KR101518571B1

KR101518571B1 - Non heat treated wire rod having excellent high strength and impact toughness and method for manafacturing the same

Info

Publication number: KR101518571B1
Application number: KR1020130100456A
Authority: KR
Inventors: 이재승
Original assignee: 주식회사 포스코
Priority date: 2013-08-23
Filing date: 2013-08-23
Publication date: 2015-05-07
Also published as: KR20150022492A

Abstract

More particularly, the present invention relates to a non-flattened wire having high strength and toughness, and a method of manufacturing the same.
To this end, the present invention provides a non-flattened wire which can secure high strength and high toughness without being subjected to a softening heat treatment and a method of manufacturing the same.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength and high toughness non-cored wire and a method of manufacturing the same. More particularly,

More particularly, the present invention relates to a non-flattened wire having high strength and toughness, and a method of manufacturing the same.

Most of the structural steels used for mechanical structural parts and automobile parts are subjected to reheating, quenching and briquetting processes after hot working, and quenching and tempering steel, which has increased strength and toughness, is used.

On the other hand, unlike crude steel, Non-Heat Treated Steel refers to steel that can obtain toughness and strength similar to those of heat treated (tempered steel) without heat treatment after hot working. Alloy steel is also called Micro-Alloyed Steel.

Conventional wire products are produced through a process of [hot rolling - cold drawing - spheroidizing heat treatment - cold drawing - cold pressing - quenching - sieve], while non-tempering wire products [hot rolling - cold drawing - cold pressing ], The final product is produced.

As described above, the non-flattened wire material is a product having excellent economical efficiency by lowering the manufacturing cost of the material by omitting the heat treatment process involved in the manufacturing of the existing crude wire material. In addition, since the final quenching and sintering are not performed, defects due to heat treatment And thus it is applied to many products.

However, since the non-heat-treated wire is omitted from the heat treatment process and given continuous cold working, there is a problem that the strength of the product increases as the process progresses, while the ductility is continuously deteriorated.

In order to solve the above-mentioned problems, there have been proposed a method in which a bainite-based microstructure is obtained by using crystal precipitates using precipitates, high-cost precipitates such as molybdenum (Mo) and boron (B) Technology has been proposed.

However, the addition of Mo has an advantage of exhibiting a high incineration property, while the cost increase of the material is greatly increased as the addition amount of the expensive element increases. In addition, although boron is low in cost, it is pointed out that there is a problem such as limit of ingestion.

An aspect of the present invention is to provide a non-flattened wire which can secure high strength and high toughness without performing a softening heat treatment and a method for manufacturing the non-flattened wire.

One aspect of the present invention provides a method of manufacturing a semiconductor device, comprising: 0.05 to 0.30% of carbon, 0.1 to 0.4% of silicon, 0.3 to 1.5% of manganese (Mn), 0.3 to 2.0% of chromium (Cr) (Ti): 0.005 to 0.040%, and vanadium (V): 0.05 to 0.50%, the balance Fe and unavoidable impurities, and the high strength and high toughness Thereby providing a non-tempered wire rod.

According to another aspect of the present invention, there is provided a method of manufacturing a billet, comprising: obtaining a billet satisfying the above-described composition of components;

Heating the billet at 1100-1250 < 0 > C for 60 minutes or less;

Subjecting the heated billets to wire rolling at a bimodal temperature interval of Ar ₁ to Ar ₃ ;

Cooling the wire rod at a cooling rate of 10 to 40 DEG C / s after the wire rod is rolled;

After the cooling, air cooling

To provide a method for producing a high strength and high toughness non-cored wire.

According to the present invention, it is possible to provide a non-tempered wire rod which can be suitably provided for parts requiring high strength and high toughness even if the heat treatment is omitted.

The inventors of the present invention have made intensive studies on securing high strength and high toughness even if a separate softening heat treatment is omitted. As a result, it has been found that (001) set The present inventors have found that, when inducing the formation of a texture, the strength can be increased and the toughness can be ensured even when a separate heat treatment is not performed, and the present invention has been accomplished.

