KR102424956B1 - low-carbon boron steel wire with improved hardenability and softening resistance and method for manufacturing the same - Google Patents

Abstract

Disclosed are a low-carbon boron steel wire having improved hardenability and softening resistance, and a method for manufacturing the same.
The low-carbon boron steel wire rod according to the present invention is, by weight, C: 0.2 to 0.3%, Si: more than 0 0.8% or less, Mn: more than 0 0.5% or less, Cr: 0.5 to 1.5%, P: 0.03% or less, S : 0.03% or less, sol.Al: more than 0 0.07% or less, V: 0.02 to 0.5%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0.01% or less, remaining Fe and unavoidable impurities, and , including ferrite and pearlite as microstructures, and satisfy the following Relations 1 to 3.
[Relational Expression 1] Cr/Mn ≥ 2.0
[Relational Expression 2] 0.9 ≤ Mn+Cr ≤ 1.6
[Relational Expression 3] Segregation zone Mn/Cr ≤ 0.6

Description

Low-carbon boron steel wire with improved hardenability and softening resistance and method for manufacturing the same

The present invention relates to a low-carbon boron steel wire having improved hardenability and softening resistance and a method for manufacturing the same, and more particularly, to a low-carbon boron steel wire suitable for use as a material for mechanical parts and a method for manufacturing the same.

The cold working method is widely used in the manufacture of mechanical parts such as bolts and nuts because the cold working method is excellent in productivity as well as the heat treatment cost reduction is large compared with the hot working method or the machining method.

However, in order to manufacture mechanical parts using the cold working method, it is essentially required to have excellent cold workability of the steel, and more specifically, it is required to have low deformation resistance and excellent ductility during cold working. This is because, when the deformation resistance of steel is high, the life of the tool used during cold working is reduced, and when the ductility of steel is low, it is easy to fracture during cold working, which can cause defective products.

Accordingly, conventional steel for cold forging is subjected to softening annealing heat treatment before cold forging. This is because the steel material is softened during the softening annealing heat treatment to reduce deformation resistance and improve ductility to improve cold forgeability. However, in this case, additional cost is incurred and manufacturing efficiency is lowered. Therefore, development of a heat treatment omitted wire rod capable of securing excellent cold forgeability without additional heat treatment is required.

However, if the heat treatment is omitted and cold forging is performed, there is a problem in that the strength of the die increases compared to the conventional heat treatment material, thereby reducing the life of the die. Since this reduction in die life reduces the cost saving effect through omitting the heat treatment, the heat treatment omitted steel may not be advantageous compared to the heat treatment material. In addition, when the alloy component is reduced in order to improve the life of the die, the hardenability and softening resistance are reduced, so that there is a high possibility that the desired physical properties cannot be secured. Therefore, in order to develop an economical heat treatment omitted boron steel, it is required to derive component design and manufacturing conditions that comprehensively consider hardenability, die life, and softening resistance.

An object of the present invention is to provide a low-carbon boron steel wire rod capable of ensuring hardenability and softening resistance, and a method for manufacturing the same.

The low-carbon boron steel wire rod capable of ensuring hardenability and softening resistance according to an embodiment of the present invention is, by weight, C: 0.2 to 0.3%, Si: more than 0 and less than 0.8%, Mn: more than 0 and less than or equal to 0.5%, Cr: 0.5 to 1.5%, P: 0.03% or less, S: 0.03% or less, sol.Al: more than 0 0.07% or less, V: 0.02 to 0.5%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0.01% or less, including the remaining Fe and unavoidable impurities, including ferrite and pearlite as microstructures, and satisfying the following Relations 1 to 3.

[Relational Expression 1] Cr/Mn ≥ 2.0

[Relational Expression 2] 0.9 ≤ Mn+Cr ≤ 1.6

[Relational Expression 3] Segregation zone Mn/Cr ≤ 0.6

The tensile strength of the wire rod may be 650 MPa or less.

The wire rod may have a Rockwell hardness of 40 HRc or more at 6mm in the ASTM A 255 Jominy test.

The tensile strength of the wire rod after drawing processing may be 720 MPa or less.

