Wire rod and component with improved delayed fracture resistivity, and method for manufacturing the same

Abstract

To provide a wire rod and a component with improved delayed fracture resistivity, and a method for manufacturing them.SOLUTION: A component with improved delayed fracture resistivity contains C:0.15 to 0.30%, Si:0.15 to 0.25%, Mn:0.95 to 1.35%, P:0.030% or less, S:0.030% or less, Ti:0.015 to 0.030%, B:0.0010 to 0.0040%, and N:0.0010 to 0.0080% by wt.%, the balance consists of Fe and inevitable impurities, and the component satisfies the following relational expression 1 and the following relational expression 2. [Relational expression 1] 2.0≤5.5×[Si]+[Mn]≤2.4 [Relational expression 2] 1.0<[Ti] / 3.42[N]<2.0 (in the relational expressions 1 and 2, each of [Si], [Mn], [Ti] and [N] denotes the content (wt.%) of the element).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a wire rod and a part having improved delayed fracture resistance, and a manufacturing method thereof, and more particularly to a wire rod, a part, and a manufacturing method thereof that can be used for fastening bolts for automobiles and structures exposed to various stresses and corrosive environments. [Background technology]

[0002] Wire rods used in fastening bolts for automobiles and structures are required to have higher strength as automobiles become lighter and structures become smaller. Generally, the strength of steel is increased by utilizing metal strengthening mechanisms such as cold working, grain refinement, martensite strengthening, and precipitation strengthening.

[0003] However, the cold working, grain boundaries, martensite lath boundaries, and fine precipitate boundaries that are used as strengthening mechanisms act as hydrogen traps within the steel and can also cause delayed fracture. For these reasons, high-strength bolts with a tensile strength of 1 GPa or more are prone to delayed fracture.

[0004] To solve these problems, Cr-Mo alloy steel with Mo added was used for bolts with a strength of 1 GPa or higher and having a tempered martensite structure, but attempts have been made to replace Cr-Mo steel with Cr-B steel to meet the need for cost reduction due to advances in bolt manufacturing technology. As a result, cost reductions were achieved by using Cr-B steel for bolts used in structures that do not have a significant impact on safety, and after confirming its safety, Cr-B steel is now also being used for some fastening bolts in automobiles.

[0005] Furthermore, in the automotive industry, there is a need to develop bolt materials that can be even more cost-effective than Cr-B steel in order to reduce costs to the utmost.To meet this need, technological development has recently been underway to apply Mn-B steel, which utilizes Mn, which is cheaper than Cr, to high-strength bolt materials of 1 GPa or more.

[0006] However, Mn induces greater solid solution strengthening in the ferrite matrix than Cr, so Mn-B steel can cause cracks in the threads of bolts during bolt manufacturing. Therefore, steel with a high Mn content added to manufacture high-strength bolts of 1 GPa or more has the disadvantage of causing delayed fracture due to cracks in the bolt threads, making it unsuitable for use in high-strength bolts. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide wire rod for high-strength bolts, bolts, and methods for manufacturing the same, which have improved delayed fracture resistance by optimizing the solid solution strengthening effect of Mn-B steel through control of alloying elements and improving formability. [Means for solving the problem]

[0008] The wire rod of the present invention with improved delayed fracture resistance contains, by weight, C: 0.15 to 0.30%, Si: 0.15 to 0.25%, Mn: 0.95 to 1.35%, P: 0.030% or less, S: 0.030% or less, Ti: 0.015 to 0.030%, B: 0.0010 to 0.0040%, N: 0.0010 to 0.0080%, with the balance being Fe and unavoidable impurities, and is characterized by satisfying the following relational expression 1: [Relationship 1] 2.0≦5.5×[Si]+[Mn]≦2.4 (In Relational Formula 1, [Si] and [Mn] each represent the content (wt%) of the corresponding element.)

[0009] In addition, the following relational expression 2 can be satisfied. [Relationship 2] 1.0<[Ti] / 3.42[N]<2.0 (In Relational Formula 2, [Ti] and [N] each represent the content (wt%) of the corresponding element.)