First, the composition of the wire of the present invention will be described in detail (hereinafter, the content of the composition means weight%).

C: 0.05 to 0.30%

Carbon (C) is an element that affects the strength of the wire rod. When the content is less than 0.05%, it is difficult to sufficiently secure the tensile strength of the wire rod after hot rolling. On the other hand, when the content exceeds 0.30%, the pearlite microstructure fraction is excessively increased, and the toughness of the material is lowered. In the reheating of the billet, the undissolved Nb, Ti, Al and V- Grain refinement may be limited during rolling.

Therefore, the content of C in the present invention is preferably limited to 0.05 to 0.30%.

Si: 0.1 to 0.4%

In the present invention, the content of silicon (Si) is preferably limited to 0.1 to 0.4%. If the content of Si exceeds 0.4%, the work hardening phenomenon occurs rapidly during the cold drawing and pressing process, which may cause a lot of problems in the workability. Therefore, the content is limited to 0.4% or less. On the other hand, if the content is too small, it is impossible to reach the sufficient strength required for the hot-rolled wire rod and the final product, and therefore, it is preferable to add it at 0.1% or more.

Mn: 0.3 to 1.5%

Manganese (Mn) is an element which forms solid solution by forming a substitutional solid solution in the matrix, and is a useful element which can secure the required strength without lowering the ductility.

If the content of Mn exceeds 1.5%, there is a problem that Mn segregation has a more detrimental effect on the product characteristics rather than the solid solution strengthening effect. In other words, when the steel is solidified, macro segregation and micro segregation occur depending on the segregation mechanism, and Mn segregation promotes the segregation zone due to the relatively low diffusion coefficient as compared with the other elements. As a result, It is the main cause of generation. In this case, the tensile strength is greatly increased, but the ductility is rapidly reduced.

On the other hand, if the content of Mn is less than 0.3%, the effect of the segregation due to Mn segregation hardly appears, but it is difficult to secure the strength of the material required in the present invention, and the cold drawability may be lowered.

Cr: 0.3 to 2.0%

Chromium (Cr), when added in a small amount, is present as a substitute solid solution in the matrix and thus acts to enhance the strength of the material.

If the content of Cr is less than 0.3%, the strength of the material is lowered and it is difficult to secure the final target strength. On the other hand, if the content exceeds 2.0%, it is solidified in the cementite or other carbonitride to form coarse carbonitride Which may adversely affect grain refinement. Therefore, the content of Cr in the present invention is preferably limited to 0.3 to 2.0%.

Nb: 0.005 to 0.040%

Niobium (Nb) is a carbonitride-forming element, and is an element useful for precipitating during wire rolling to induce crystal grain refinement. Nb carbonitride is a strong precipitate that extends the non-recrystallized reverse temperature to about 900 ° C, which is lower than that of titanium (Ti) or vanadium (V) carbonitride, suppressing recrystallization and inducing refinement of ferrite grains .

If the content of Nb is less than 0.005%, the amount of formed carbonitride is small and the recrystallization and restoration ability deteriorate. On the other hand, when the Nb content exceeds 0.040%, the Nb is not dissolved in the heating furnace to form a coarse precipitate, .

The present invention includes at least one of titanium (Ti) and vanadium (V) in addition to the aforementioned component system.

Ti: 0.005 to 0.040%

Titanium (Ti) is a strong carbonitride-forming element. It precipitates at a temperature of 1100 ° C or higher to interfere with grain growth in a heating furnace, finely maintaining the initial grain size, and influencing non-recrystallization inversion during wire rolling.

If the content of Ti is less than 0.005%, the carbonitride formation amount is small and the initial grain refinement and recrystallization inhibiting ability is decreased. On the other hand, if the Ti content is more than 0.040%, the precipitation effect can be reduced by coarsening by micro segregation during performance.

V: 0.05 to 0.50%

Vanadium (V) is a strong carbide-forming element together with the above-mentioned Ti, and its effect is similar to that of Ti.

