KR20160031644A - Medium carbon steel wire rod and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a medium carbon steel wire rod widely used as a material for high strength bolts and nuts, and more particularly to a medium carbon steel wire rod having excellent tensile strength and impact toughness, and a method for manufacturing the same.
An aspect of the present invention is a steel sheet comprising, by weight%, 0.3 to 0.45% of C, 0.005 to 0.4% of Si, 0.5 to 1.5% of Mn, 0.03% or less of P, 0.005 to 0.05%, B: 0.0005 to 0.005%, and N: 0.001 to 0.01%, and the balance of Fe and other unavoidable impurities, wherein the carbon equivalent (Ceq) satisfies the relationship of 0.6? Ceq? A medium carbon steel wire having a tensile strength of 1000 MPa or more and a method for producing the same.
Ceq (%) = C + Mn / 5 + Cr / 9
(Here, C, Mn and Cr each means% by weight).
INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a medium carbon steel wire rod having excellent tensile strength and impact toughness at a low cost without adding Mo by controlling an alloy component and a quenching-annealing heat treatment condition.

Description

TECHNICAL FIELD [0001] The present invention relates to a carbon steel wire rod and a method of manufacturing the carbon steel wire rod.

The present invention relates to a medium carbon steel wire rod widely used as a material for high strength bolts and nuts, and more particularly, to a medium carbon steel wire rod having excellent tensile strength and impact toughness and a method of manufacturing the same.

Generally, high strength bolts and nuts with tensile strength of 90kgf / mm ² or more used for fastening components in automobiles and industrial machines are widely used as SCM435 / 440, a medium carbon alloy steel wire. The SCM 435/440 material contains a large amount of large incombustible elements such as Cr and Mo.

For the high strength bolt material, the greatest effect of Mo addition is high temperature stability at the time of tempering heat treatment. In general, the high temperature safety is the tempering. When the temperature is increased, the strength drops sharply. When the molybdenum is added, the strength is not lowered even at the high temperature tempering. Therefore, the material having high toughness and excellent toughness is manufactured It is possible.

However, in recent years, the prices of the hardenability improving elements have soared, and the manufacturing cost is very high. In particular, the price of Mo is ten times higher than the prices of Mn and Cr, which is a major cause of the increase in manufacturing cost. Therefore, in order to reduce the manufacturing cost, it is required to develop a replacement steel of SCM435 / 440 which is a Mo-abbreviated type of Mo, but when the Mo is simply omitted, the high temperature stability against the ingotability and tempering that can be obtained by adding Mo is very poor .

An aspect of the present invention is to provide a medium carbon steel wire rod having excellent tensile strength and impact toughness without adding Mo.

Another aspect of the present invention is to provide a method for producing a medium carbon steel wire rod having excellent tensile strength and impact toughness without adding Mo.

An aspect of the present invention provides a method for producing a steel sheet, comprising: 0.3 to 0.45% of C, 0.005 to 0.4% of Si, 0.5 to 2.0% of Mn, 0.03% or less of P, 0.03% or less of S, 0.005 to 0.05% of Ti, 0.0005 to 0.005% of B, 0.001 to 0.01% of N, the balance of Fe and other inevitable impurities, and satisfying the relationship of 0.6? Ceq? 0.85 , And a tensile strength of 1000 MPa or more.

Ceq (%) = C + Mn / 5 + Cr / 9

(Where C, Mn and Cr mean the weight% of each of these components).

In another aspect of the present invention, there is provided a method of producing a carbon steel wire rod by cold pressing followed by heat treatment, the method comprising the steps of: 0.3 to 0.45% of C, 0.005 to 0.4% of Si, 0.5 to 2.0% of Mn, 0.03% or less, S: not more than 0.03%, Cr: 0.1 to 1.5%, Ti: 0.005 to 0.05%, B: 0.0005-0.005%, N: 0.001 to 0.01%, and the balance Fe and other unavoidable impurities , And a carbon equivalent (Ceq) satisfies a relationship of 0.6? Ceq? 0.85, is annealed at a temperature of 800 to 900 占 폚 and then cooled; And a pest heat treatment step of subjecting the pavement to heat treatment at a searing road satisfying the following formula (1) after the quenching heat treatment.