The wire rod may have a carbon equivalent (C _eq ) defined by the following formula (1) of 0.35 to 0.50.

(1) C _eq = [C] + [Si]/9 + [Mn]/5 + [Cr]/12

(Here, each of [C], [Si], [Mn] and [Cr] means the weight % of the corresponding element)

The wire rod may further include Nb: 0.1% or less (including 0%).

According to an embodiment of the present invention, the method for manufacturing a low-carbon boron steel wire rod capable of ensuring quenchability and softening resistance is weight %, C: 0.2 to 0.3%, Si: more than 0 and less than 0.8%, Mn: more than 0 0.5 % or less, Cr: 0.5 to 1.5%, P: 0.03% or less, S: 0.03% or less, sol.Al: 0 to 0.07% or less, V: 0.02 to 0.5%, Ti: 0.005 to 0.05%, B: 0.0005 to reheating the billet containing 0.005%, N: 0.01% or less, remaining Fe and unavoidable impurities; Final rolling the reheated billet in a temperature range of 880°C to 1050°C; A first cooling step of cooling from the final finish rolling temperature to 850 °C at an average cooling rate of 3 °C / s or more; a second cooling step of cooling from 850°C to 750°C at an average cooling rate of 1 to 3°C/s after the first cooling; and a third cooling step of cooling from 750° C. to 600° C. at an average cooling rate of less than 1° C./s after the second cooling.

The billet may satisfy Relations 1 to 3 below.

[Relational Expression 1] Cr/Mn ≥ 2.0

[Relational Expression 2] 0.9 ≤ Mn+Cr ≤ 1.6

[Relational Expression 3] Segregation zone Mn/Cr ≤ 0.6

The billet may have a carbon equivalent (C _eq ) defined by the following formula (1) of 0.35 to 0.50.

(1) C _eq = [C] + [Si]/9 + [Mn]/5 + [Cr]/12

(Here, each of [C], [Si], [Mn] and [Cr] means the weight % of the corresponding element)

The billet may further include Nb: 0.1% or less (including 0%).

The low-carbon boron steel wire according to the embodiment of the present invention secures a ferrite and pearlite composite structure even if softening heat treatment is omitted by controlling the Cr/Mn ratio of the matrix and the segregation part, but deformation resistance during cold working can be sufficiently suppressed.

In addition, it is possible to provide a low-carbon boron steel wire rod having sufficient hardenability and softening resistance after Q/T heat treatment, and the increase in strength is suppressed even after wire-drawing, so that die life can be secured.

This specification does not describe all elements of the embodiments, and general content in the technical field to which the present invention pertains or content that overlaps among the embodiments is omitted.

Also, when a part "includes" a certain component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.

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

Hereinafter, the present invention will be described in detail.

The present inventors studied from various angles to provide a wire rod capable of securing sufficient hardenability and softening resistance after the wire rod-drawing-cold forging-Q/T heat treatment process, and as a result, the alloy composition and manufacturing method were optimized Thus, the ferrite and pearlite composite structure is secured as the microstructure of the wire rod, but by appropriately controlling the Cr/Mn ratio of the matrix and segregation part, the increase in strength is suppressed even after wire drawing, so that the life of the die can be secured and sufficient after Q/T heat treatment It has been found that a wire rod capable of ensuring hardenability and softening resistance can be provided, and thus the present invention has been completed.

The low-carbon boron steel wire rod with improved quenchability and softening resistance according to an embodiment of the present invention is, by weight, C: 0.2 to 0.3%, Si: more than 0 and less than 0.8%, Mn: more than 0 and less than or equal to 0.5%, Cr: 0.5 to 1.5%, P: 0.03% or less, S: 0.03% or less, sol.Al: more than 0 0.07% or less, V: 0.02 to 0.5%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0.01 % or less, including remaining Fe and unavoidable impurities, and including ferrite and pearlite as microstructures.

In addition, the low-carbon boron steel wire rod according to the present invention may further include Nb in an amount of 0.1% or less (including 0%).

Hereinafter, the reason for numerical limitation of the content of alloying elements in an embodiment of the present invention will be described. Hereinafter, unless otherwise specified, the unit is % by weight.

The content of C is 0.2 to 0.3%.

