CN118318058A

CN118318058A - Cold-forged wire and steel part with improved delayed fracture resistance and method for manufacturing the same

Info

Publication number: CN118318058A
Application number: CN202280078714.7A
Authority: CN
Inventors: 全英洙; 卢政杓; 金丹碧; 金永泰
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2021-12-01
Filing date: 2022-12-01
Publication date: 2024-07-09
Also published as: KR20230082090A; EP4424861A1; WO2023101487A1

Abstract

Disclosed are a cold-forged wire and steel part in which a microstructure is controlled by an alloy composition and a manufacturing method, and a method for manufacturing the same, whereby it is possible to reduce costs and enhance delayed fracture resistance. A steel part with improved delayed fracture resistance according to one embodiment of the present invention comprises, in weight percent: 0.18% to 0.25% of C, 0.30% to 0.50% of Si, 0.35% to 0.50% of Mn, more than 0% and not more than 0.03% of P, more than 0% and not more than 0.03% of S, 0.45% to 0.60% of Cr, 0.015% to 0.03% of Ti and 0.001% to 0.004% of B, and the remainder containing Fe and unavoidable impurities, and may contain at least 90% by volume of self-tempered martensite.

Description

Cold-forged wire and steel part with improved delayed fracture resistance and method for manufacturing the same

Technical Field

The present disclosure relates to a wire and steel part for cold forging and a method of manufacturing the same, and more particularly, to a wire and steel part for cold forging and a method of manufacturing the same, which improve delayed fracture resistance by controlling a microstructure.

Background

Wires for vehicles and structural fastening bolts require steel materials having high strength to lighten the vehicles and miniaturize the structures. In order to improve the strength of steel, metal strengthening mechanisms such as cold working, grain refinement, martensite strengthening, precipitation hardening, and the like are used. However, dislocations, grain boundaries, martensite lath boundaries, fine precipitate boundaries, and the like for the strengthening mechanism act as hydrogen trapping sites in the steel material, and as a cause of delayed fracture degradation. Accordingly, the delayed fracture degradation of the high strength bolt having a tensile strength of at least 1 GPa.

To solve this problem, a steel material for a bolt of at least 1GPa having a conventional tempered martensite structure uses a cr—mo alloy steel to which Mo is added. However, in response to the demand for cost reduction with the development of bolt manufacturing process technology, efforts have been made to replace high strength steel of at least 1GPa with steel to which boron is added. As a result, steel added with boron is used to achieve cost reduction, and after confirming safety, the steel added with boron is applied to some fastening bolts for vehicles.

However, when the steel added with boron is used at 1.1GPa or more, hydrogen induced delayed cracking occurs (refer to n.uno et al, nippon STEEL TECHNICAL Report No.97 (2008)). Therefore, for high-strength steel of at least 1.1GPa, standard steel to which Mo is added, or steel grades to which Mo or V is added, which are proprietary to respective steel companies, are applied. However, in order to be cost competitive, it is necessary to develop high strength steel omitting expensive carbide elements such as Mo and V.

Disclosure of Invention

Technical problem

In order to solve the above-described problems, the present disclosure aims to provide a wire and steel member for cold forging and a method for manufacturing the same, which enable cost reduction and improvement in delayed fracture resistance by controlling a microstructure through an alloy composition and a manufacturing method.

Technical proposal

According to one embodiment of the present disclosure, a steel part having improved delayed fracture resistance comprises, in weight-%: 0.18% to 0.25% of C, 0.30% to 0.50% of Si, 0.35% to 0.50% of Mn, greater than 0% to 0.03% of P, greater than 0% to 0.03% of S, 0.45% to 0.60% of Cr, 0.015% to 0.03% of Ti, 0.001% to 0.004% of B, the remainder having Fe and other unavoidable impurities, wherein the self-tempered martensite is at least 90% by volume fraction.

In one embodiment of the present disclosure, a steel part having improved delayed fracture resistance may have carbides having an average thickness of 15nm or less in prior austenite grains.

In one embodiment of the present disclosure, a steel component having improved delayed fracture resistance may have a tensile strength of at least 1200 MPa.