Ceq (%) = C + Mn / 5 + Cr / 9

(Where C, Mn and Cr mean the weight% of each of these components).

 [Formula (1)

825-0.31xA? Tt? 825-0.27xA

(In the above formula (1), A represents the tensile strength after the quenching and blanket heat treatment, and Tt represents the blanket heat treatment temperature (占 폚) corresponding to A.)

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a medium carbon steel wire rod having excellent tensile strength and impact toughness at a low cost without adding Mo by controlling an alloy component and a quenching-annealing heat treatment condition.

The inventors of the present invention conducted researches and experiments to solve the problem that physical properties were lost when Mo was omitted. As a result, it was found that when the content of Mn and Cr in the carbon steel wire was appropriately controlled and the heat treatment conditions for quenching and plowing were controlled, Tensile strength and impact toughness, and the present invention has been made based on the results of these studies.

A carbon steel wire rod which is one aspect of the present invention will be described in detail.

C (carbon): 0.3 to 0.45 wt%

When the C content is less than 0.3% by weight, the strength required by the final product can not be obtained. When the C content is more than 0.45% by weight, the strength after crushing and sintering becomes higher than necessary. Therefore, the content of C is preferably limited to 0.3 to 0.45%.

Si (silicon): 0.005 to 0.4 wt%

Si is added for deoxidation and necessary strength. If the content of Si is less than 0.005 wt%, such effect is not exhibited. If it exceeds 0.4 wt%, the activity of carbon is increased to promote surface decarburization, which may cause surface decarburization during the cooling process of the wire rod Therefore, the Si content is preferably limited to 0.005 to 0.4 wt%.

Mn (manganese): 0.5 to 2.0 wt%

Mn is an element necessary for deoxidation of steel. In order to improve the incombustibility and strength, it is preferable to add the Mn content by 0.5 wt% or more. However, when Mn is added in an amount of 2.0 wt%, it is more harmful to the product characteristics . Accordingly, the content of Mn is preferably limited to 0.5 to 2.0% by weight.

Cr (chrome): 0.1 to 1.5 wt%

Cr is added in order to secure the hardenability and strength at the time of heat treatment. If the content of Cr is less than 0.1% by weight, such effect is difficult to exhibit. If the content of Cr is more than 1.5% by weight, cracking is easy to occur in the quenching treatment and the strength is excessively increased. Therefore, the content of Cr is preferably limited to 0.1 to 1.5% by weight.

Ti (titanium): 0.005 to 0.05 wt%

Ti bonds with nitrogen in the steel to form titanium nitride (TIN). The nitride is very stable at high temperatures and is formed in the austenite grain boundaries to inhibit the growth of austenite grains and to refine the structure. The transformation of ferrite and pearlite, which are soft tissues, is promoted during cooling by the micronized austenite structure, so that the effect of softening the wire rod is obtained. If the content of Ti is less than 0.005% by weight, such an effect can not be exhibited. If it exceeds 0.05% by weight, coarse titanium nitride is precipitated excessively and toughness is decreased. Therefore, the content of Ti is preferably limited to 0.005 to 0.05 wt%.

B (boron): 0.0005 to 0.005 wt%

B is a grain strengthening element for improvement of incombustibility and delayed fracture resistance. When the content of B is less than 0.0005% by weight, the boron atoms in the heat treatment are insufficient in improving the grain boundary strengthening effect and the entanglement improving effect due to grain boundary segregation. When the content is more than 0.005% by weight, boron carbide precipitates in the grain boundaries, do. Therefore, the content of B is preferably limited to 0.0005 to 0.005 weight.

N (nitrogen): 0.001-0.01 wt%

N, as mentioned above, combines with titanium to form a nitride and serves to refine austenite. If the content of N is less than 0.001 wt%, it is difficult to exhibit such an effect. If the N content is more than 0.01 wt%, coarse titanium nitride is excessively produced, and the toughness of the steel is reduced. Therefore, the content of N is preferably limited to 0.001 to 0.01 wt%.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities can be known by anyone skilled in the art of manufacturing, and therefore, not all of the details are specifically mentioned in this specification.