C is an element that improves the strength of the wire rod. When the content of C is less than 0.2%, the above-mentioned effect cannot be exhibited, and when the content of C exceeds 0.3%, the deformation resistance of the steel rapidly increases, which may deteriorate cold workability, so the content thereof is limited. .

The content of Si is more than 0 and 0.8% or less.

Si is an element added to secure a certain level of strength. However, when the Si content exceeds 0.8, the deformation resistance of the steel rapidly increases due to solid solution strengthening, which may deteriorate the cold workability. Therefore, the upper limit of the Si content is preferably 0.8%.

The content of Mn is more than 0 and 0.5% or less.

Mn is an element added to improve hardenability. However, when the content of Mn exceeds 0.5%, the strength of the steel itself becomes excessively high, so that the deformation resistance of the steel rapidly increases, which may deteriorate cold workability. Therefore, the upper limit of the Mn content is preferably 0.5%. The upper limit of the Mn content is more preferably 0.4%.

The content of Cr is 0.5 to 1.5%.

Cr is an element that promotes ferrite and pearlite transformation during hot rolling. In addition, Cr precipitates carbides in the steel without increasing the strength of the steel itself more than necessary to reduce the amount of solid solution carbon, and greatly contributes to the reduction of dynamic strain aging due to the solid solution carbon. When the content of Cr is less than 0.5%, the above-mentioned effect cannot be exhibited, and when the content of Cr exceeds 1.5%, the strength of the steel itself becomes excessively high and the deformation resistance of the steel rapidly increases, which causes the cold workability to deteriorate. can Accordingly, the upper limit of the Cr content is preferably 1.5%, and the upper limit of the Cr content is more preferably 1.4%.

The content of P is 0.03% or less.

P is an unavoidably contained impurity and is an element that segregates at grain boundaries to reduce the toughness of steel and is a major factor in reducing delayed fracture resistance. Therefore, it is desirable to control the content as low as possible. In theory, it is advantageous to control the content of P to 0%, but since it is inevitably contained during the manufacturing process, it is important to manage the upper limit of P, and in the present invention, the upper limit of the P content is managed as 0.03%.

The content of S is 0.03% or less.

S is an unavoidable impurity that is segregated at grain boundaries to greatly reduce the ductility of steel, and forms an emulsion in steel, which is a major cause of deterioration of delayed fracture resistance and stress relaxation characteristics. Therefore, it is desirable to control the content as low as possible. Theoretically, it is advantageous to control the content of S to 0%, but since it is inevitably contained during the manufacturing process, it is important to manage the upper limit, and in the present invention, the upper limit of the S content is managed as 0.03%.

The content of sol.Al is greater than 0 and less than or equal to 0.07%.

sol.Al is an element that plays a role in refining the grain size of austenite by forming aluminum nitride (AlN). However, when the content of Sol.Al exceeds 0.07%, the above-described austenite particle size refining effect is increased and cold workability is lowered. Therefore, in the present invention, the upper limit of the sol.Al content is managed as 0.07%.

The content of V is 0.02 to 0.5%.

V is an element that forms carbides and carbonitrides to limit grain boundary movement of austenite and ferrite. However, since the carbonitride of V acts as a fracture origin and may reduce impact toughness, it is preferable to add it by keeping the solubility limit. In the present invention, the addition of V is an element that plays a key role in improving the softening resistance by forming a large amount of nano-sized VC precipitates during Q/T heat treatment. When the content of V is less than 0.02%, there is a problem in that it is difficult to secure strength due to insufficient VC precipitation. It is preferable to limit the content to 0.5% or less.

The content of Ti is 0.005 to 0.05%.

Ti combines with nitrogen in steel to form titanium nitride (TIN). Titanium nitride is very stable at high temperatures, and it is generated at the austenite grain boundary to suppress the growth of austenite grains, thereby refining the structure. Refined austenite structure promotes the transformation of ferrite and pearlite, which are soft structures, during cooling, so that the effect of softening the wire rod can be obtained. In addition, the formation of titanium nitride contributes to the reduction of solid solution nitrogen in steel, and has an effect of enabling the securing of solid solution B. When the content of Ti is less than 0.005%, the aforementioned effect cannot be exhibited, and when the content of Ti exceeds 0.05%, coarse titanium nitride is excessively precipitated to reduce toughness, so in the present invention, the content of Ti is reduced from 0.005% to 0.005%. It is preferable to limit it to 0.05%.

