Wire rod for cold heading, processed product using same, and method for manufacturing same

Abstract

The present invention relates to: a wire rod for cold heading, which has improved hydrogen delayed fracture resistance while ensuring cold forging characteristics by reducing the Si content and adding Mo and V; a processed product using the same; and a method for manufacturing the same. A wire rod for cold heading according to one embodiment of the present invention comprises: 0.3 wt% to 0.5 wt% of C; from 0.1 wt% to 0.3 wt% of Si; mn in the range of 0.5 wt% to 1.0 wt%; at least two or more of 0.5 wt% to 1.5 wt% Cr, 0.5 wt% to 1.5 wt% Mo, and 0.01 wt% to 0.2 wt% V; and the remainder being Fe and other impurities, and having a value of 3.56 or more obtained by the following formula (1). Formula (1)

Description

[0001] This application is a divisional application of the Chinese patent application with the application date of August 29, 2019, the application number of "201980060544.8", and the invention name of "Wire rod for cold heading, processed product using the same, and method of manufacturing the same", and the original application is the Chinese national stage application of international application PCT / KR2019 / 011086. TECHNICAL FIELD

[0002] The present disclosure relates to a Cold Heading Quality (CHQ) wire rod, a processed product using the same, and a method of manufacturing the same, and more particularly, to a CHQ wire rod having improved resistance to hydrogen delayed fracture while securing cold forging properties by reducing Si content and adding Mo and V, a processed product using the same, and a method of manufacturing the same. BACKGROUND

[0003] A general CHQ wire rod product is manufactured into mechanical structures and automobile parts through wire rod, cold drawing, spheroidizing heat treatment, cold drawing, cold heading, quenching, and tempering.

[0004] The latest technical development trend of the cold heading product is focused on the development of a high-strength cold heading product that can achieve weight reduction of parts in response to global automobile fuel economy regulations, while achieving a processed wire rod that omits heat treatment and processing. For example, in response to global automobile fuel economy regulations to improve the atmospheric environment, weight reduction of vehicles is being pursued, and for this purpose, parts such as engines are being miniaturized and high-powered. In order to manufacture such miniaturized and high-powered parts, a high-strength cold heading product is required.

[0005] Such a high-strength cold heading processed product undergoes rapid cooling and tempering heat treatment after cold heading, and the tempering martensite structure formed at this time as a microstructure is very sensitive to hydrogen delayed fracture at a high strength of 1300 MPa or more, and is difficult to use. Therefore, it is necessary to develop a wire rod having cold forging properties and improved resistance to hydrogen delayed fracture, and a processed product using the same. SUMMARY

[0006] TECHNICAL PROBLEM

[0007] The present disclosure aims to provide a CHQ wire rod having improved resistance to hydrogen delayed fracture without compromising cold forging properties, a processed product using the same, and a method of manufacturing the same.

[0008] TECHNICAL SOLUTION

[0009] According to one aspect of the present disclosure, a cold heading quality wire rod includes, in weight percent (%), of the total composition, C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, V: 0.01% to 0.2% of at least two or more; the remainder of iron (Fe) and other inevitable impurities, and a value of the following formula (1) is 3.56 or more.

[0010] (1)

[0011] Here, [Cr], [Mo], and [V] respectively mean weight % of Cr, Mo, and V.

[0012] The wire rod can include bainite, martensite, and pearlite as a microstructure, and the bainite can be 85% or more, the martensite can be 2% to 10%, and the pearlite can be 1% to 5% in area fraction.

[0013] The average austenite grain size of the wire rod can be 30 μm or less.

[0014] According to another aspect of the present disclosure, a processed product includes, in weight percent (%), of the total composition, C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, V: 0.01% to 0.2% of at least two or more; the remainder of iron (Fe) and other inevitable impurities, and a value of the following formula (1) is 3.56 or more.

[0015] (1)

[0016] The processed product can include tempered martensite as a microstructure.

[0017] The tensile strength of the processed product can be 1400 Mpa or more, and the impact toughness of the processed product can be 50 J or more.