According to one embodiment of the present disclosure, a method of manufacturing a steel part having improved delayed fracture resistance includes: the following steels were prepared in weight percent: 0.18% to 0.25% C, 0.30% to 0.50% Si, 0.35% to 0.50% Mn, greater than 0% to 0.03% P, greater than 0% to 0.03% S, 0.45% to 0.60% Cr, 0.015% to 0.03% Ti, 0.001% to 0.004% B, the remainder having Fe and other unavoidable impurities; preparing a wire rod by finish rolling a steel material; winding the wire rod; drawing the wound wire rod and then performing spheroidizing heat treatment; forming the spheroidized wire rod into a part; and austenitizing the component and then quenching.

In one embodiment of the present disclosure, in the method of manufacturing a steel part having improved delayed fracture resistance, finish rolling may be performed at 880 to 980 ℃, and coiling may be performed at 830 to 930 ℃.

In one embodiment of the present disclosure, in a method of manufacturing a steel part having improved delayed fracture resistance, the spheroidizing heat treatment may be performed at a maximum temperature ranging from 745 ℃ to 765 ℃.

In one embodiment of the present disclosure, austenitizing may be performed at 870 ℃ to 940 ℃ in a method of manufacturing a steel part having improved delayed fracture resistance.

In one embodiment of the present disclosure, in a method of manufacturing a steel part having improved delayed fracture resistance, quenching may be performed with a coolant of 10 ℃ to 80 ℃.

According to one embodiment of the present disclosure, a wire rod for cold forging includes, in weight%: 0.18% to 0.25% of C, 0.30% to 0.50% of Si, 0.35% to 0.50% of Mn, more than 0% to 0.03% of P, more than 0% to 0.03% of S, 0.45% to 0.60% of Cr, 0.015% to 0.03% of Ti, 0.001% to 0.004% of B, the remainder having Fe and other unavoidable impurities, wherein the wire has a diameter of 5.5mm to 20mm.

Advantageous effects

According to one embodiment of the present disclosure, a wire and steel member for cold forging and a method for manufacturing the same may be provided, which enable cost reduction and improvement of delayed fracture resistance by controlling a microstructure through an alloy composition and a manufacturing method.

Detailed Description

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The following embodiments are provided as examples to convey the full spirit of the disclosure to those of ordinary skill in the art to which embodiments of the disclosure pertain. The present disclosure is not limited to the embodiments described but may be specified in any other form. In the drawings, irrelevant parts of the specification are not shown to clarify the disclosure, and the size of elements may be slightly exaggerated to aid understanding.

Throughout this specification, unless the context requires otherwise, the term "comprise" or "comprise" is inclusive or open-ended and does not exclude additional unrecited components, elements or method steps.

It is to be understood that unless the context clearly dictates otherwise, nouns without quantitative word modifications include one and more.

The reasons for numerical limitation of the content of alloy compositions in one embodiment of the present disclosure will now be described. Unless otherwise mentioned, units of weight% will now be used.

According to one embodiment of the present disclosure, a steel part having improved delayed fracture resistance comprises, in weight-%: 0.18% to 0.25% of C, 0.30% to 0.50% of Si, 0.35% to 0.50% of Mn, more than 0% to 0.03% of P, more than 0% to 0.03% of S, 0.45% to 0.60% of Cr, 0.015% to 0.03% of Ti, 0.001% to 0.004% of B, the remainder having Fe and other unavoidable impurities.

The content of C (carbon) may be 0.18% to 0.25%.

C is an element added to ensure the strength of the product. With this in mind, at least 0.18% C may be added. However, when the content of C is too high, it may cause delayed fracture due to the increase in strength. In view of this, the upper limit of the content of C is limited to 0.25%.

The content of Si (silicon) may be 0.30% to 0.50%.

Si may be added for deoxidizing the steel. Si is an element effective in securing strength by solid solution strengthening. In view of this, si may be added at least 0.30%. However, when the content of Si is too high, impact characteristics and formability may be deteriorated. In view of this, the upper limit of the content of Si may be limited to 0.50%. Preferably, the Si content may be 0.31% to 0.48%.

The content of Mn (manganese) may be 0.35% to 0.50%.