However, in general, impurities to be mentioned more frequently will be briefly described as follows.

P (phosphorus): not more than 0.03% by weight

P is an element which is inevitably contained at the time of manufacture, and is segregated in the grain boundaries, which is a main cause of lowering toughness and reducing delayed fracture resistance. Therefore, it is preferable to limit the upper limit of P to 0.03 wt%.

S (sulfur): not more than 0.03% by weight

S is an element which is inevitably contained at the time of manufacture and is segregated in the grain boundaries in the steel to greatly decrease toughness and form emulsions, which adversely affect the delayed fracture resistance and stress relaxation characteristics. Therefore, it is preferable to limit the upper limit of S to 0.03 wt%.

O (oxygen): 0.005 wt% or less

It is preferable that oxygen is not contained, but when it is inevitably contained, its content is preferably 0.005% or less. If the content exceeds 0.005%, the fatigue life due to oxide-based nonmetal inclusions may be reduced.

According to an aspect of the present invention, it is possible to provide a medium carbon steel wire rod having excellent tensile strength and impact toughness by satisfying the component system. In order to further improve the effect of the present invention, one or two selected from the group consisting of Nb and V may be further included.

Nb (niobium): 0.01 to 0.05 wt%

Nb is added for the purpose of improving delayed fracture resistance and softening resistance. When the content of Nb is less than 0.01% by weight, the effect of delayed fracture resistance and softening resistance is less expected as the distribution of precipitates is decreased. When the content of Nb is more than 0.05% by weight, coarse carbonitrides So that it acts like a nonmetallic inclusion, resulting in a decrease in fatigue characteristics. Therefore, the content of Nb is preferably limited to 0.01 to 0.05% by weight.

V (vanadium): 0.05 to 0.2 wt%

V is an element that acts like Nb and is added for the purpose of improving delayed fracture resistance and softening resistance. When the content of V is less than 0.05% by weight, the effect of delayed fracture resistance and softening resistance is less expected as the distribution of precipitates is decreased. When the content of V is more than 0.2% by weight, coarse carbonitrides So that it acts like a nonmetallic inclusion, resulting in a decrease in fatigue characteristics. Therefore, the content of V is preferably limited to 0.05 to 0.2% by weight.

The carbon equivalent (Ceq) preferably satisfies the relationship of 0.6? Ceq? 0.85. The carbon equivalent should be not less than 0.6 in order to have the incombustibility (90% martensite hardness depth) at the level of SCM435 / 440 and not more than 0.85 in order to avoid center segregation of C, Mn and Cr. Therefore, it is preferable that the carbon equivalent is limited to 0.6? Ceq? 0.85.

At this time, the carbon equivalent is as follows.

Ceq (%) = C + Mn / 5 + Cr / 9

Here, C, Mn and Cr mean the weight percent of each of these components.

The wire rod may have a tensile strength of 1000 MPa or more after the quench-pore heat treatment.

Further, the wire rod may have a 90% martensite hardness depth (fine depth) of 13 mm or more after the quenching heat treatment.

A method of manufacturing a medium carbon steel wire rod according to another aspect of the present invention will be described in detail.

A method of manufacturing a medium carbon steel wire rod according to one aspect of the present invention comprises the steps of: 0.3 to 0.45% of C, 0.005 to 0.4% of Si, 0.5 to 0.5% of Mn, Of Ti, 0.005 to 0.005% of B, 0.001 to 0.01% of N and 0.001 to 0.01% of N, and the balance of Fe and Cr in an amount of 0.1 to 1.5%, P of 0.03% or less, S of 0.03% or less, Cr of 0.1 to 1.5% And other unavoidable impurities, and cooling the medium carbon steel wire rod having a carbon equivalent (Ceq) satisfying the relationship of 0.6? Ceq? 0.85 after the heat treatment at a temperature of 800 to 900 占 폚; And a soaking heat treatment step of performing a soaking heat treatment on the searched road satisfying the following formula (1) after the quenching heat treatment.

Ceq (%) = C + Mn / 5 + Cr / 9

(Where C, Mn and Cr mean the weight% of each of these components).