[0018] According to another aspect of the present disclosure, a manufacturing method of a cold heading quality wire rod includes: heating a billet at 900 to 1200 °C, the billet including, in weight percent (%) of the total composition, C: 0.3 to 0.5%; Si: 0.1 to 0.3%; Mn: 0.5 to 1.0%; at least two or more of Cr: 0.5 to 1.5%, Mo: 0.5 to 1.5%, V: 0.01 to 0.2%; the remainder of iron (Fe) and other inevitable impurities, and having a value of 3.56 or more in the following formula (1); finish-rolling the heated billet at 850 to 1150 °C; and controlling an average austenite grain size to 30 µm or less by cooling the rolled billet at a rate of 0.2 to 0.5 °C / sec.

[0019] (1)

[0020] According to another aspect of the present disclosure, the manufacturing method of a processed product further includes: heating the cold heading quality wire rod at 850 to 1050 °C; cooling the heated wire rod to 40 to 70 °C; and heating the cooled wire rod at 500 to 600 °C for 5000 to 10000 seconds.

[0021] Advantageous Effects

[0022] The cold heading quality wire rod according to one embodiment of the present disclosure, the processed product using the same, and the manufacturing method thereof can provide a wire rod having improved resistance to hydrogen delayed fracture while ensuring cold forging characteristics and a processed product using the same. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a graph measuring tensile strength according to tempering temperature of the inventive example and comparative examples of the present disclosure.

[0024] Figure 2 is a graph measuring impact toughness according to tempering temperature of the inventive example and comparative examples of the present disclosure. DETAILED DESCRIPTION

[0025] According to one aspect of the present disclosure, the cold heading quality wire rod includes, in weight percent (%) of the total composition, C: 0.3 to 0.5%; Si: 0.1 to 0.3%; Mn: 0.5 to 1.0%; at least two or more of Cr: 0.5 to 1.5%, Mo: 0.5 to 1.5%, V: 0.01 to 0.2%; the remainder of iron (Fe) and other inevitable impurities, and a value of the following formula (1) is 3.56 or more.

[0026] (1)

[0027] (Here, [Cr], [Mo], and [V] each mean weight % of Cr, Mo, and V, respectively.)

[0028] Embodiment of Invention

[0029] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to convey the technical idea of the present disclosure to those skilled in the art. However, the present disclosure is not limited to these embodiments, and can be implemented in another form. In the drawings, parts irrelevant to the description can not be shown to clarify the present disclosure, and in addition, the sizes of components are shown more or less exaggerated for easy understanding.

[0030] According to one aspect of the present disclosure, a cold heading quality wire rod contains, in weight percent (%), of the total composition, C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; at least two or more of Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, V: 0.01% to 0.2%; the remainder of iron (Fe) and other inevitable impurities, and a value of the following formula (1) is 3.56 or more.

[0031] (1)

[0032] (Here, [Cr], [Mo], and [V] each mean weight % of Cr, Mo, and V, respectively.)

[0033] Hereinafter, the effects and contents of each component contained in the cold heading quality wire rod according to the present disclosure and a processed product using the same will be described as follows. The percentages of the following components mean weight percentages.

[0034] The content of C (carbon) is 0.3% to 0.5%.

[0035] C is an element added to secure the strength of a product. If the content of C is less than 0.3%, it is difficult to secure the target strength, and it is not easy to secure sufficient hardenability after quenching and tempering heat treatment. On the contrary, if the content of C exceeds 0.5%, it can reduce the fatigue life due to excessive generation of carbides. Therefore, the upper limit is set to 0.5%. Therefore, according to one embodiment of the present disclosure, the content of C is set to 0.3% to 0.5%.

[0036] The content of Si (silicon) is 0.1% to 0.3%.

[0037] Si is not only used for deoxidization of steel, but also is an element that is advantageous to ensure strength by solid solution strengthening. Therefore, 0.1% or more is added. However, since working is difficult when added in excess, the upper limit is limited to 0.3%. Therefore, according to one embodiment of the present disclosure, the content of Si is 0.1% to 0.3%.

[0038] The content of Mn (manganese) is 0.5% to 1.0%.

[0039] Mn is advantageous to ensure strength by improving the hardenability of a worked product, and is an element that increases rollability and reduces brittleness. In order to ensure sufficient strength, 0.5% or more is added. However, if added in excess, a hardened structure can occur during cooling after hot rolling, and a large amount of MnS inclusions can be generated, resulting in a decrease in fatigue properties. Therefore, the upper limit is limited to 1.0%. Therefore, according to one embodiment of the present disclosure, the content of Mn is set to 0.5% to 1.0%.

