KR101053305B1 - Low decarburized wire and its manufacturing method - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low decarburized wire rod and a method for manufacturing the same. The present invention relates to a wire rod and a method for manufacturing the wire rod, which are uniformly maintained at high temperature and then quenched to prevent decarburization and suppress low temperature structure.

The present invention is in weight%, C: 0.2-0.8%, Si: 1.5-3.0%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0%, Cu: 0.01-1.0%, B : 0.005 ~ 0.02%, Al: 0.1% or less, O (oxygen): 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, V: 0.005 ~ 0.5% and Ti: 0.005 ~ One or both of 0.5% provides a low decarburized wire rod containing the balance Fe and other unavoidable impurities.

Further, after rolling the steel strip having the composition, the wire is maintained at a temperature of 800 ° C. or more for 40 to 50 seconds, and the wire is maintained for 40 to 50 seconds from the end of the maintenance to 50 to 60 seconds after 5 ° C./s or more. It provides a method for producing a low decarburized wire, comprising the step of quenching at a cooling rate.

 Decarburization, Pearlite, Low Temperature Structure, Cold Workability

Description

Low decarburized wire and its manufacturing method {STEEL FOR LOW DECARBURIZATION AND MANUFACTURING METHOD OF THE SAME}

The present invention relates to a low decarburized wire and a method for manufacturing the same, and more particularly, to a low decarburized wire and a method for manufacturing the same to minimize the decarburization and low-temperature structure.

In recent years, car manufacturers are trying to manufacture a lighter body by increasing the strength of the components constituting the vehicle to improve fuel efficiency. Wire rod products are manufactured in various forms such as CHQ, bearing steel and spring steel, and are used as automotive parts. The higher strength of the wire rod product can have a greater impact on the compactness of each part through the change of design concept such as engine block and steering device, rather than the effect of direct weight reduction of the parts themselves. Therefore, for the miniaturization of such parts, the technology of the material itself and the flawlessness of the surface state of products are important.

Since decarburization, one of the representative surface defects, is determined by the diffusion reaction, it is important to control the conditions essential for the carbon diffusion reaction of the steel surface. Therefore, many studies have been made in terms of being able to control at least one or more of the factors of temperature, time and concentration gradient required for the diffusion of carbon atoms.

So far, control cooling has been introduced as an approach to solve this problem. In order to avoid ferrite generation in CCT (continuous cooling transformation) and to quickly pass through the region of initiation and termination of pearlite phase transformation, it was mainly studied to suppress low temperature structure and surface decarburization.

However, as the strength of the material increases, the ferrite region on the CCT is expanded or the pearlite phase transformation region is reduced due to the addition of various alloying elements, and the temperature deviation in the material is increased. And there was a problem in controlling the surface decarburization completely.

As described above, a method for effectively controlling low temperature tissue and surface decarburization at the same time has not been proposed yet.

The present invention is to solve the above-mentioned problems of the prior art, according to one aspect of the present invention, to provide a high-strength low-decarburized wire rod and a method of manufacturing the minimized decarburization and low-temperature structure.

Low carbonaceous wire for solving the problems of the present invention is by weight%, C: 0.2 ~ 0.8%, Si: 1.5 ~ 3.0%, Mn: 0.3 ~ 1.0%, Cr: 0.01 ~ 1.5%, Ni: 0.01 ~ 1.0%, Cu: 0.01 to 1.0%, B: 0.005 to 0.02%, Al: 0.1% or less, O (oxygen): 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, One or both of V: 0.005 to 0.5% and Ti: 0.005 to 0.5% are characterized by having a composition containing the balance Fe and other unavoidable impurities.

The wire is preferably an internal structure of the pearlite containing an area fraction of more than 90% and the balance is made of ferrite. Moreover, it is preferable that the sum of the area fractions of bainite and martensite structure is made 1% or less.

In addition, the wire rod is preferably a maximum depth of the ferrite decarburization layer is 30㎛ or less.

Another aspect of the present invention is a method for producing a low decarburized wire, in weight%, C: 0.2-0.8%, Si: 1.5-3.0%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01 -1.0%, Cu: 0.01-1.0%, B: 0.005-0.02%, Al: 0.1% or less, O (oxygen): 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less , V: 0.005 ~ 0.5% and Ti: 0.005 ~ 0.5%, one or both of them, after the wire rods containing the remaining Fe and other unavoidable impurities, the wire is maintained for 40 to 50 seconds at a temperature above 800 ℃ And a step of quenching the wire rod maintained for 40 to 50 seconds from the end of the maintenance to 50 to 60 seconds at a cooling rate of 5 ° C./s or more.