Mn is an element that is very effective in improving hardenability and producing a solid solution strengthening effect by forming a substitutional solid solution in a matrix structure. When the content of Mn is low, it may not be sufficiently combined with S (sulfur) as an impurity in steel, thereby causing casting cracks. In view of this, at least 0.35% Mn may be added. However, when the content of Mn is too high, it may form MnS, which may deteriorate the delayed fracture resistance. In view of this, the upper limit of the content of Mn may be limited to 0.50%. Preferably, the Mn content may be 0.36% to 0.49%.

The content of P (phosphorus) may be more than 0% to 0.03%.

P segregates at grain boundaries, thereby reducing toughness and serving as a cause of reduced delayed fracture resistance. Thus, it may be managed as an impurity in the present disclosure. In view of this, the upper limit of the P content may be limited to 0.03%, and it is desirable to be as close to 0% as possible.

The content of S (sulfur) may be more than 0% to 0.03%.

Like P, S segregates at grain boundaries, thereby reducing toughness and serving as a cause of impeding hot rolling by forming low melting point sulfides. Thus, it may be managed as an impurity in the present disclosure. In view of this, the upper limit of the S content may be limited to 0.03%, and it is desirable to be as close to 0% as possible.

The content of Cr (chromium) may be 0.45% to 0.60%.

Cr is an element that is very effective in improving hardenability and producing a solid solution strengthening effect by forming a substitutional solid solution in a matrix structure. In view of this, cr may be added at least 0.45%. However, when the Cr content is too high, the c/a ratio of the corrosion pits increases due to the formation of a chromium oxide layer on the surface, and thus the delayed fracture resistance may be deteriorated due to the notch effect. In view of this, the upper limit of the content of Cr may be limited to 0.60%. Preferably, the content of Cr may be 0.46% to 0.59%.

The Ti (titanium) content may be 0.015% to 0.03%.

Ti is an effective element for preventing the combination of B (boron) and N (nitrogen) by combining with N carried into steel into titanium nitride. In view of this, ti may be added at least 0.015%. However, when the content of Ti is too high, it may form coarse carbonitrides, which may deteriorate delayed fracture resistance. In view of this, the upper limit of the Ti content may be limited to 0.03%. Preferably, the Ti content may be 0.023% to 0.026%.

The content of B (boron) may be 0.001% to 0.004%.

B is an effective element in improving hardenability. In view of this, B may be added at least 0.001%. However, when the content of B is too high, it may form Fe ₂₃(CB)₆ carbide, which may cause embrittlement of austenite grain boundaries and thus deteriorate delayed fracture resistance. In view of this, the upper limit of the content of B may be limited to 0.004%. Preferably, the content of B may be 0.0018% to 0.0023%.

The remainder of the disclosure is iron (Fe). However, during normal manufacturing, unexpected impurities may be inevitably mixed from raw materials or the surrounding environment, and thus they may not be excluded. These impurities may be known to any person skilled in the ordinary manufacturing process, and thus not all of these impurities are specifically mentioned in the present specification.

In one embodiment of the present disclosure, a steel component having improved delayed fracture resistance may have at least 90% self-tempered martensite by volume fraction.

When the self-tempered martensite is less than 90%, it is difficult to secure sufficient toughness, which may deteriorate the delayed fracture resistance. Thus, in the present disclosure, the self-tempered martensite can be controlled to at least 90% by the alloy composition and manufacturing method. In particular, the self-tempered martensite structure is characterized by self-tempering during quenching without an additional tempering heat treatment process.

Further, the steel part having improved delayed fracture resistance according to one embodiment of the present disclosure may be automatically tempered during quenching without an additional tempering heat treatment process, so that the average thickness of carbides in prior austenite grains may be controlled to 15nm or less.

It is known that the surface direction of carbides precipitated in the plate shape during autotempering has high commonality and thus cannot be effectively used as hydrogen trapping sites; while the plate-type side surface has a low commonality, thereby acting as a non-diffusible hydrogen trapping site and thus improving hydrogen-induced delayed fracture resistance.

Therefore, when the average thickness of carbides in the prior austenite grains is thin, the commonality of carbide interfaces increases, making it difficult to improve the hydrogen-induced delayed fracture resistance. On the other hand, when the average thickness of the carbide in the prior austenite grains is thick, the number of carbides decreases, making it difficult to improve the delayed fracture resistance. Therefore, in the present disclosure, the average thickness of carbides in prior austenite grains is controlled to 15nm or less to improve the delayed fracture resistance.