If the content of V is less than 0.05%, the amount of carbide formation is small, and the effect is decreased. On the other hand, if the content exceeds 0.50%, there is a problem that the ductility and toughness are largely lowered due to the formation of coarse precipitates.

And the remainder includes Fe and unavoidable impurities, and the wire of the present invention does not exclude the inclusion of other elements other than the above-mentioned composition.

Hereinafter, the microstructure of the wire rod of the present invention will be described in detail.

The microstructure of the wire according to the present invention satisfying the above-described composition is preferably a ferrite single phase or a composite structure of ferrite and pearlite, wherein the pearlite is preferably contained in an area fraction of 20% or less. The average grain size of the ferrite is preferably 15 탆 or less.

The wire rod of the present invention having the microstructure as described above is excellent in ductility, strength and toughness without special heat treatment, and thus is useful for cold rolling. The microstructure can be produced more effectively when the advantageous manufacturing conditions of the present invention are satisfied as described later.

Hereinafter, a method of manufacturing a wire according to an aspect of the present invention will be described in detail.

The present invention relates to a method of controlling the temperature range of a wire to control the temperature range in order to induce the development of the texture during rolling while suppressing the recrystallization through subsequent cooling process so as to secure the strength and toughness of the wire, And a method of forming fine ferrite therein is used. That is, the present invention optimizes the heating, rolling, and cooling conditions in a series of processes comprising [heating-rolling-cooling] a billet satisfying the above-mentioned component system.

Hereinafter, each step will be described in more detail.

Billet heating: less than 60 minutes at 1100 ~ 1250 ℃

The steel material such as billet is heated under the above-mentioned conditions to reuse the carbonitride formed by Nb, V, etc. in the constituent system into the base material. If the precipitate formed by Nb, V, or the like remains in the heating furnace without dissolving in heating, continuous refinement during high temperature makes it difficult to refine the grain in the subsequent wire rolling process, have. The initial grain size at the time of heating also affects the grain diameter after the final wire rolling. The temperature range is a condition in which Ti fine carbonitride is not dissolved and the initial grain refinement can be controlled to 100 탆 or less.

If the heating temperature exceeds 1250 deg. C, the Ti-based carbonitride is coarsened and it becomes difficult to control the initial grain size and the surface scale amount is increased, which may cause defects such as surface flaws in the final material, which is not preferable.

Therefore, it is preferable to control the heating temperature range to 1100 to 1250 ° C when heating the billet, and it is more preferable to control the heating holding time at the heating temperature.

The heating and holding time is intended to make the temperature inside and outside of the billet uniform. When the heating is maintained for a sufficient time after heating, the aforementioned carbonitride can be sufficiently dissolved. However, if the heating and holding time is excessively long, there is a possibility that the remaining carbonitride is coarsened. Therefore, the heating is preferably performed within 60 minutes, more preferably within 40 to 60 minutes.

Rolling of wire after heating: carried out in the 2-phase temperature range of Ar ₁ to Ar ₃

The rolling of the billets heated in the above manner in the two-phase temperature zone of Ar ₁ to Ar ₃ is for forming a texture together with the grain refinement. The Nb or V system carbonitride , The austenite phase is elongated to increase the area fraction and the ferrite phase is deformed to form a substructure to affect the strength of the material as well as the texture of the aggregate.

More specifically, the austenite phase and the ferrite phase, which are composed of the austenite phase and the ferrite phase, exhibit austenitic phase due to the continuous introduction of deformation band along with dynamic recrystallization, Fine ferrite crystal grains can be secured. Such grain refinement is an almost unique method capable of securing strength and impact toughness at the same time. The high strength value of the pearlite structure is canceled by grain refinement of the ferrite and the low impact value of the pearlite structure is overcome through the ferrite structure can do.