[0010] Furthermore, the size of one TiN inclusion of the present invention may be 15 μm or less.

[0011] A method for producing a wire rod having improved delayed fracture resistance according to the present invention includes the steps of finish-rolling, at 880 to 980°C, a steel material containing, by weight %, C: 0.15 to 0.30%, Si: 0.15 to 0.25%, Mn: 0.95 to 1.35%, P: 0.030% or less, S: 0.030% or less, Ti: 0.015 to 0.030%, B: 0.0010 to 0.0040%, N: 0.0010 to 0.0080%, with the balance being Fe and unavoidable impurities, and satisfying the following relational expression 1: and winding the sheet at 830 to 930°C. [Relationship 1] 2.0≦5.5×[Si]+[Mn]≦2.4 (In Relational Formula 1, [Si] and [Mn] each represent the content (wt%) of the corresponding element.)

[0012] Furthermore, the steel material of the present invention can satisfy the following relational expression 2. [Relationship 2] 1.0<[Ti] / 3.42[N]<2.0 (In Relational Formula 2, [Ti] and [N] each represent the content (wt%) of the corresponding element.)

[0013] The method for manufacturing a part with improved delayed fracture resistance of the present invention is characterized by comprising the steps of drawing a wire rod manufactured by the present invention, subjecting the drawn wire rod to a spheroidizing heat treatment at 745 to 770°C, forming the spheroidizing heat-treated wire rod into a part and heating it in a temperature range of 870 to 940°C, quenching the heated part in a temperature range of 50 to 80°C, and tempering the quenched part in a temperature range of 400 to 600°C.

[0014] The part of the present invention with improved delayed fracture resistance contains, by weight%, 0.15 to 0.30% C, 0.15 to 0.25% Si, 0.95 to 1.35% Mn, 0.030% or less P, 0.030% or less S, 0.015 to 0.030% Ti, 0.0010 to 0.0040% B, 0.0010 to 0.0080% N, with the remainder being Fe and unavoidable impurities, and is characterized by satisfying the following relational expression 1: [Relationship 1] 2.0≦5.5×[Si]+[Mn]≦2.4 (In Relational Formula 1, [Si] and [Mn] each represent the content (wt%) of the corresponding element.)

[0015] Furthermore, the component of the present invention satisfies the following relational expression 2. [Relationship 2] 1.0<[Ti] / 3.42[N]<2.0 (In Relational Formula 2, [Ti] and [N] each represent the content (wt%) of the corresponding element.)

[0016] The part of the present invention also contains, by volume fraction, 0.3 to 2% retained austenite and the remainder tempered martensite structure. Furthermore, the components of the present invention have TiN inclusions of 15 μm or less in size. [Effects of the Invention]

[0017] According to the present invention, high-strength bolt components with improved delayed fracture resistance improve the formability of the Mn-B steel bolt threads during processing, thereby preventing cracks from occurring in the bolt threads, and thus suppressing delayed fracture in 1 Gpa-class high-strength bolts. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a photograph showing the observation of the threaded portion of Comparative Example 3 before evaluation of delayed fracture resistance. DETAILED DESCRIPTION OF THE INVENTION

[0019] This specification does not describe all elements of the embodiments, and content that is common in the technical field to which the present invention belongs or that is overlapping between the embodiments will be omitted.

[0020] Furthermore, when any part is said to "comprise" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified.

[0021] The singular expression includes the plural expression unless the context clearly dictates otherwise.

[0022] The present invention will be described in detail below. The following embodiments are presented to fully convey the concept of the present invention to those skilled in the art, and the present invention is not limited to the examples presented herein and may be embodied in other forms.

[0023] The inventors of the present invention have found that when the Si and Mn contents are controlled, the solid solution strengthening effect can be optimized to ensure strength while improving formability, and the occurrence of cracks due to deterioration in forming of the threaded portion can be suppressed, thereby improving delayed fracture resistance.