The content of B is 0.0005 to 0.005%.

B is a grain boundary strengthening element for improving hardenability and delayed fracture resistance. When the B content is less than 0.0005%, the grain boundary strengthening effect or hardenability improvement effect due to grain boundary segregation of B atoms during heat treatment is insufficient. Therefore, in the present invention, it is preferable to limit the content of B to 0.0005 to 0.005%.

The content of N is 0.01% or less.

N is an inevitably contained impurity, and when the content is excessive, the amount of solid solution N increases, and the deformation resistance of the steel rapidly increases, which may deteriorate cold workability. Theoretically, it is advantageous to control the content of N to 0%, but it is important to manage the upper limit because it is inevitably contained during the manufacturing process. It is more preferable to manage it as %, and it is more preferable to manage it at 0.007%.

The content of Nb is 0.1% or less (including 0%).

Since Nb is an element that forms carbides and carbonitrides to limit grain boundary movement of austenite and ferrite, it may be contained in 0.1% or less. However, when the content of Nb exceeds 0.1%, the carbonitride of Nb acts as a fracture origin to reduce impact toughness, so the content of Nb is limited to 0.1% or less.

The balance other than the alloy composition is Fe. The low-carbon boron steel wire rod of the present invention may contain other impurities that may be included in the industrial production process of ordinary steel. These impurities are not particularly limited in the present invention because the content can be known by anyone having ordinary knowledge in the technical field to which the present invention pertains.

The low-carbon boron steel wire with improved quenchability and softening resistance according to an embodiment of the present invention may have a carbon equivalent (C _eq ) of 0.35 to 0.50. Here, the carbon equivalent (C _eq ) may be defined by Equation 1 below. If the carbon equivalent (C _eq ) is less than 0.35, it may be difficult to secure strength during Q/T heat treatment.

[Equation 1]

C _eq = [C] + [Si]/9 + [Mn]/5 + [Cr]/12

(Here, each of [C], [Si], [Mn], and [Cr] means the content (%) of the corresponding element)

The low-carbon boron steel wire having improved quenchability and softening resistance according to an embodiment of the present invention may satisfy the following Relational Expressions 1 to 3.

[Relational Expression 1] Ratio of [Cr]/[Mn]

Relation 1 is an expression related to the tensile strength of the wire rod and the wire rod, and in the present invention, the strength of the wire rod and the wire rod after the wire rod is lowered through the control of the Cr/Mn ratio to secure the formability during cold forging and to suppress die wear as much as possible. did. In general, when Mn is added excessively, since Mn is an austenite stabilizing element and has a very large solid solution strengthening effect, a low-temperature structure may occur in the segregation part, and excessive strength increase may occur during wire drawing. To solve this problem, on the contrary, when Cr is added instead of Mn, Cr is a ferrite stabilizing element and the solid solution strengthening effect is very small compared to Mn. It has the effect of suppressing the increase in strength by lowering the Therefore, it is preferable that the Cr/Mn ratio to satisfy the tensile strength of the wire rod and the wire rod proposed in the present invention is 2.0 or more.

[Relational Expression 2] 0.9 ≤ [Mn]+[Cr] ≤ 1.6

Relation 2 is an expression related to the depth of hardenability, and when the sum is less than 0.9, the quenching depth suggested in the present invention is not satisfied, and when it exceeds 1.6, component segregation is deepened and the occurrence of low-temperature structures is high. The content of Mn+Cr is preferably limited to 0.9 to 1.6.

[Relational Expression 3] Ratio of [Mn]/[Cr] (segregation part)

Relation 3 is an expression related to the generation of low-temperature tissue in the segregation area. Omission of heat treatment In the case of cold forging steel, low-temperature structure should not occur in the state of the wire rod. When low-temperature tissues are generated, the possibility of cracks occurring around low-temperature tissues during fresh processing increases. In particular, in the case of Mn, the segregation index is high and austenite is stabilized, which increases the possibility of low-temperature structure formation. Therefore, when the Mn/Cr ratio is appropriately controlled in the wire rod segregation part, stability to such a low-temperature structure can be improved.