In addition, the ferrite phase in the bimodal temperature range is easier to recover than the recrystallization due to the BCC crystal structure, which leads to the introduction of dislocations by the transformation during wire rod rolling and the formation of the substructure obtained by recovery, resulting in grain refinement and (001) fiber tecture. The aggregate structure secured in wire rolling develops in the rolling direction, and a high impact toughness value is secured in the direction perpendicular to the rolling direction. Such a texture is continuously developed during cold drawing, and it is possible to secure the strength according to the increase of the drawing process, as well as to secure the impact toughness due to the texture development.

In addition, the present invention limits the content of carbon to 0.05 to 0.3% by weight, where the production of pearlite is facilitated and the fraction thereof is increased as the carbon content is increased. As the carbon content increases, the increase of the pearlite in the microstructure is one of the important factors for securing the strength of the material. However, if the pearlite fraction is excessively increased, the toughness of the material rapidly deteriorates. Therefore, in the present invention, when pearlite is included in the microstructure, it is preferable to limit the fraction to 20% or less.

More preferably, the bimetallic temperature range is preferably 650 to 850 ° C., and when the temperature is lower than 650 ° C., the phase transformation occurs and is favorable for formation of aggregate structure. On the other hand, the rolling load is greatly increased due to deformation at low temperature, there is a problem. On the other hand, when the temperature exceeds 850 DEG C, there is a problem that it is difficult to form an aggregate structure in a single phase of austenite.

Cooling after rolling the wire: cooling at a cooling rate of 10 to 40 ° C / s

It is intended to suppress the recovery and recrystallization of the rolled steel by cooling the wire rods in the above manner at a cooling rate of 10 to 40 DEG C / s.

When the cooling rate is lower than 10 ° C / s when cooling after rolling, there is a problem that the strength of aggregate structure generated due to the recovery and recrystallization of the deformed structure is reduced or the crystal grains are coarsened, while the cooling rate exceeds 40 ° C / s And if it is excessively rapid, it may be transformed into an abnormal structure such as martensite or bainite, and deterioration of physical properties may occur.

When cooling at the above-mentioned cooling rate, it is preferable to terminate the cooling at 400 to 650 ° C. If the cooling termination temperature is lower than 400 ° C, a low temperature tempered structure such as a bainite structure There is a problem that the control of the aggregate structure becomes difficult. On the other hand, when the temperature exceeds 650 ° C., the ferrite transformation is not terminated, resulting in deterioration of physical properties There is a concern.

It is possible to perform air cooling after rough cooling according to the above description because there is no influence of change in cooling rate on the structure in the state where the transformation is completed.

The wire rod of the present invention has a tensile strength of 400 to 700 MPa. After cooling and drawing the wire rod, the wire rod has a tensile strength of 800 to 1400 MPa and a V-notch Charpy impact toughness of 80 J or more.

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 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 )

In order to homogenize the steel material satisfying the composition shown in the following Table 1 and to remove the segregated structure during the performance, the steel sheet was heated to a temperature of 1100 to 1250 캜 for 60 minutes or less and then rolled to 20 mm under the conditions shown in Table 2 below. Cooled at a cooling rate, and made into wire rods.

The microstructural changes (presence of low-temperature structure and average grain size) of the wire rod were observed, and the respective tensile strength and impact toughness were measured. The results are shown in Table 2 below.

division
Component composition (% by weight) C Si Mn Cr Nb Ti V Inventory 1 0.12 0.21 0.71 0.42 0.03 0.005 - Inventory 2 0.11 0.25 1.40 0.47 0.02 0.005 - Inventory 3 0.19 0.19 1.20 0.71 0.03 0.006 0.2 Comparison 1 0.41 0.21 0.73 0.32 0.03 0.003 - Comparative material 2 0.12 0.30 0.75 0.44 0.06 0.005 - Comparative material 3 0.13 0.25 1.40 0.47 0.02 0.002 -

division
Manufacturing conditions Properties Heating temperature
(° C) Rolling temperature
(° C) Cooling rate
(° C / s) Cold tissue
The presence or absence FGS
(탆) The tensile strength
(MPa) Shock
(J) Inventory 1 1120 750 12 × 8 690 90 Inventory 2 1150 770 13 × 7 715 93 Inventory 3 1130 780 21 × 8 740 100 Comparison 1 1100 830 13 ○ 27 820 13 Comparative material 2 1110 800 10 × 23 671 21 Comparative material 3
1120 900 5 × 33 662 38 1010 850 8 × 28 693 33

(FGS in Table 2 means ferrite mean grain size).

