025	0.02	0.0022	0.004
Comparative Example 1-2	0.52	0.42	0.47	0.52	0.023	0.03	0.0021	0.004
Comparative Example 1-3	0.38	0.25	0.45	0.50	0.022	0.03	0.0023	0.004
Comparative Example 1-4	0.34	0.40	0.78	0.51	0.026	0.02	0.0021	0.004
Comparative Example 1-5	0.43	0.37	0.49	0.38	0.023	0.03	0.0022	0.004
Comparative Example 1-6	0.42	0.42	0.47	0.74	0.023	0.04	0.0019	0.004

Also, in Table 2 below, the hardness was measured using a Vickers hardness tester. The thickness of carbide was measured in 5 fields using a transmission electron microscope (TEM) using a replica specimen and expressed as an average thickness. The thickness of the carbide was measured by defining the short axis of the plate-type carbide as the thickness.

The delayed fracture performance evaluation test method was conducted by a delayed fracture simulation method to observe the presence or absence of cracks in the screw thread, which is the stress concentration part, before and after immersing in 5% hydrochloric acid + 95% distilled water solution for 10 minutes after fastening to the target part after heat treatment of the final product. .

	Hardness (HV)	carbide thickness (nm)	presence or absence of cracks
Example 1	525	15	X
Example 2	622	16	X
Example 3	587	14	X
Example 4	592	14	X
Example 5	546	15	X
Example 6	596	16	X
Example 7	602	17	X
Example 8	589	13	X
Comparative Example 1-1	488	17	X
Comparative Example 1-2	634	22	O
Comparative Example 1-3	492	21	O
Comparative Example 1-4	559	17	O
Comparative Example 1-5	602	17	O
Comparative Example 1-6	609	18	O

Referring to Tables 1 and 2, the alloy composition of Examples 1 to 8 is included in the scope of the present invention, the carbide thickness after heat treatment is 20 nm or less, and the hardness is 500 HV or more. On the other hand, in Comparative Examples 1-1 to 1-6, the alloy components were out of the range of the present invention, the carbide thickness exceeded 20 nm after heat treatment, the hardness was less than 500 HV, or cracks occurred due to delayed fracture. Further, In Table 3 below, in order to confirm the effect of the present invention in which only quenching heat treatment was performed without austempering heat treatment, screw parts were manufactured by performing austempering heat treatment with the alloy composition of Table 1 above, and their hardness was measured using a Vickers hardness machine measured.

Specifically, in Table 3 below, Comparative Examples 2-1 to 2-8 are the hardness of parts manufactured by performing austempering heat treatment with the compositions of Examples 1 to 8 in Table 1, Comparative Examples 3-1 to 3 -6 is the hardness of the parts manufactured by performing austempering heat treatment with the compositions of Comparative Examples 1-1 to 1-6 in Table 1 above.

(weight%)	Hardness when austempered (HV)
Comparative Example 2-1	432
Comparative Example 2-2	461
Comparative Example 2-3	456
Comparative Example 2-4	455
Comparative Example 2-5	443
Comparative Example 2-6	457
Comparative Example 2-7	469
Comparative Example 2-8	462
Comparative Example 3-1	402
Comparative Example 3-2	479
Comparative Example 3-3	441
Comparative Example 3-4	453
Comparative Example 3-5	458
Comparative Example 3-6	456

Referring to Tables 2 and 3, it can be seen that the hardness is lowered when austempering is performed compared to the case where only quenching heat treatment is performed without austempering heat treatment. As described above, exemplary embodiments of the present invention Although described, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible without departing from the concept and scope of the claims described below.

According to an example of the present invention, in manufacturing high-strength cold-rolling wire rods and screw parts with excellent drilling characteristics, only quenching heat treatment is applied instead of conventionally used austempering heat treatment to secure hardness for reducing wear during drilling Manufacturing provides a way

Claims (7)

1. By weight %, carbon (C): 0.30 to 0.50%, silicon (Si): 0.30 to 0.50%, manganese (Mn): 0.35 to 0.75%, chromium (Cr): 0.40 to 0.70%, titanium (Ti): 0.010 to 0.050%, aluminum (Al): 0.01 to 0.05%, boron (B): 0.0010 to 0.0050%, nitrogen (N): 0.002 to 0.020%, the balance to provide a billet containing iron (Fe) and other impurities step;

   heating the billet to a temperature of 900 to 1200° C., and performing finish hot rolling at a temperature of 800 to 1000° C. to manufacture a wire rod;

   Winding the wire rod at 700 to 900° C. to control the average austenite grain size to be 30 μm or less; and

   A method of manufacturing a wire rod for cold forging having excellent drilling characteristics, comprising the step of cooling the wound wire at a rate of 0.4 to 1.0 ° C / s.
2. According to claim 1,

   The microstructure of the prepared wire rod is a method for producing a wire rod for cold forging, including 45 to 60% of ferrite and 40 to 55% of pearlite in area fraction.
3. By weight %, carbon (C): 0.30 to 0.50%, silicon (Si): 0.30 to 0.50%, manganese (Mn): 0.35 to 0.75%, chromium (Cr): 0.40 to 0.70%, titanium (Ti): 0.010 ~0.050%, aluminum (Al): 0.01 ~ 0.05%, boron (B): 0.0010 ~ 0.0050%, nitrogen (N): 0.002 ~ 0.020%, the balance includes iron (Fe) and other impurities, and the microstructure is Softening heat treatment of a wire rod having 45 to 60% of ferrite and 40 to 55% of pearlite in area fraction and having an average austenite grain size of 30 μm or less at a temperature of 600 to 800 ° C;

   forming a screw part shape having a body diameter of 3 to 6 mm by cold forging the wire rod subjected to the softening heat treatment;

   heating for 500 to 4,000 seconds at a temperature of 850 to 950° C. so that the average austenite grain size is 15 μm or less after the cold forging; and

   After the heating step of quenching in a refrigerant of 20 ~ 100 ℃; manufacturing method of a screw part comprising a.
4. The method of claim 3,

   In the quenching step, the area fraction is controlled to have a microstructure of 70% or more of auto-tempered martensite, 0.1 to 5.0% of bainite, 1 to 28% of fresh martensite, and 0.1 to 1.0% of retained austenite, Manufacturing method of screw parts.
5. The method of claim 3,

   In the quenching step, the average thickness of carbides precipitated in prior austenite crystal grains is controlled to be 20 nm or less, a method for manufacturing a screw part.
6. The method of claim 3,

   The screw part is a method of manufacturing a screw part having a hardness of 500 HV or more at room temperature.
7. By area fraction, it has a microstructure of 70% or more of autotempered martensite, 0.1-5% of bainite, 1-28% of fresh martensite, and 0.1-1% of retained austenite,

   A screw component having an average thickness of carbides precipitated in prior austenite crystal grains of 20 nm or less and a hardness of 500 HV or more at room temperature.

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