The ratio of Mn/Cr in the wire rod segregation part can be controlled by a light pressure reduction process when molten steel having a predetermined chemical composition is produced into bloom by continuous casting. In the light reduction process, it is preferable to reduce the bloom during solidification with the roll of a continuous casting machine. By performing under light pressure, Mn concentration in the central segregation portion is reduced, so that it is easy to obtain a segregation profile of a desired Mn/Cr ratio.

As a result of many experiments in the present invention, it was confirmed that when the ratio of Mn/Cr in the segregation part was controlled to 0.6 or less, the possibility of low-temperature structure generation was reduced and stable ferrite and pearlite structures were secured in the state of the wire rod.

The low-carbon boron steel wire rod having improved hardenability and softening resistance according to an embodiment of the present invention may have a strength of 650 MPa or less. The tensile strength of the wire rod is a physical property that is closely related to the tensile strength of the wire rod, and when Relations 1 to 3 proposed in the present invention are satisfied, it is necessary to manage the tensile strength of the wire rod to 650 MPa or less in order to satisfy 720 MPa or less after wire drawing.

The strength of the wire drawn with the low-carbon boron steel wire with improved hardenability and softening resistance according to the present invention may be 720 MPa or less. The tensile strength of the wire rod is a physical property related to formability and die life during cold forging. In the present invention, through numerous tests, when the tensile strength of the wire drawing material is lower than 720 MPa, sufficient formability can be secured during cold forging and the die life is significantly reduced. It came to find out that it is not possible and propose the present invention.

The low-carbon boron steel wire with improved hardenability and softening resistance according to the present invention may have a logwell hardness of 40HRc or more at 6mm in the Jomini test. Jomini hardness is a physical property related to hardenability, and having a hardness value of 40HRc or higher at a depth of 6mm means that it can be applied to products with a diameter of 12mm. Since most of the 10T-class high-strength bolts for automobiles are 12 mm or less, the low-carbon boron steel wire of the present invention exhibiting a sufficient martensite hardness value of 40 HRc or more at a quenching depth of 6 mm can be applied to automobile bolts and the like.

Next, a method for manufacturing a steel wire having improved wire drawing ability according to an embodiment of the present invention will be described.

The low-carbon boron steel wire rod with improved quenchability and softening resistance according to the present invention can be manufactured by various methods, and the manufacturing method is not particularly limited. However, as an embodiment, it may be manufactured by the following method.

The low-carbon boron steel wire rod with improved quenchability and softening resistance according to an embodiment of the present invention has C: 0.2 to 0.3%, Si: more than 0 and less than 0.8%, Mn: more than 0 and less than 0.5%, Cr: 0.5 to 1.5%, P: 0.03% or less, S: 0.03% or less, sol.Al: more than 0 0.07% or less, V: 0.02 to 0.5%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0.01% or less, remainder reheating the billet containing Fe and unavoidable impurities; Final rolling the reheated billet in a temperature range of 880°C to 1050°C; A first cooling step of cooling from the final finish rolling temperature to 850 °C at an average cooling rate of 3 °C / s or more; a second cooling step of cooling from 850°C to 750°C at an average cooling rate of 1 to 3°C/s after the first cooling; and a third cooling step of cooling from 750° C. to 600° C. at an average cooling rate of less than 1° C./s after the second cooling.

In addition, the billet according to the present invention may further include Nb in an amount of 0.1% or less (including 0%).

First, the bloom (bloom) satisfying the above-described component system is heated, and then the slab is rolled to obtain a billet.

The rolled billet is then reheated. The reheating of the present invention may be performed under normal conditions, but may be performed in a temperature range of 1000 to 1200° C. in order to reduce the rolling load. After reheating, the reheated billet is hot rolled to manufacture a wire rod. At this time, the final finish rolling may be performed in a temperature range of 880 ℃ to 1050 ℃. If the finish rolling temperature is less than 880 ℃, the deformation resistance may increase due to strength increase due to ferrite grain refinement, whereas, if it exceeds 1050 ℃, the ferrite grains may be excessively coarse and toughness may be reduced.