[0024] Furthermore, the inventors have found that by controlling the Ti and N contents and the size of TiN inclusions, it is possible to refine the crystal grains, thereby improving formability and ensuring delayed fracture resistance, and have completed the present invention.

[0025] A wire rod with improved delayed fracture resistance according to one embodiment of the present invention contains, by weight, C: 0.15 to 0.30%, Si: 0.15 to 0.25%, Mn: 0.95 to 1.35%, P: 0.030% or less, S: 0.030% or less, Ti: 0.015 to 0.030%, B: 0.0010 to 0.0040%, N: 0.0010 to 0.0080%, and the remainder being Fe and unavoidable impurities.

[0026] The reasons for limiting the numerical values ​​of the alloying element contents in the embodiments of the present invention will be explained below. Unless otherwise specified, the units are % by weight.

[0027] The carbon (C) content is 0.15 to 0.30%. C is an element added to ensure product strength. If the carbon content is less than 0.15%, it is difficult to achieve the target strength in the present invention. If the carbon content exceeds 0.30%, it can interfere with the formation of retained austenite, which has excellent mechanical stability and is formed by hydrostatic pressure at the lath martensite boundary during quenching, thereby degrading delayed fracture resistance. Therefore, in the present invention, the C content is limited to 0.15 to 0.30%.

[0028] The silicon (Si) content is 0.15 to 0.25%. Silicon is not only useful for deoxidizing steel, but is also an effective element for ensuring strength through solid solution strengthening. If the Si content is less than 0.15%, the strength achieved through deoxidation and solid solution strengthening of the steel is insufficient, while if it exceeds 0.25%, the formability and impact properties due to solid solution strengthening may deteriorate. Therefore, in the present invention, the Si content is limited to 0.15 to 0.25%.

[0029] The manganese (Mn) content is 0.95 to 1.35%. Mn is an element that improves hardenability and is a very useful element that forms a substitutional solid solution in the matrix structure and exerts a solid solution strengthening effect. If the Mn content is less than 0.95%, the above-mentioned solid solution strengthening effect and hardenability are insufficient, making it difficult to ensure the strength targeted in the present invention. If the Mn content exceeds 1.35%, the solid solution strengthening effect can deteriorate formability. Therefore, in the present invention, the Mn content is limited to 0.95 to 1.35%.

[0030] The phosphorus (P) content is 0.030% or less (0% is excluded). P is an element that segregates at grain boundaries, lowering toughness and reducing delayed fracture resistance, so in the present invention, the upper limit of P is set to 0.030%.

[0031] The sulfur (S) content is 0.030% or less (0% is excluded). Like P, S is an element that not only segregates at grain boundaries to reduce toughness but also forms low-melting-point sulfides that hinder hot rolling. Therefore, in the present invention, the upper limit of S is set to 0.030%.

[0032] The titanium (Ti) content is 0.015 to 0.030%. Ti is an element that combines with N that flows into the steel to form titanium carbonitride (TiN). In the present invention, TiN refines crystal grains, thereby suppressing cracking due to forming deterioration during part forming and improving delayed fracture resistance. Furthermore, Ti forms TiN, which prevents free N from combining with B and suppresses the formation of BN, which deteriorates formability. If the Ti content is less than 0.015%, as mentioned above, sufficient TiN is not formed and the free N forms BN, making it difficult to utilize the hardening effect of B. If the Ti content exceeds 0.03%, coarse carbonitrides are formed, which can deteriorate delayed fracture resistance. Therefore, in the present invention, the Ti content is limited to 0.015 to 0.03%.

[0033] The content of boron (B) is 0.0010 to 0.0040%. B is an element that improves hardenability. If the B content is less than 0.0010%, it is difficult to expect the aforementioned effect of improving hardenability, and if it exceeds 0.0040%, Fe is present at the grain boundaries. 23 B forms (CB)6 carbides, which induces embrittlement of austenite grain boundaries, and forms BN, which deteriorates formability and therefore deteriorates delayed fracture resistance. Therefore, in the present invention, the B content is limited to 0.0010 to 0.0040%.