As shown in Table 2, it can be seen that the impact value is significantly different between the inventive material satisfying the present invention and the comparative material devoid of the present invention. This is due to the fact that the inventive material according to the present invention has an optimal composition of the composition and developed a texture due to the modified ferrite structure during the rolling of the wire in the bimodal region.

Further, it can be confirmed that the comparative materials 1 to 3 have a ferrite average crystal grain size of 20 mu m or more in all, which means that the composition of the alloy component does not satisfy the optimum conditions proposed in the present invention or does not satisfy the manufacturing conditions , The aggregate structure was not sufficiently formed and recrystallization was not suppressed.

Table 3 shows the properties of the steel material having the properties shown in Table 2 above, and the change in physical properties before and after the sintering was measured.

division
Wire rod Drawing process Tensile Strength (MPa) Shock value (J) Freshness (%) Tensile Strength (MPa) Shock value (J) Inventory 1 690 90 60 1000 89 Inventory 2 715 93 30 860 90 Inventory 3 740 100 70 1100 93 Comparison 1 820 13 (monorail) - - Comparative material 2 671 21 40 870 15 Comparative material 3
662 38 60 943 22 693 33 60 870 23

As shown in Table 3, the inventive materials 1 to 3 according to the present invention have increased strength according to the amount of freshness, and impact toughness is similar to that before the drawing process.

On the other hand, the comparative materials 1 to 3, which do not satisfy the present invention, show only an increase in the strength after drawing, and the impact toughness is rather reduced. Particularly, the comparative material 1 had too high a strength due to the low-temperature structure, resulting in breakage during drawing.

Therefore, when satisfying the component system, component composition and production conditions proposed in the present invention, it can be used directly for cooling and pressing, and excellent strength and toughness can be secured without performing any additional heat treatment.

Claims

(C): 0.05 to 0.30%, silicon (Si): 0.1 to 0.4%, manganese (Mn): 0.3 to 1.5%, chromium (Cr): 0.3 to 2.0%, niobium (Nb): 0.005 To about 0.040%, and at least one of titanium (Ti): 0.005 to 0.040% and vanadium (V): 0.05 to 0.50%, the remainder Fe and unavoidable impurities, and the ferrite single phase or ferrite and pearlite composite Wherein the pearlite has an area fraction of 20% or less.

delete

The method according to claim 1,
The ferrite is a high strength and high toughness non-temperate wire having an average grain size of 15 탆 or less.

The method according to claim 1,
The wire rod has a tensile strength of 400 to 700 MPa and a high strength and high toughness nonconditioning wire.

The method according to claim 1,
The wire rod has a tensile strength of 800 to 1400 MPa and a V-impact toughness of 80 J or more after cold drawing.

(C): 0.05 to 0.30%, silicon (Si): 0.1 to 0.4%, manganese (Mn): 0.3 to 1.5%, chromium (Cr): 0.3 to 2.0%, niobium (Nb): 0.005 Obtaining a billet containing at least 0.040% of at least one of titanium (Ti): 0.005 to 0.040% and vanadium (V): 0.05 to 0.50%, the balance Fe and unavoidable impurities;
Heating the billet at 1100-1250 < 0 > C for 60 minutes or less;
Subjecting the heated billets to wire rolling at a bimodal temperature interval of Ar ₁ to Ar ₃ ;
Cooling the wire rod at a cooling rate of 10 to 40 DEG C / s after the wire rod is rolled; And
After the cooling, air cooling
Wherein the method comprises the steps of:

The method according to claim 6,
Wherein the two-phase temperature range of Ar ₁ to Ar ₃ is 650 to 850 ° C.

The method according to claim 6,
Wherein the cooling step ends the cooling at 400 to 650 占 폚.