In one embodiment of the present disclosure, a steel part having improved delayed fracture resistance may have a tensile strength of at least 1200MPa by controlling the alloy composition and manufacturing method.

Next, a method of manufacturing a steel part having improved delayed fracture resistance according to another aspect of the present disclosure will now be described.

In one embodiment of the present disclosure, a method of manufacturing a steel part having improved delayed fracture resistance includes: the following steels were prepared in weight percent: 0.18% to 0.25% C, 0.30% to 0.50% Si, 0.35% to 0.50% Mn, greater than 0% to 0.03% P, greater than 0% to 0.03% S, 0.45% to 0.60% Cr, 0.005% to 0.03% Ti, 0.001% to 0.004% B, the remainder having Fe and other unavoidable impurities; preparing a wire rod by finish rolling a steel material; winding the wire rod; drawing the wound wire rod and then performing spheroidizing heat treatment; forming the spheroidized wire rod into a part; the component is austenitized and then quenched.

The reasons for numerical limitation of the constituent ranges of each alloy composition are the same as described above, and each manufacturing step will now be described in more detail.

After the steel material satisfying the alloy composition is first prepared, it may undergo a series of finish rolling, coiling, spheroidizing heat treatment, forming, austenitizing, and quenching processes.

First, the wire rod may be prepared by finish rolling a steel material at 880 to 980 ℃, and the wire rod may be wound at 830 to 930 ℃.

When the finish rolling temperature or the winding temperature is low, the surface layer is a quasi-two-phase region, and thus it is possible to form a surface ferrite decarburized layer by phase transformation. Therefore, when the finish rolling temperature or the winding temperature is low, a ferrite decarburized layer is formed on the surface even during the heat treatment of the steel part, thereby deteriorating the delayed fracture resistance. In this regard, the finish rolling temperature may be at least 880 ℃ or the winding temperature may be at least 830 ℃.

On the other hand, when the finish rolling temperature or the winding temperature is high, since diffusion accelerates decarburization, a ferrite decarburized layer is formed on the surface, thereby deteriorating delayed fracture resistance. In view of this, the finish rolling temperature may be 980 ℃ or less, or the winding temperature may be 930 ℃ or less.

Subsequently, the wound wire rod may be drawn to suit the purpose and subjected to a spheroidizing heat treatment at a maximum temperature ranging from 745 ℃ to 765 ℃.

When the highest temperature for the spheroidizing heat treatment is too low or too high, the hardness of the material to be subjected to the spheroidizing heat treatment increases, which may cause cracks due to deterioration of formability when the steel part is processed. In view of this, the highest temperature at which the spheroidizing heat treatment is performed may be 745 to 765 ℃.

The spheroidized heat treated wire rod is formed into a steel part suitable for the purpose, and the steel part may be austenitized at 870 ℃ to 940 ℃.

When the austenitizing temperature is low, the reverse transformation of austenite cannot sufficiently occur, and thus toughness may be deteriorated due to the non-uniformity of the martensitic structure after quenching. In this regard, the austenitizing temperature may be at least 870 ℃. On the other hand, when the austenitizing temperature is high, austenite grain size becomes coarse, which may deteriorate delayed fracture resistance. In view of this, the upper limit of the austenitizing temperature may be limited to 940 ℃.

The austenitized steel component may then be quenched with a coolant at 10 ℃ to 80 ℃.

When the temperature of the quenching coolant is low, fine quench cracks are formed due to thermal deformation of the steel part, thereby causing delayed fracture. In this regard, the temperature of the quench coolant may be at least 10 ℃. On the other hand, when the temperature of the quenching coolant is high, the self-tempering effect may increase, making it difficult to achieve the target strength. In view of this, the upper limit of the temperature of the quenching coolant may be limited to 80 ℃.

According to the foregoing process, the final microstructure of the steel part can achieve a self-tempered martensitic structure of at least 90% without a tempering process, and a structure in which carbides having an average thickness of 15nm or less are precipitated within prior austenite grains. Thus, delayed fracture resistance can be improved by controlling the microstructure.

Next, a wire rod for cold forging according to another aspect of the present disclosure will now be described.