Then, the hot-rolled wire rod may be cooled in stages to manufacture a low-carbon boron steel wire rod according to the present invention. The cooling process of the present invention is a first cooling step of cooling from the final finish rolling temperature to 850 ℃ at an average cooling rate of 3 ℃ / s or more; a second cooling step of cooling from 850°C to 750°C at an average cooling rate of 1 to 3°C/s after the first cooling; and a third cooling step of cooling from 750° C. to 600° C. at an average cooling rate of less than 1° C./s after the second cooling.

The first cooling step may be performed at a cooling rate of 3°C/s or more from the final finish rolling temperature to 850°C. The temperature range of the first cooling step is a temperature range in which crystal grains grow, and when the cooling rate is slowed, there is a risk that the ferrite grains are coarsened. Therefore, it is preferable that the first cooling step be performed at a cooling rate of 3° C./s or more.

The second cooling step may be performed at an average cooling rate of 1 to 3° C./s from 850° C. to 750° C. after the first cooling. The temperature range of the second cooling step is a temperature range related to grain boundary segregation of boron. At a cooling rate of 1°C/s or less, the effect of refining the particle size maintained in the first cooling is likely to decrease, and at a cooling rate of 3°C/s or more, Due to the grain boundary strengthening, there is a possibility that a bainite structure may be formed due to a transformation delay in the third cooling step. Therefore, the second cooling step is preferably performed at a cooling rate of 1 °C / s to 3 °C / s.

The third cooling step may be performed at an average cooling rate of less than 1°C/s from 750°C to 600°C after the second cooling. The third cooling step is a temperature range related to ferrite-pearlite transformation, and there is a possibility that the transformation may not be 100% complete at a cooling rate of more than 1° C./s. Therefore, the third cooling step is preferably performed at a cooling rate of less than 1 °C / s.

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for illustrating the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

Example

After heating a bloom having an alloy composition of Table 1 at a temperature of 1,250° C. for 4 hours, the steel slab was rolled at a finish rolling temperature of 1,150° C. to obtain a billet. Thereafter, the billet was reheated at 1,100° C. for 3 hours, and then the reheated billet was hot-rolled at a final finish rolling temperature of 900° C. with a roll of Ψ25 mm to prepare a low-carbon boron steel wire rod specimen. Thereafter, step cooling over three steps of CR1-CR2-CR3 was applied under the conditions shown in Table 2 below, and the microstructure and tensile strength of the cooled wire rod were measured and shown in Table 2 below.

In addition, the specimen of the cooled wire rod was subjected to Q/T heat treatment, and the physical properties of the Q/T heat treatment are shown in Table 3 below. Q/T heat treatment was reheated at 900°C for 60 minutes and tempered at 400-600°C for 90 minutes. Thereafter, tensile properties and impact toughness were evaluated for each specimen in the same strength class and are shown in Table 3 below.

steel grade Alloy composition (wt%) C Si Mn P S Cr Al Nb V Ti B N note Invention lecture 1 0.20 0.15 0.45 0.010 0.0050 1.14 0.024 - 0.112 0.022 0.0018 0.0042 Invention Example 1 Invention lecture 2 0.23 0.25 0.42 0.012 0.0063 0.92 0.028 0.005 0.096 0.024 0.0020 0.0045 Invention Example 2 Invention lecture 3 0.26 0.24 0.36 0.011 0.0057 0.78 0.032 - 0.075 0.028 0.0022 0.0053 Invention example 3 Invention lecture 4 0.30 0.30 0.27 0.009 0.0044 0.55 0.035 - 0.058 0.019 0.0023 0.0040 Invention Example 4 Comparative lecture 1 0.18 0.17 0.62 0.012 0.0062 0.20 0.040 - - 0.025 0.0021 0.0042 Comparative Example 1 Comparative lecture 2 0.22 0.26 0.86 0.010 0.0051 0.17 0.035 - - 0.021 0.0017 0.0053 Comparative Example 2 Comparative lecture 3 0.25 0.21 1.08 0.011 0.0048 0.15 0.028 - - 0.023 0.0016 0.0056 Comparative Example 3 Comparative lecture 4 0.28 0.26 0.48 0.010 0.0054 0.94 0.032 - 0.093 0.015 0.0020 0.0044 comparative example
4 Comparative steel 5 0.29 0.33 1.32 0.011 0.0046 0.10 0.031 0.010 0.105 0.018 0.0024 0.0047 Comparative Example 5 Comparative lecture 6 0.33 0.20 0.48 0.010 0.0052 0.67 0.033 - - 0.021 0.0025 0.0041 Comparative Example 6 where Ceq=[C]+[Si]/9+[Mn]/5+[Cr]/12,
Each of [C], [Si], [Mn] and [Cr] means the content (wt%) of the corresponding element