[0034] The nitrogen (N) content is 0.0010 to 0.0080%. N is an element that forms carbonitrides. If the N content is less than 0.0010%, TiN precipitates that refine grains cannot be sufficiently formed. If the N content exceeds 0.0080%, the amount of dissolved nitrogen increases, which can deteriorate the toughness and softness of the steel. Free N can combine with B to form BN, which deteriorates formability. Therefore, in the present invention, the N content is limited to 0.0010 to 0.0080%.

[0035] The balance other than the alloy composition is iron (Fe). The wire rod with improved delayed fracture resistance of the present invention may contain other impurities that are usually contained in the industrial production process of steel. Since such impurities are known to anyone with ordinary skill in the art to which the present invention pertains, the type and content of such impurities are not particularly limited in the present invention.

[0036] The wire rod with improved delayed fracture resistance according to one embodiment of the present invention satisfies the following relational expression 1. [Relationship 1] 2.0≦5.5×[Si]+[Mn]≦2.4

[0037] In Relational Formula 1, [Si] and [Mn] each represent the content (wt %) of the corresponding element.

[0038] In the present invention, the Si and Mn contents are controlled to ensure strength through the solid solution strengthening effect, while suppressing excessive solid solution strengthening to improve wire formability and delayed fracture resistance. Relational Formula 1 was derived from this study to optimize the solid solution strengthening effect. If the value of 5.5 × [Si] + [Mn] in Relational Formula 1 is less than 2.0, the target strength cannot be achieved in the present invention. If the value of 5.5 × [Si] + [Mn] exceeds 2.4, excessive solid solution strengthening can cause cracks due to forming deterioration during the forming of high-strength parts, leading to delayed fracture. Therefore, in the present invention, the value of 5.5 × [Si] + [Mn] is limited to 2.0 to 2.4 in order to improve delayed fracture resistance.

[0039] Furthermore, the wire rod with improved delayed fracture resistance according to one embodiment of the present invention satisfies the following relational expression 2.

[0040] [Relationship 2] 1.0<[Ti] / 3.42[N]<2.0

[0041] In Relational Formula 2, [Ti] and [N] each represent the content (wt %) of the corresponding element.

[0042] In the present invention, we aimed to improve the delayed fracture resistance of wire rod by refining crystal grains and improving formability. Through extensive research, the inventors of the present invention have found that by forming TiN inclusions, controlling their size to refine crystal grains, and suppressing BN, we were able to ensure formability and delayed fracture resistance. This led to the derivation of Relational Equation 2, which controls the size of TiN inclusions and suppresses BN formation. If the [Ti] / 3.42[N] value in Relational Equation 2 is 1.0 or less, formability may be degraded due to BN formed from free N that does not bond with Ti. If the [Ti] / 3.42[N] value is 2.0 or more, excess Ti causes TiN to coarsen, preventing the grain refinement effect from being achieved. Therefore, in the present invention, the [Ti] / 3.42[N] value is limited to more than 1.0 but less than 2.0.

[0043] In the present invention, the size of TiN inclusions for refining crystal grains may be 15 μm or less. As mentioned above, if the maximum size of TiN inclusions exceeds 15 μm, it is difficult to ensure delayed fracture resistance due to grain refinement.

[0044] Furthermore, parts with improved delayed fracture resistance manufactured from the wire rod according to the present invention contain, by volume fraction, 0.3 to 2% retained austenite and the remainder tempered martensite. If the retained austenite fraction is less than 0.3%, it is difficult to expect it to act as an obstacle to delay hydrogen diffusion, which deteriorates delayed fracture resistance. If it exceeds 2%, the retained austenite is formed thickly not only at lath boundaries but also at austenite grain boundaries, making it difficult to delay hydrogen diffusion, thereby reducing the effect of improving delayed fracture resistance.

[0045] Next, a method for manufacturing a wire rod and a part having improved delayed fracture resistance according to one embodiment of the present invention will be described.

[0046] The wire rod and parts having improved delayed fracture resistance according to the present invention may be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an embodiment, the wire rod and parts may be manufactured by the following method.