[0040] In addition, according to one embodiment of the present disclosure, at least two or more of Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, and V: 0.01% to 0.2% are included.

[0041] Cr is effective together with Mn for improving hardenability, and Cr is an element that improves the corrosion resistance of steel. Therefore, when added, 0.5% or more is added. However, if Cr is added beyond a certain level, impact toughness decreases, and since a carbide that is poor in resistance to hydrogen delayed fracture is formed, the upper limit is limited to 1.5%.

[0042] Mo is an element that improves hardenability by precipitation strengthening and solid solution strengthening caused by precipitation of fine carbides. The improvement in hardenability due to Mo is more effective than Mn and Cr. When Mo is added, if the content is less than 0.5%, sufficient hardening cannot be performed, and thus it is not easy to ensure sufficient strength after quenching and tempering heat treatment. In contrast, when more than 1.5% of Mo is added, the shape of a worked product can be deformed after quenching due to excessively high hardenability. Therefore, there is a problem that an additional process is required to correct this, and the upper limit is set to 1.5%. When Mo is added, the content is set to 0.5% to 1.5%.

[0043] V is an element that refines the structure of steel by forming fine carbides such as VC, VN, and V(C,N). When V is added, if the content is less than 0.01%, the distribution of V precipitates in the base steel is small, so that the austenite grain boundary cannot be fixed. Therefore, the grains are coarsened during tempering in the heat treatment process, resulting in a decrease in strength. When V is added, 0.01% or more is added. In contrast, when V is excessively added, coarse carbonitrides are formed, which decreases the toughness, limiting the upper limit to 0.2%. Therefore, according to one embodiment of the present disclosure, when V is added, the content is set to 0.01% to 0.2%.

[0044] The value of the following formula (1) is 3.56 or more.

[0045] (1)

[0046] To improve the resistance to hydrogen delayed fracture, it is necessary to obtain fine carbides capable of capturing diffused hydrogen. The fine carbides capable of capturing hydrogen include CrC, MoC, and VC carbides, each of which is mainly composed of Cr, Mo, and V. When these carbides exist in a certain amount or more, it is possible to ensure a strength of 1400 MPa or more at a tempering temperature of 500°C to 600°C, and the hydrogen capturing effect can be maximized. In particular, by controlling the value of formula (1) which is a combination of the contents of Cr, Mo, and V to 3.56 or more, it is possible to increase the strength of the cold heading steel and improve the resistance to hydrogen delayed fracture.

[0047] The cold heading quality wire rod according to one embodiment of the present disclosure has a microstructure including bainite, martensite, and pearlite, and the bainite is 85% or more, the martensite is 2% to 10%, and the pearlite is 1% to 5% in terms of area fraction.

[0048] Further, according to one embodiment of the present disclosure, the average austenite grain size can be 30 μm or less.

[0049] Further, the processed product according to one embodiment of the present disclosure can include tempered martensite.

[0050] Further, the tensile strength of the processed product according to one embodiment of the present disclosure can be 1400 MPa or more, and the impact toughness of the processed product can be 50 J or more.

[0051] Hereinafter, a manufacturing method of a cold heading quality wire rod and a processed product using the same according to one embodiment of the present disclosure will be described.

[0052] A billet satisfying the above components is heated. The heating of the billet is performed at 900°C to 1200°C.

[0053] The heated billet is finish-rolled at 850°C to 1150°C. The billet can be coiled after the rolling. The rolling reduction can be 80% or more.

[0054] The rolled billet is cooled at a rate of 0.2°C / sec to 0.5°C / sec, and the average austenite grain size is controlled to 30 μm or less. The cooling can be performed with air cooling. After the cooling, the microstructure of the wire includes bainite, martensite, and pearlite, and includes, in terms of area fraction, 85% or more of bainite, 2% to 10% of martensite, and 1% to 5% of pearlite.

[0055] The cooled wire is then heated at 850°C to 1050°C. The heating time can be 3000 seconds to 4000 seconds.

[0056] The heated wire is cooled to 40°C to 70°C, that is, quenched. The cooling can be performed by immersion in oil.