steel grade cooling rate
(℃/s) microstructure
type Ceq Mn+Cr Cr/Mn Mn/Cr
(segregation part) wire rod
ts 10% fresh TS 6mm
Jomini hardness value note CR1 CR2 CR3 Invention lecture 1 5.5 2.1 0.6 F+P 0.403 1.59 2.533 0.27 585 652 40 Invention Example 1 Invention lecture 2 6.2 1.4 0.7 F+P 0.418 1.34 2.190 0.36 612 687 43 Invention Example 2 Invention lecture 3 4.3 1.2 0.5 F+P 0.424 1.14 2.167 0.41 624 696 46 Invention example 3 Invention lecture 4 3.1 1.1 0.6 F+P 0.433 0.92 2.407 0.38 633 705 47 Invention Example 4 Comparative lecture 1 8.5 3.3 1.2 F+P 0.340 0.82 0.323 3.55 552 638 33 Comparative Example 1 Comparative lecture 2 3.8 1.9 1.1 F+P 0.439 1.05 0.193 5.41 604 693 44 Comparative Example 2 Comparative lecture 3 4.1 2.7 1.6 F+P 0.478 1.11 0.156 6.78 615 708 45 Comparative Example 3 Comparative lecture 4 3.9 3.2 2.1 F+P 0.483 1.42 1.958 0.76 653 749 46 Comparative Example 4 Comparative steel 5 1.4 1.2 0.9 F+P 0.545 1.15 0.095 11.20 676 768 48 Comparative Example 5 Comparative lecture 6 2.6 1.5 0.8 F+P 0.504 1.15 1.396 0.87 643 729 50 Comparative Example 6

steel grade TS(MPa) El. (%) RA (%) Hardness
(HV) impact toughness
(Room temperature, J/cm2) note Invention lecture 1 1031 18.8 73.5 304 162 Invention Example 1 Invention lecture 2 1046 18.4 72.3 315 157 Invention Example 2 Invention lecture 3 1044 17.7 71.4 312 153 Invention example 3 Invention lecture 4 1053 17.2 70.7 318 144 Invention Example 4 Comparative lecture 1 - - - - - Comparative Example 1 Comparative lecture 2 1034 13.5 64 308 82 Comparative Example 2 Comparative lecture 3 1056 12.9 62 321 77 Comparative Example 3 Comparative lecture 4 1024 12.7 63 325 92 Comparative Example 4 Comparative steel 5 1048 14.6 66 317 97 Comparative Example 5 Comparative lecture 6 1052 11.8 59 316 68 Comparative Example 6

As can be seen from Tables 2 and 3, in the case of Inventive Examples 1 to 4 that satisfy the alloy composition and manufacturing conditions of the present invention, the tensile strength of the wire rod of 650 MPa or less, the tensile strength after wire drawing of 720 MPa or less, and 6mm Jomini hardness of 40 HRC or more All values were satisfied, and it was confirmed that the softening resistance was excellent.

On the other hand, in the case of Comparative Examples 1 to 5, which did not satisfy at least one or more of the conditions suggested by the present invention, any one of softening resistance, tensile strength after drawing, and hardenability was inferior to that of the invention example.

Specifically, Comparative Example 1 does not satisfy both C _eq and Relations 1 to 3 of the alloy composition, and does not satisfy the second cooling rate and the third cooling rate, so that the tensile strength after drawing is within the scope of the present invention, It was confirmed that the mini hardness value was insufficient and the hardenability was inferior.