[0047] The wire rod having improved delayed fracture resistance according to the present invention includes a step of finish-rolling a steel material containing, by weight %, C: 0.15 to 0.30%, Si: 0.15 to 0.25%, Mn: 0.95 to 1.35%, P: 0.030% or less, S: 0.030% or less, Ti: 0.015 to 0.030%, B: 0.0010 to 0.0040%, N: 0.0010 to 0.0080%, with the balance being Fe and unavoidable impurities, at 880 to 980°C, and a step of coiling at 830 to 930°C.

[0048] First, a steel material satisfying the above-mentioned alloy composition is prepared and subjected to finish wire rolling at 880 to 980°C. Thereafter, the rolled wire material is wound into a coil shape at 830 to 930°C.

[0049] In this case, if the wire rod rolling temperature is below 880°C or the coiling temperature is below 830°C, the surface layer is in the quasi-two-phase region, so a surface ferrite decarburization layer can form due to phase transformation. This ferrite decarburization layer can also form on the surface during heat treatment of the bolt, degrading delayed fracture resistance. Furthermore, the prior austenite grain size of the bolt product becomes finer, increasing the fraction of retained austenite, degrading delayed fracture resistance. Furthermore, if the wire rod finish rolling temperature exceeds 980°C or the coiling temperature exceeds 930°C, decarburization is accelerated by diffusion, forming a ferrite decarburization layer on the surface, coarsening the prior austenite grain size, and degrading delayed fracture resistance.

[0050] The coiled wire can then be drawn, spheroidized, coated, bolt-formed, austenitized, quenched, and tempered as desired to produce the final bolt component, although in one embodiment it can be produced by the following method:

[0051] A method for manufacturing a bolt part according to one embodiment of the present invention includes the steps of drawing a wire rod manufactured according to the present invention, subjecting the drawn wire rod to a spheroidizing heat treatment at 745 to 770°C, forming the spheroidizing heat-treated wire rod into a part and heating it at 870 to 940°C, quenching the heated part at 50 to 80°C, and tempering it at 400 to 600°C.

[0052] In this case, the spheroidizing heat treatment may be performed at a temperature of 745 to 770°C. If the heat treatment temperature is lower than 745°C or higher than 770°C, the spheroidizing rate decreases and the hardness of the spheroidized heat-treated material increases, which deteriorates formability during thread processing after bolt formation, and may lead to cracks in the thread portion.

[0053] The austenitizing heat treatment can be performed at 870 to 940°C. If the heat treatment temperature is less than 870°C, the austenite reverse transformation does not occur sufficiently, resulting in the formation of a non-uniform martensite structure after quenching, which can deteriorate toughness. If the heat treatment temperature exceeds 940°C, the prior austenite grain size becomes coarse, which can deteriorate delayed fracture resistance.

[0054] Furthermore, quenching can be performed at a temperature range of 50 to 80°C. If the quenching coolant temperature is below 50°C, fine quenching cracks may occur in the bolt threads due to thermal deformation, which can lead to delayed fracture. If the temperature exceeds 80°C, sufficient quenching is not achieved, and in addition to the mechanically stable retained austenite in the lath, retained austenite is formed at the prior austenite grain boundaries, which acts as a hydrogen trapping site and can lead to delayed fracture.

[0055] Tempering can be performed at a temperature range of 400 to 600°C to impart strength and toughness to suit the intended use and purpose of the final product. If the tempering temperature is less than 400°C, embrittlement may occur, and if it exceeds 600°C, it may be difficult to achieve the strength intended in the present invention.

[0056] The part with improved delayed fracture resistance manufactured according to the present invention contains, by volume fraction, 0.3 to 2% retained austenite and the remainder tempered martensite structure.

[0057] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention, as the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred from them.