In the case of manufacturing the high strength wire rod according to the present invention, low-temperature structure and surface decarburization can be effectively suppressed at the same time, so that processes such as softening heat treatment, decarburization and peeling can be omitted.

Hereinafter, the present invention will be described in detail.

The present invention provides a high strength low decarburized wire rod and a method of manufacturing the same to minimize decarburization and low temperature structure.

EMBODIMENT OF THE INVENTION Hereinafter, the component system of this invention is demonstrated in detail.

C: 0.2-0.8 wt%

C is an essential element added to secure the strength of the wire rod. If the content of C is less than 0.2% by weight, sufficient strength is not secured, and thus the strength required for the high strength wire rod cannot be secured. In addition, when the C content is more than 0.8% by weight, the cementitious cementite structure is formed and cracks occur in the material, thereby significantly reducing the fatigue strength. In addition, it is difficult to secure sufficient toughness due to the high strength and to suppress the decarburization of the material generated by the addition of high Si. Therefore, the C content is preferably limited to 0.2 to 0.8% by weight.

Si: 1.5-3.0 wt%

Si is dissolved in ferrite to strengthen the base material strength. If the Si content is less than 1.5% by weight, the effect of strengthening the base material strength is not sufficient because Si is dissolved in the ferrite. And if the Si content exceeds 3.0% by weight, the possibility of the central segregation is increased, and the C content is increased to 1.5 to 3.0% by weight since it promotes surface decarburization by increasing the activity of C during heat treatment. desirable.

Mn: 0.3 ~ 1.0 wt%

Mn is an element that is beneficial to secure strength by improving the hardenability of steel when present in steel. Therefore, when the Mn content is less than 0.3% by weight, it is difficult to obtain sufficient strength and hardenability required as a high strength wire rod, whereas when the Mn content exceeds 1.0% by weight, toughness is lowered. Therefore, the content of Mn is preferably limited to 0.3 to 1.0% by weight.

Cr: 0.01 ~ 1.5 wt%

Cr is an element useful for preventing surface decarburization, oxidation resistance, temper softening and hardening. However, when the Cr content is less than 0.01% by weight, it is difficult to secure sufficient oxidation resistance, temper softening property, surface decarburization and quenching effect. On the other hand, when the content exceeds 1.5% by weight, the deformation resistance may be lowered and the strength may be lowered. Therefore, the content of Cr is preferably limited to 0.01 to 1.5% by weight.

Ni: 0.01-1.0 wt%

Ni is an element which forms a scale layer with high adhesiveness with a base material, and suppresses surface decarburization efficiently. In addition, the toughness of steel can be improved efficiently. If the content of Ni is less than 0.01% by weight, the effect is not sufficient. If the content of Ni is more than 1.0% by weight, the amount of retained austenite is increased to reduce the fatigue life. do. Therefore, the content of Ni is preferably limited to 0.01 to 1% by weight.

Cu: 0.01 ~ 1.0 wt%

The addition of Cu is effective to prevent decarburization and to improve corrosion resistance by forming a scale layer having high adhesion with the base material together with Ni. This effect is weak at less than 0.01% by weight, and more than 1.0% by weight is likely to cause rolling defects due to embrittlement. Therefore, the content of Cu is preferably limited to 0.01 to 1.0% by weight.

B: 0.005 to 0.02 wt%

The addition of B has an effect of densifying rust generated on the surface, increasing corrosion resistance, and enhancing the strength of the grain boundary by improving hardenability. If less than 0.005% by weight, the hardenability is not secured and the strength required for the high strength wire rod cannot be secured. On the other hand, when it exceeds 0.02% by weight, the carbonitride-based precipitates are coarsened, which adversely affects the fatigue characteristics. Therefore, the content of B is preferably limited to 0.005 to 0.02% by weight.

O (oxygen): 0.0015% by weight or less

The content of O is limited to 0.0015% by weight or less. When it exceeds 0.0015% by weight, oxide-based nonmetallic inclusions are coarsened, and fatigue life is drastically reduced. Therefore, the content of O is preferably limited to 0.0015% by weight or less.

Al: 0.1 wt% or less

When the Al is added, the grain size is refined and the toughness is improved. When the Al content exceeds 0.1% by weight, the amount of oxide-based precipitates is increased and the size thereof is coarsened to adversely affect the fatigue characteristics. Therefore, the content of Al is preferably limited to 0.1% by weight or less.

P and S: 0.02% by weight or less

The content of P and S is limited to 0.02% by weight or less, respectively, because P is segregated at grain boundaries to lower toughness, so the upper limit thereof is limited to 0.01% by weight, and S is low-melting element, and the grain boundary is segregated to lower toughness. Since the formation of an emulsion has a detrimental effect on the spring properties, it is preferable to limit the upper limit to 0.02% by weight, respectively.