According to one embodiment of the present disclosure, a wire rod for cold forging includes, in weight%: 0.18% to 0.25% of C, 0.30% to 0.50% of Si, 0.35% to 0.50% of Mn, more than 0% to 0.03% of P, more than 0% to 0.03% of S, 0.45% to 0.60% of Cr, 0.005% to 0.03% of Ti, 0.001% to 0.004% of B, with the remainder of Fe and other unavoidable impurities, wherein the wire has a diameter of 5.5mm to 20mm.

The reasons for limiting the numerical values of the respective constituent ranges of the alloy composition are as described above, and the wire rod for cold forging according to one embodiment of the present disclosure may be manufactured to have a diameter of 5.5mm to 20 mm. However, it is not limited thereto, and may be manufactured to have various diameters for the purpose.

Embodiments of the present disclosure will now be described in more detail. The embodiments may be for illustration only and the disclosure is not limited thereto. The scope of the disclosure is defined by the claims and their equivalents.

{ Implementation }

Steel was manufactured with various alloy composition ranges shown in table 1 below, wire rods having a diameter of 15mm were prepared by finish rolling the steel at 910 ℃ and then wound into a coil form at 880 ℃. The wound wire may be subjected to spheroidizing heat treatment at a maximum temperature in the range of 755 ℃, formed into a bolt of screw-M12 standard, austenitized at 890 ℃, and quenched with a coolant at 60 ℃. The spheroidizing heat treatment temperature refers to the highest heating temperature.

TABLE 1

Table 2 below shows the tensile strength, carbide thickness and crack presence from the delayed fracture performance evaluation of the bolts produced. Tensile strength was measured by Zwick Z250 tensile tester from Zwick/Roell. Tensile strength testing was performed with tensile specimens having a diameter of 10mm and a marked diameter of 6.25 mm.

Carbide thickness measurements were made with a FEI TECNAI OSIRIS Transmission Electron Microscope (TEM). In this case, carbide thicknesses of 5 random points on the replica sample were measured and expressed as average thicknesses, and as thicknesses, short axes of carbides formed in the plate shape were defined and measured.

The delayed fracture performance evaluation test was performed by the following delayed fracture simulation method: the bolts were fastened to the structure with a clamping force of yield strength, and the presence or absence of cracks (which are areas of stress concentration) on the threads was observed before/after immersing the bolts fastened to the structure in 5% hydrochloric acid+95% distilled water solution for 10 minutes.

As a result of the delayed fracture performance evaluation, it is marked with "O" when a crack is present, or with "X" when no crack is present.

TABLE 2

Differentiation of	Tensile strength (MPa)	Carbide thickness (nm)	Crack presence
				Embodiment 1	1213	12	X
Embodiment 2	1680	13	X
				Embodiment 3	1365	11	X
Embodiment 4	1391	11	X
				Embodiment 5	1265	12	X
Embodiment 6	1354	13	X
				Embodiment 7	1566	14	X
Embodiment 8	1421	10	X
				Comparative example 1	1185	13	X
Comparative example 2	1693	16	O
				Comparative example 3	1193	16	O
Comparative example 4	1655	14	O
				Comparative example 5	1586	13	O
Comparative example 6	1543	15	O

Referring to table 2, embodiments 1 to 8 satisfy the alloy compositions, compositional ranges, and manufacturing methods set forth in the present disclosure. Therefore, embodiments 1 to 8 satisfy a tensile strength of at least 1200MPa and a carbide thickness of 15nm or less, and no cracks were generated as a result of the delayed fracture performance evaluation. On the other hand, comparative example 1 did not satisfy the tensile strength of 1200MPa due to the low C content.

In comparative example 2, the carbide thickness exceeded 15nm due to the high C content, and cracks occurred as a result of the delayed fracture performance evaluation.

Comparative example 3 does not satisfy the tensile strength of 1200MPa due to the low Si content.

Comparative example 4 has a high Mn content and forms coarse MnS, which results in the occurrence of cracks as a result of delayed fracture performance evaluation.

In comparative example 5, the Cr content was low, and a bainite mixed structure was formed in the microstructure, resulting in the occurrence of cracks as a result of the delayed fracture performance evaluation.