[0058] Example Wire rods of Examples 1 to 6 and Comparative Examples 1 to 5, satisfying the alloy compositions in Table 1 below, were manufactured under manufacturing conditions according to the present invention to obtain bolts for final testing. Specifically, steel billets satisfying the alloy compositions in Table 1 below were finish-rolled at 880 to 980°C and wound into a coil at 830 to 930°C. The wound wire rod was then subjected to spheroidizing heat treatment at a maximum temperature of 745 to 770°C. The spheroidizing heat-treated wire rod was then formed into a bolt, austenitized at 870 to 940°C, quenched in a refrigerant at 50 to 80°C, and then tempered at 400 to 600°C to ensure a tensile strength of 1050±16 MPa, to obtain the final bolt product.

[0059] [Table 1]

[0060] Next, the bolt products of Examples 1 to 6 and Comparative Examples 1 to 5 were evaluated for the maximum size of TiN precipitates and the presence or absence of delayed fracture cracks, and the results are shown in Table 2 together with the values ​​of Relational Formula 1 and Relational Formula 2. The maximum size of TiN precipitates was measured by cutting the bolt products in the L cross section (longitudinal direction) and measuring the size of the precipitates at 160 mm. 2 The area was observed over 30 fields, and the size of the inclusion measured through extreme value analysis was defined as the maximum inclusion size, and the values ​​are shown in Table 2 below.

[0061] Delayed fracture resistance was evaluated by a delayed fracture simulation in which the bolt product was fastened to a structure with a fastening force of yield strength, then immersed in 5% hydrochloric acid + 95% distilled water solution for 10 minutes, and the presence or absence of cracks in the threads, where stress is concentrated, was observed. If no cracks occurred, this was indicated by an X, and if cracks occurred, this was indicated by a ○.

[0062] [Table 2]

[0063] As can be seen from Table 2, in Examples 1 to 6, which satisfy the alloy composition, relational expression, and TiN size proposed in the present invention, no delayed fracture cracks occurred in the threads of the bolt products before or after the evaluation of delayed fracture resistance. On the other hand, in Comparative Example 1, the [Ti] / 3.42[N] value was 2.506, which exceeded the upper limit of 2.0 proposed in the present invention, and coarse TiN was formed, which resulted in the occurrence of delayed fracture cracks.

[0064] In Comparative Example 2, the [Ti] / 3.42[N] value was 3.070, which exceeded the upper limit of 2.0 proposed in the present invention, and coarse TiN was formed, which resulted in the occurrence of delayed fracture cracks.

[0065] In Comparative Example 3, the Si content was 0.26%, exceeding the upper limit of 0.25% proposed in the present invention, and the 5.5 × [Si] + [Mn] value was 2.58, exceeding the upper limit of 2.4 proposed in the present invention. This excessive solid solution strengthening effect resulted in poor formability of the bolt threads after spheroidizing heat treatment, and delayed fracture cracks occurred. Figure 1 is a photograph of the threads of Comparative Example 3 before evaluation of delayed fracture resistance. Referring to Figure 1, it can be seen that Comparative Example 3 did not satisfy the conditions proposed in the present invention, and therefore delayed fracture cracks occurred, indicating that delayed fracture resistance was not achieved.

[0066] In Comparative Example 4, the Mn content was 1.45%, which exceeded the upper limit of 1.35% proposed in the present invention, and the 5.5 × [Si] + [Mn] value was 2.61, which exceeded the upper limit of 2.4 proposed in the present invention. As a result, the formability of the bolt thread portion deteriorated after spheroidizing heat treatment due to an excessive solid solution strengthening effect, and delayed fracture cracks occurred.

[0067] In Comparative Example 5, the C content was 0.33%, which exceeded the upper limit of 0.30% proposed in the present invention, and the formation of a retained austenite structure with excellent mechanical stability was suppressed, resulting in the occurrence of delayed fracture cracks.

[0068] Next, Invention Example 3 and Comparative Examples 6-1 to 6-6, which satisfy the alloy composition of Invention Example 3 in Table 1 according to the present invention, were manufactured under the manufacturing conditions shown in Table 3 below to obtain final bolt products.