N: 0.02% by weight or less

N is an element that easily reacts with B to form BN and reduces the quenching effect. Therefore, the content of N is preferably as low as possible, but considering the process load is preferably limited to 0.02% by weight or less.

V: 0.005 to 0.5% by weight, Ti: 0.005 to 0.5% by weight

It is an element that improves the spring characteristics by forming carbon / nitride by single or multiple addition to cause precipitation hardening. When the content of V and Ti is less than 0.005% by weight, respectively, the precipitation of V and Ti-based carbon / nitride is reduced, and the effect of improving grain size control and spring characteristics (fatigue and permanent strain resistance) is not sufficient.

When the content exceeds 0.5% by weight, the manufacturing cost rises sharply, and the spring property improvement effect due to the precipitate is saturated, and the coarse alloy carbide which is not dissolved in the base metal during the austenite heat treatment increases, such as nonmetallic inclusions. Because of this action, fatigue properties and precipitation strengthening effects are lowered. It is preferable to limit the content in the range of 0.005 to 0.5% by weight, respectively.

Hereinafter, the method of manufacturing the low decarburization type wire rod which has a component system as mentioned above is demonstrated.

In order to manufacture spring steel with less decarburization, it is necessary to first control the cooling rate during cooling after rolling the steel having the component system in the above-described range. That is, during cooling, it is necessary to maintain the surface layer of steel without decarburization in the austenitic temperature range of 800 ° C. or more, and rapidly cool to start and end phase transformation to suppress low temperature structure and surface decarburization.

1. Maintain over 800 ℃ for 40 ~ 50 seconds after wire rod rolling

For 40-50 seconds after the wire is rolled, keep the wire above 800 ° C, the austenizing temperature range. Maintaining more than 800 ℃ as described above is to minimize the low-temperature tissue generated due to rapid cooling.

2. Rapid cooling of the wire rod maintained for 40 to 50 seconds from the end of the maintenance to 50 to 60 seconds at a cooling rate of 5 ℃ / s or more

After the wire is rolled, it is maintained at 800 ° C. or higher for 40 to 50 seconds, and when the cooling is performed for 50 to 60 seconds after maintaining the 40 to 50 seconds, the cooling rate is accelerated to quickly pass through the ferrite generation and growth zones, where decarburization occurs. Minimize time and pass through pearlite start and end points quickly. Therefore, the cooling rate needs to be higher than a certain level. When the wire rod is manufactured by the component system of the present invention, an appropriate cooling rate is preferably 5 ° C / sec or more.

Hereinafter, the internal structure of the wire rod provided in the process will be described.

1. Organizational fraction

In addition to the above component system, the wire rod preferably has an internal structure composed of ferrite and pearlite. This is because the cold workability is very poor when the internal structure of the steel is a low temperature structure such as martensite or bainite, which is not the above structure. Therefore, the pearlite contains a fraction of 90% or more and the balance includes ferrite.

In order to secure the cold workability of the wire rod, it is preferable not to include a low-temperature structure such as martensite or bainite, but when it is inevitably included by segregation, ferrite having an area fraction of 1% or less is one of the martensite and bainite structures. It is preferred to be replaced by two or two species.

2. Depth of Ferrite Decarburization

When the depth of ferrite decarburization exceeds 30 mu m, it is preferable to keep it at 30 mu m or less because it is difficult to remove the ferrite decarburization.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

After casting steels having a composition as shown in Table 1 and fabricating steel strips, wires were manufactured through various cooling conditions shown in Table 2 below. At this time, the initial temperature of the cooling head and the temperature after 50 seconds and 100 seconds were measured in the laying head, and the occurrence of surface ferrite decarburization of the final product steel was observed. In addition, by selecting any site from the prepared wire rod after cooling to room temperature, the low-temperature tissue fraction and the depth of the deepest ferrite decarburization were measured, and are shown in Table 2 below.

division C Si Mn Ni Cr V Ti Cu B P S Al N O Comparative Example 1 0.53 1.3 0.7 - 0.7 - - - - 0.01 0.03 0.001 50 16 Comparative Example 2 0.50 2.2 0.5 0.4 1.0 0.1 - - - 0.008 0.008 0.01 49 16 Comparative Example 3 0.55 2.2 0.7 0.4 1.0 0.2 0.07 0.4 0.002 0.009 0.007 0.06 55 14 Inventive Example 1 0.45 2.5 0.7 0.5 1.0 0.2 0.05 0.3 0.002 0.008 0.009 0.03 49 18 Inventive Example 2 0.48 2.2 0.7 0.4 1.2 0.14 0.04 0.4 0.003 0.012 0.008 0.02 51 19 Inventive Example 3 0.52 2.1 0.5 0.5 1.0 0.10 0.02 0.5 0.003 0.009 0.015 0.05 53 17 Honorable 4 0.53 2.4 0.7 0.3 0.7 0.13 0.05 0.4 0.005 0.015 0.009 0.06 52 17 Inventory 5 0.55 2.2 0.7 0.4 1.0 0.2 0.07 0.4 0.002 0.007 0.006 0.06 50 15