Comparative Examples 2 and 3 satisfy C _eq but do not satisfy Relations 1 and 3, and do not satisfy the third cooling rate. It was confirmed that it was inferior to this invention example. Comparative Example 4 satisfies all components, but does not satisfy Relations 1 and 3, and does not satisfy the first and third cooling rates. it can be seen that

Comparative Examples 5 and 6 did not satisfy both C _eq and Relations 1 and 3, and did not satisfy the first cooling rate, so it was confirmed that the tensile strength after drawing was exceeded and the Q/T heat treatment properties were inferior to the invention steel. .

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art may not depart from the concept and scope of the following claims. It will be appreciated that various modifications and variations are possible.

Claims (13)

By weight%, C: 0.2 to 0.3%, Si: more than 0 0.8% or less, Mn: more than 0 0.5% or less, Cr: 0.5 to 1.5%, P: 0.03% or less, S: 0.03% or less, sol.Al: More than 0 0.07% or less, V: 0.02 to 0.5%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0.01% or less, the remainder Fe and unavoidable impurities,
It contains ferrite and pearlite as a microstructure,
A low-carbon boron steel wire with improved hardenability and softening resistance, satisfying the following Relations 1 to 3, and having a carbon equivalent (C _eq ) defined by the following Formula (1) of 0.35 to 0.50.
[Relational Expression 1] Cr/Mn ≥ 2.0
[Relational Expression 2] 0.9 ≤ Mn+Cr ≤ 1.6
[Relational Expression 3] Segregation zone Mn/Cr ≤ 0.6
(1) C _eq = [C] + [Si]/9 + [Mn]/5 + [Cr]/12
(Here, each of [C], [Si], [Mn] and [Cr] means the weight % of the corresponding element) delete delete The method of claim 1,
The tensile strength of the wire rod is,
Low carbon boron steel wire with improved hardenability and softening resistance of 650 MPa or less. The method of claim 1,
The wire is
Low-carbon boron steel wire with improved hardenability and softening resistance with a Rockwell hardness of 40 HRc or higher at 6.0 mm in the Jominy test. The method of claim 1,
The tensile strength of the wire rod after drawing is,
Low carbon boron steel wire with improved hardenability and softening resistance of 720 MPa or less. delete The method of claim 1,
Low-carbon boron steel wire with improved hardenability and softening resistance that further contains Nb: 0.1% or less (excluding 0). By weight%, C: 0.2 to 0.3%, Si: more than 0 0.8% or less, Mn: more than 0 0.5% or less, Cr: 0.5 to 1.5%, P: 0.03% or less, S: 0.03% or less, sol.Al: More than 0 and less than or equal to 0.07%, V: 0.02 to 0.5%, Ti: 0.005 to 0.05%, B: 0.0005 to 0.005%, N: 0.01% or less, remaining Fe and unavoidable impurities, and satisfying the following Relations 1 to 3, Reheating the billet having a carbon equivalent (C _eq ) of 0.35 to 0.50 defined by the following formula (1);
final finishing rolling the reheated billet at a temperature range of 880°C to 1050°C;
a first cooling step of cooling from the final finishing rolling temperature to 850°C at an average cooling rate of 3°C/s or more;
a second cooling step of cooling from 850°C to 750°C at an average cooling rate of 1 to 3°C/s after the first cooling; and
After the second cooling, a third cooling step of cooling from 750°C to 600°C at an average cooling rate of less than 1°C/s;
[Relational Expression 1] Cr/Mn ≥ 2.0
[Relational Expression 2] 0.9 ≤ Mn+Cr ≤ 1.6
[Relational Expression 3] Segregation zone Mn/Cr ≤ 0.6
(1) Ceq = [C] + [Si]/9 + [Mn]/5 + [Cr]/12
(Here, each of [C], [Si], [Mn] and [Cr] means the weight % of the corresponding element) delete delete delete 10. The method of claim 9,
The billet is a method of manufacturing a low-carbon boron steel wire rod with improved quenchability and softening resistance further comprising Nb: 0.1% or less (excluding 0).