[0057] The cooled wire is heated at 500°C to 600°C for 5000 seconds to 10000 seconds, that is, tempered. After the tempering, the microstructure of the processed product can consist of tempered martensite. Since the tempering is performed at a high temperature of 500°C or more, this prevents the formation of film-like carbides at the austenite grain boundaries, and spheroidized carbides are dispersed and distributed inside and outside the grain boundaries. This can improve the resistance to hydrogen delayed fracture of the processed product.

[0058] Hereinafter, the present disclosure will be described in detail through examples, but the following examples serve to explain the present disclosure in more detail, and the scope of the present disclosure is not limited to these examples.

[0059] Inventive Example

[0060] A billet having the composition of the following [Table 1] is heated to 900°C to 1200°C, hot-rolled at a finish-rolling temperature of 1000°C and a rolling reduction of 80% or more. Thereafter, air cooling is performed at a cooling rate of 0.2°C / sec to 0.5°C / sec. After the hot-rolled wire is processed into a tensile test specimen according to the ASTM E8 standard, it is heated at 920°C for 3600 seconds, then immersed in oil at 50°C for rapid cooling, and then tempered at 500°C to 600°C for 5000 seconds to 10000 seconds. Then, a tensile test is performed. The tensile test results of Comparative Examples 1 to 5 and Inventive Examples 1 to 5 are shown in Figure 1 .

[0061] [Table 1]

[0062]

[0063] As can be seen from [Table 1], the values of the formula (1) in Inventive Examples 1 to 5 of the present disclosure are all 3.56 or more, but the values of the formula (1) in Comparative Examples 1 to 5 of the comparative examples according to the present disclosure are all less than 3.56.

[0064] Further, referring to the tensile test results shown in Figure 1 , Inventive Examples 1 to 5 all show a tensile strength of 1400 MPa or more, but in Comparative Examples 1 to 5, it can be seen that the tensile strength decreases around 600℃, and thus the tensile strength is less than 1400 MPa.

[0065] Further, referring to the impact toughness results shown in Figure 2 , it can be seen that Inventive Examples 1 to 5 all have an impact toughness of 50 J or more.

[0066] Therefore, according to the inventive examples of the present disclosure, by minimizing the content of Si, which causes solid solution strengthening to suppress cold forging characteristics, adding Mo to prevent a decrease in strength, and adding V to increase strength and refine grains, cold forging characteristics can be ensured. Further, at the same time, by performing a tempering heat treatment at a high temperature of 500℃ or more, and by adding V to refine grains, resistance to hydrogen delayed fracture can be improved. Therefore, the processed product can have a tensile strength of 1400 MPa or more and an impact toughness of 50 J or more.

[0067] As described above, although exemplary embodiments of the present disclosure have been described, the present disclosure is not limited thereto, and those of ordinary skill in the art will understand that various changes and modifications can be made without departing from the concept and scope of the appended claims.

[0068] Industrial applicability

[0069] The cold-upset quality wire rod and the processed product according to the present disclosure provide a 1.4 GPa high-strength CHQ steel that has both cold forging characteristics and resistance to hydrogen delayed fracture and can be used as an automobile component.

[0070] The present application also relates to the following aspects:

[0071] 1. A cold-upset quality wire rod, comprising, in weight percentages (%) of the total composition: C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, V: 0.01% to 0.2% of at least two or more; the remainder of iron (Fe) and other inevitable impurities, and

[0072] the value of the following formula (1) is 3.56 or more,

[0073] (1)

[0074] Here, [Cr], [Mo], and [V] each mean the weight% of Cr, Mo, and V, respectively.

[0075] 2. The cold-upset quality wire rod according to aspect 1, wherein the wire rod includes bainite, martensite, and pearlite as a microstructure, and

[0076] the bainite is 85% or greater, the martensite is 2% to 10%, and the pearlite is 1% to 5% in terms of area fraction.

[0077] 3. The cold-upset quality wire rod according to aspect 1, wherein the average austenite grain size of the wire rod is 30 μm or less.

[0078] 4. A processed product including, in terms of weight percentage (%) of the entire composition: C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; at least two or more of Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, V: 0.01% to 0.2%; a remaining portion of iron (Fe) and other inevitable impurities, and

[0079] a value of the following formula (1) is 3.56 or greater,

[0080] (1)

[0081] Here, [Cr], [Mo], and [V] each mean the weight% of Cr, Mo, and V, respectively.