[Formula (1)

825-0.31xA? Tt? 825-0.27xA

(In the above formula (1), A represents the tensile strength after the quenching and blanket heat treatment, and Tt represents the blanket heat treatment temperature (占 폚) corresponding to A.)

The cold-pressing (cold working) of the above medium carbon steel wire rod is a step of forging according to the shape of the final product at room temperature and is carried out under commonly used conditions, and the manufacturing conditions may be changed according to the shape to be manufactured.

Submission  Heat treatment

The quenching heat treatment temperature is preferably limited to 800 to 900 ° C. When the quenching heat treatment temperature is higher than 900 ° C, the austenite grain size sharply increases and the delayed fracture resistance becomes lower. When the quenching heat treatment temperature is lower than 800 ° C, the retained austenite increases in the material after quenching This is because the strength is lowered. At this time, the heat treatment time and the cooling condition after the heat treatment are performed under ordinary conditions.

Heat treatment

In order to obtain the same level of strength and toughness as SCM435 / 440, it is necessary to control the bake treatment temperature. In the case of medium-carbon steels having a carbon equivalent (Ceq) satisfying the relationship of 0.6? Ceq? 0.85, the tempering heat treatment temperature is limited to the range of the following formula (1) to secure strength and toughness equivalent to those of SCM435 / 440 can do. When the value calculated by the following formula (1) is less than 825-0.31xA, there is a problem of deterioration of ductility and toughness, and when it exceeds 825-0.27xA, the strength is lowered. Therefore, in the case of medium carbon steels satisfying the carbon equivalent (Ceq) of not less than 0.6 and not more than 0.85, it is preferable to limit the tempering heat treatment temperature to the range of the following formula (1).

[Formula (1)

825-0.31xA? Tt? 825-0.27xA

Here, A represents the tensile strength after the quenching and blanket heat treatment, and Tt represents the blanket heat treatment temperature (占 폚) corresponding to A

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the present invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

[ Example ]

A comparative steel having the composition shown in Table 1 below was manufactured as a small ingot (160 mm x 160 mm x 250 mm), heated at 1200 ° C for 6 hours, and then hot rolled to obtain a hot rolled steel sheet having a thickness of 20 mm. In the following Table 1, Inventive Examples 1 and 2 show SCM435 alternative steel, Inventive Examples 3 and 4 show SCM440 alternative steel components, Comparative Example 1 shows the SCM435 component system, and Comparative Example 2 shows the SCM440 component system.

The hot-rolled steel sheets were used as test specimens to fabricate mini, tensile, and impact specimens. In the mini-test, 90% martensite hardness depth (depth of penetration) was measured after water quenching to room temperature at 900 ° C. The fineness depth means a depth at which martensite is generated from the surface, that is, a depth having a certain degree of martensite hardness. Thus, in the present invention, the depth of penetration was measured with 90% martensite hardness depth, which is the most frequently used index, as an index.

Tensile and impact tests were carried out at a quenching temperature and a bending temperature as shown in Table 2, and tensile strength and impact value were measured at room temperature after heat treatment. In this case, SCM435 alternate grade steel sheets 1 and 2 satisfy the above formula (1) and SCM440 steel grade steel sheets 3 and 4 satisfy the above formula (2). Table 2 shows the tensile strength and impact value according to the ingotability, the quenching and the heat treatment conditions of the inventive steel and the comparative steel.

weight (%) C Si Mn P S Cr Mo B Ti N Inventory 1 0.35 0.25 1.0 0.01 0.006 0.9 - 0.002 0.03 0.0050 Inventory 2 0.35 0.25 1.2 0.01 0.007 0.7 - 0.0022 0.03 0.0048 Inventory 3 0.37 0.25 1.3 0.01 0.007 0.9 - 0.0019 0.03 0.0048 Honorable 4 0.37 0.25 1.5 0.01 0.007 0.7 - 0.0020 0.03 0.0048 Comparative Example 1 0.35 0.25 0.75 0.01 0.007 1.0 0.2 - 0.0048 Comparative Example 2 0.4 0.25 0.75 0.01 0.008 1.0 0.2 - 0.0050