Comparative example 6 has a high Cr content, thereby forming sharp corrosion pitting when corroded by hydrochloric acid, and thus causing occurrence of cracks as a result of delayed fracture performance evaluation.

Next, steel materials were produced with the alloy composition of embodiment 5 in table 1 and were produced into bolts at the finish rolling temperature, the winding temperature, the highest temperature of the spheroidizing heat treatment, and the austenitizing temperature shown in table 3 below, and then the presence or absence of cracks was marked in table 3 as a result of the delayed fracture performance evaluation.

TABLE 3

Comparative example 7 has a high finish rolling temperature and winding temperature, which causes the prior austenite grain size to grow, resulting in the occurrence of cracks as a result of the delayed fracture performance evaluation. Comparative example 8 has a low finish rolling temperature and winding temperature, which results in forming a ferrite decarburized layer on the wire rod, thereby resulting in the occurrence of cracks as a result of the delayed fracture performance evaluation.

Comparative example 9 has a high austenitizing temperature, which causes the prior austenite grain size to grow, resulting in the appearance of cracks as a result of the delayed fracture performance evaluation.

Comparative example 10 has a low austenitizing temperature so that it enters a quasi-two-phase region, and a ferrite decarburized layer is formed during heating, resulting in the occurrence of cracks as a result of delayed fracture performance evaluation.

Comparative examples 11 and 12 have low and high maximum temperatures for the spheroidizing heat treatment, respectively, and thus the spheroidizing heat treatment is not sufficiently performed, resulting in deterioration of formability. Accordingly, comparative examples 11 and 12 formed cracks during the screw member forming, resulting in the occurrence of cracks as a result of the delayed fracture performance evaluation.

TABLE 4

Comparative examples 13 to 16 used steels having the same alloy composition and composition range as in embodiment 5, but compared to embodiment 5, they had self-martensitic fraction and carbide thickness that did not satisfy the range of the present disclosure, which resulted in the occurrence of cracks as a result of delayed fracture performance evaluation.

[ INDUSTRIAL APPLICABILITY ]

According to the present disclosure, it is possible to provide a wire rod and a steel part for cold forging and a method for manufacturing the same, which enable cost reduction and improvement of delayed fracture resistance by controlling a microstructure through an alloy composition and a manufacturing method, thus confirming industrial applicability.

Claims

1. A steel component having improved delayed fracture resistance comprising, in weight-%:

0.18 to 0.25% C, 0.30 to 0.50% Si, 0.35 to 0.50% Mn, more than 0 to 0.03% P, more than 0 to 0.03% S, 0.45 to 0.60% Cr, 0.015 to 0.03% Ti, 0.001 to 0.004% B, the remainder having Fe and other unavoidable impurities,

Wherein the self-tempered martensite is at least 90% by volume fraction.

2. The steel component of claim 1, wherein the average thickness of carbides in prior austenite grains is 15nm or less.

3. The steel component of claim 1, wherein the tensile strength is at least 1200MPa.

4. A method of manufacturing a steel component having improved delayed fracture resistance, the method comprising:

The following steels were prepared in weight percent: 0.18% to 0.25% C, 0.30% to 0.50% Si, 0.35% to 0.50% Mn, greater than 0% to 0.03% P, greater than 0% to 0.03% S, 0.45% to 0.60% Cr, 0.015% to 0.03% Ti, 0.001% to 0.004% B, the remainder having Fe and other unavoidable impurities;

Preparing a wire rod by finish rolling the steel material;

winding the wire;

Drawing the wound wire rod and then performing spheroidizing heat treatment;

forming the spheroidized wire rod into a part; and

The component is austenitized and then quenched.

5. The method according to claim 4, wherein:

The finish rolling is carried out at 880 to 980 ℃, and

Wherein the winding is performed at 830 ℃ to 930 ℃.

6. The method of claim 4, wherein the spheroidizing heat treatment is performed at a maximum temperature in the range of 745 ℃ to 765 ℃.

7. The method of claim 4, wherein the austenitizing is performed at 870 ℃ to 940 ℃.

8. The method of claim 4, wherein the quenching is performed with a coolant of 10 ℃ to 80 ℃.

9. A wire for cold forging comprising, in weight percent:

Wherein the wire has a diameter of 5.5mm to 20mm.