[0082] 5. The processed product according to aspect 4, wherein the processed product includes tempered martensite as a microstructure.

[0083] 6. The processed product according to aspect 4, wherein the tensile strength of the processed product is 1400 MPa or greater, and the impact toughness of the processed product is 50 J or greater.

[0084] 7. A manufacturing method of the cold-upset quality wire rod according to any one of aspects 1 to 3, the method including:

[0085] a blank including, in terms of weight percentage (%), of the entire composition, C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; at least two or more of Cr: 0.5% to 1.5%, Mo: 0.5% to 1.5%, V: 0.01% to 0.2%; the remainder of iron (Fe) and other inevitable impurities, and having a value in the following formula (1) of 3.56 or more;

[0086] subjecting the heated blank to finish rolling at 850°C to 1150°C; and

[0087] controlling the average austenite grain size to 30 μm or less by cooling the rolled blank at a rate of 0.2°C / sec to 0.5°C / sec,

[0088] (1)

[0089] Here, [Cr], [Mo], and [V] respectively mean the weight % of Cr, Mo, and V.

[0090] 8. The manufacturing method of a processed product according to aspect 7, the method further comprising:

[0091] heating the cold heading quality wire at 850°C to 1050°C;

[0092] cooling the heated wire to 40°C to 70°C; and

[0093] heating the cooled wire at 500°C to 600°C for 5000 seconds to 10000 seconds.

Claims

1. A cold-headed quality wire, comprising, by weight percentage (%) of all components: C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; Cr: 0.5% to 1.5%; Mo: 0.5% to 1.5%; V: 0.01% to 0.2%; the remainder being iron (Fe) and other unavoidable impurities, as well as The wire contains CrC, MoC, and VC carbides, and The value of the following equation (1) is 3.56 or greater. (1) Here, [Cr], [Mo], and [V] refer to the weights of Cr, Mo, and V, respectively.

2. The cold-forged quality wire according to claim 1, wherein the wire comprises bainite, martensite, and pearlite as its microstructure, and The bainite comprises 85% or more, the martensite comprises 2% to 10%, and the pearlite comprises 1% to 5% by area fraction.

3. The cold-forged quality wire according to claim 1, wherein the average austenite grain size of the wire is 30 μm or less.

4. A processed product, comprising, by weight percentage (%) of all components: C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; Cr: 0.5% to 1.5%; Mo: 0.5% to 1.5%; V: 0.01% to 0.2%; the remainder being iron (Fe) and other unavoidable impurities, as well as The processed products mentioned above contain CrC, MoC, and VC carbides, and The value of the following equation (1) is 3.56 or greater. (1) Here, [Cr], [Mo], and [V] refer to the weights of Cr, Mo, and V, respectively.

5. The processed product according to claim 4, wherein the processed product comprises tempered martensite as a microstructure.

6. The processed product according to claim 4, wherein the tensile strength of the processed product is 1400 MPa or greater, and the impact toughness of the processed product is 50 J or greater.

7. A method for manufacturing cold-headed quality wire according to any one of claims 1 to 3, the method comprising: The billet is heated at 900°C to 1200°C, and the billet contains, by weight percentage (%) of the total composition, C: 0.3% to 0.5%; Si: 0.1% to 0.3%; Mn: 0.5% to 1.0%; Cr: 0.5% to 1.5%; Mo: 0.5% to 1.5%; V: 0.01% to 0.2%; the remainder being iron (Fe) and other unavoidable impurities, and having a value of 3.56 or greater in the following formula (1); Finish rolling of heated billets at 850°C to 1150°C; and The wire contains CrC, MoC, and VC carbides, and The average austenite grain size is controlled to 30 μm or smaller by cooling the rolled billet at a rate of 0.2 °C / s to 0.5 °C / s. (1) Here, [Cr], [Mo], and [V] refer to the weights of Cr, Mo, and V, respectively.

8. The method for manufacturing a processed product according to claim 7, further comprising: The cold-forged quality wire is heated at 850°C to 1050°C; Cool the heated wire to 40°C to 70°C; as well as The cooled wire is heated at 500°C to 600°C for 5000 to 10000 seconds.

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