Ceq 90% martensite hardness depth
(Depth of cut) (mm) The tensile strength
(MPa) Impact toughness
(J) Quenching temperature
(° C) Sorrow temperature
(° C) Inventory 1 0.65 16 1056 58 880 510 Inventory 2 0.66 17 1045 62 880 515 Inventory 3 0.73 23 1142 52 880 485 Honorable 4 0.74 25 1155 58 880 490 Comparative Example 1 0.61 16 1050 57 880 550 Comparative Example 2 0.66 24 1152 47 880 540

In Table 2, Inventive Examples 1 and 2 show that SCM435 is an alternative steel having an equivalent level of fillability, tensile strength and impact toughness as compared with Comparative Example 1, which is a SCM435 steel. In addition, Inventive Examples 3 and 4 are SCM440 alternate grades and have comparable levels of ingotability, tensile strength and impact toughness as compared with Comparative Example 2, which is an SCM440 grader.

Ceq in Table 2 means the carbon equivalent. The carbon equivalent of 0.6 to 0.85 is satisfied so that the ingot of the same level as SCM435 / 440 is ensured and the annealing temperature is limited to the formula (1) 440 strength and toughness at the same level can be secured.

The addition of boron in Inventive Examples 1 to 4 is intended to fully exploit the effect of improving the hardenability of boron (B) by intergranular segregation during quenching heat treatment. The conventional low carbon boron steel has a 90% martensite hardness depth of less than 10 mm, which is far below that of SCM435. On the other hand, as shown in Tables 1 and 2, in Examples 1 to 4, Mn and Cr were properly added to heavy carbon-based boron steel to attain SCM435 / 440 entrapment property in the quenching heat treatment, By controlling the annealing temperature after the quenching heat treatment, it can be seen that medium carbon boron steel having the same strength and toughness as SCM435 / 440 can be developed.

While the illustrative embodiments of the present invention have been shown and described, various modifications and alternative embodiments may be made by those skilled in the art. Such variations and other embodiments will be considered and included in the appended claims, all without departing from the true spirit and scope of the invention.

Claims (5)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.3 to 0.45% of C, 0.005 to 0.4% of Si, 0.5 to 2.0% of Mn, 0.03% or less of P, 0.03% or less of S, 0.1 to 1.5% of Cr, 0.005 to 0.05% B: 0.0005-0.005%, N: 0.001-0.01%, the balance Fe and other unavoidable impurities, the carbon equivalent (Ceq) satisfies the relationship of 0.6? Ceq? 0.85, and the tensile strength is 1000 MPa or more Carbon steel wire rod.
Ceq (%) = C + Mn / 5 + Cr / 9
(Where C, Mn and Cr mean the weight% of each of these components).
The medium carbon steel wire rod according to claim 1, wherein the wire rod has a 90% martensite hardness depth of 13 mm or more.
The medium carbon steel wire rod according to claim 1 or 2, further comprising one or two selected from the group consisting of 0.01-0.05% Nb and 0.05-0.2% V, by weight.
A method for producing a carbon steel wire rod by cold pressing followed by heat treatment,
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.3 to 0.45% of C, 0.005 to 0.4% of Si, 0.5 to 2.0% of Mn, 0.03% or less of P, 0.03% or less of S, 0.1 to 1.5% of Cr, 0.005 to 0.05% B: from 0.0005 to 0.005%, N: from 0.001 to 0.01%, the balance Fe and other unavoidable impurities, and satisfying the relationship of the carbon equivalent (Ceq) 0.6? Ceq? Lt; RTI ID = 0.0 > C < / RTI > And
And a soaking heat treatment step of performing a soaking heat treatment at a searing road satisfying the following formula (1) after the quenching heat treatment.
Ceq (%) = C + Mn / 5 + Cr / 9
(Where C, Mn and Cr mean the weight% of each of these components).
[Formula (1)
825-0.31xA? Tt? 825-0.27xA
(In the above formula (1), A represents the tensile strength after the quenching and blanket heat treatment, and Tt represents the blanket heat treatment temperature (占 폚) corresponding to A.)
The method for producing a medium carbon steel wire rod according to claim 4, further comprising one or two selected from the group consisting of 0.01-0.05% Nb and 0.05-0.2% V, by weight.