However, in Table 1, the content of each component means weight% (wherein N and O are in ppm).

division Cooling
Initial temperature
(℃) 50s cooling
Temperature (℃) Temperature after cooling 100s (℃) 50s ~ 100s
Average cooling rate between
(℃ / s) ferrite
Decarburized
depth
(Μm) Cold tissue
Fraction
(%) Comparative Example 1 930 460 280 3.6 0 15 Comparative Example 2 900 500 220 5.6 0 12 Comparative Example 3 880 820 600 4.4 45 0 Inventive Example 1 900 850 320 10.6 12 0 Inventive Example 2 930 880 300 11.6 0 0 Inventive Example 3 970 890 270 12.4 0 0 Honorable 4 880 800 250 11 0 0 Inventory 5 890 830 300 12.6 2 0

However, the low temperature tissue fraction in Table 2 means the area fraction.

Comparative Examples 1 and 2 were quenched for the first 50 seconds to inhibit ferrite nucleation and growth. Therefore, no ferrite decarburized layer was observed, but low temperature tissue fractions occurred 12% and 15%. In Comparative Example 3, the austenizing region was maintained for the first 50 seconds, but was slowly cooled to 50 to 100 seconds to be maintained in the ferrite region for a long time, so that ferrite decarburization occurred on the steel surface. Figure 2 (a) is a microstructure photograph of Comparative Example 1 can be seen that the low-temperature tissue fraction was generated about 15%. In addition, Figure 2 (c) is a microstructure photograph of Comparative Example 3 it can be seen that the decarburized layer was generated about 45㎛.

On the other hand, Inventive Examples 1 to 5 maintain the austenizing region of 800 ° C. or higher for the first 50 seconds, and then rapidly quench for 50 to 100 seconds, but the nucleation of ferrite is possible, but the growth of the pearlite is suppressed to the maximum. When the start and end were controlled at high speed, decarburization and low temperature tissue formation were effectively suppressed.

Figure 2 (b) is a microstructure photograph of the invention example 2 it can be seen that the low-temperature tissue was not produced. Figure 2 (d) is a microstructure photograph of Inventive Example 4 it can be seen that the decarburization layer was not produced.

1 is a graph showing a cooling pattern capable of suppressing low-temperature structure and decarburization by rapidly passing through the pearlite starting point and ending point after maintaining a temperature of 800 ° C. or higher;

Figure 2 (a) is a microstructure photograph of Comparative Example 1, (b) is a microstructure photograph of Example 2, (c) is a microstructure photograph of Comparative Example 3, (d) is a microstructure photograph of Example 4 .

Claims (5)

By weight%, C: 0.2-0.8%, Si: 1.5-3.0%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0%, Cu: 0.01-1.0%, B: 0.005- 0.02%, Al: 0.1% or less, O (oxygen): 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, additionally V: 0.005-0.5% and Ti: A low decarburized wire, comprising one or both of 0.005 to 0.5%, consisting of the balance Fe and other unavoidable impurities, and the maximum depth of the ferrite decarburized layer is 30 μm or less. The method of claim 1, The internal structure of the wire rod is a low-decarburized wire rod, characterized in that the pearlite is contained in an area fraction of more than 90% and the remainder is made of ferrite. 3. The method of claim 2, A low-decarburized wire rod, wherein the ferrite having an area fraction of 1% or less is replaced by one or two of bainite and martensite structures. delete By weight%, C: 0.2-0.8%, Si: 1.5-3.0%, Mn: 0.3-1.0%, Cr: 0.01-1.5%, Ni: 0.01-1.0%, Cu: 0.01-1.0%, B: 0.005- 0.02%, Al: 0.1% or less, O (oxygen): 0.0015% or less, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less, additionally V: 0.005-0.5% and Ti: Maintaining the wire at a temperature of 800 ° C. or higher for 40 to 50 seconds after rolling the wire rod including one or both of 0.005 to 0.5% and containing the remaining Fe and other unavoidable impurities; Quenching the wire rod maintained for 40 to 50 seconds from the end of maintenance to 50 to 60 seconds at a cooling rate of 5 ° C./s or more. Method for producing a low decarburized wire, characterized in that it comprises a.

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