[0069] [Table 3]

[0070] Inventive Example 3, which satisfied the finish rolling temperature, coiling temperature, spheroidizing heat treatment temperature, and austenitizing temperature requirements of the present invention, no delayed fracture cracks occurred. On the other hand, in Comparative Example 6-1, the rolling temperature was 990°C, exceeding the upper limit of 980°C proposed in the present invention, and the coiling temperature was 940°C, exceeding the upper limit of 930°C proposed in the present invention, so that the prior austenite grain size in the wire rod became coarse, and as the prior austenite grain size in the bolt product also became coarse, delayed fracture cracks occurred.

[0071] In Comparative Example 6-2, the rolling temperature was 870°C, which did not reach the lower limit of 880°C proposed in the present invention, and the coiling temperature was 820°C, which did not reach the lower limit of 830°C proposed in the present invention. As the prior austenite grain size became finer in the wire rod and the prior austenite grain size in the bolt product also became finer, the retained austenite fraction increased and delayed fracture cracks occurred.

[0072] In Comparative Example 6-3, the austenitizing heat treatment temperature was 950°C, which exceeded the upper limit of 940°C proposed in the present invention, and as the prior austenite grain size of the bolt product became coarse, delayed fracture cracks occurred.

[0073] In Comparative Example 6-4, the austenitizing heat treatment temperature was 860°C, which was lower than the lower limit of 870°C proposed in the present invention. Therefore, the QT heat treatment was performed in a state in which the bolt product was not sufficiently austenitized, and undissolved pearlite was formed, which resulted in the generation of delayed fracture cracks.

[0074] Comparative Example 6-5 had a spheroidizing temperature of 740°C, which was below the lower limit of 745°C proposed in the present invention. Comparative Example 6-6 had a spheroidizing temperature of 775°C, which exceeded the upper limit of 770°C proposed in the present invention. As a result, the spheroidizing rate was low, heat treatment was insufficient, formability deteriorated, and delayed fracture cracks occurred.

[0075] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person having ordinary skill in the art will understand that various changes and modifications are possible within the scope of the concept and scope of the claims set forth in this specification.

Claims

1. The steel sheet contains, by weight, C: 0.15 to 0.30%, Si: 0.15 to 0.25%, Mn: 0.95 to 1.35%, P: 0.030% or less, S: 0.030% or less, Ti: 0.015 to 0.030%, B: 0.0010 to 0.0040%, N: 0.0010 to 0.0080%, and the balance being Fe and unavoidable impurities; A part having improved delayed fracture resistance, characterized by satisfying the following relational formula 1 and the following relational formula 2. [Relationship 1] 2.0≦5.5×[Si]+[Mn]≦2.4 [Relationship 2] 1.0<[Ti] / 3.42[N]<2.0 (In Relational Formulas 1 and 2, [Si], [Mn], [Ti], and [N] each represent the content (wt%) of the corresponding element.)

2. 2. A part with improved delayed fracture resistance according to claim 1, characterized in that the size of the TiN inclusions is 15 μm or less.

3. The steel sheet contains, by weight, C: 0.15 to 0.30%, Si: 0.15 to 0.25%, Mn: 0.95 to 1.35%, P: 0.030% or less, S: 0.030% or less, Ti: 0.015 to 0.030%, B: 0.0010 to 0.0040%, N: 0.0010 to 0.0080%, and the balance being Fe and unavoidable impurities; A step of finish rolling a steel material satisfying the following relational formula 1 and the following relational formula 2 at 880 to 980 ° C.; a step of drawing the wire rod manufactured by the method for manufacturing a wire rod having improved delayed fracture resistance, the method comprising the steps of: subjecting the drawn wire to a spheroidizing heat treatment at 745 to 770°C; forming the spheroidized heat-treated wire into a part and heating it at a temperature in the range of 870 to 940°C; quenching the heated part at a temperature in the range of 50 to 80°C; and tempering the quenched part at a temperature in the range of 400 to 600°C. [Relationship 1] 2.0≦5.5×[Si]+[Mn]≦2.4 [Relationship 2] 1.0<[Ti] / 3.42[N]<2.0 (In Relational Formulas 1 and 2, [Si], [Mn], [Ti], and [N] each represent the content (wt%) of the corresponding element.)

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