CN108220774B

CN108220774B - Wire rod and steel wire having excellent toughness, and method for producing same

Info

Publication number: CN108220774B
Application number: CN201711402072.9A
Authority: CN
Inventors: 李在胜; 林男锡; 朴仁圭
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2016-12-22
Filing date: 2017-12-22
Publication date: 2020-05-26
Anticipated expiration: 2037-12-22
Also published as: KR101940873B1; CN108220774A; KR20180072965A

Abstract

The present invention relates to a wire rod and a steel wire having excellent toughness, and a method for manufacturing the same, the wire rod and the steel wire comprising, in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05%, and N: 0.004 to 0.02%, and the balance Fe and other unavoidable impurities, wherein the wire rod or steel wire has a microstructure including ferrite having a grain size of 12 [ mu ] m or less, 20 to 50% by area of subgrain grains, and AlN precipitates. The invention provides a wire rod and a steel wire with excellent toughness.

Description

Wire rod and steel wire having excellent toughness, and method for producing same

Technical Field

The present invention relates to a wire rod and a steel wire having excellent toughness, and a method for producing the same.

Background

Wire rod products are generally classified into QT (quenched and tempered) heat-treated steel products and non-heat-treated steel products in which physical properties of the steel are secured only by drawing according to the purpose of an end customer.

The strength and toughness/ductility of the QT type steel depends on the tempering process after the austenitizing heat treatment.

The lower the tempering temperature, the stronger the material, but at the same time the ductility/toughness of the product will decrease and the hydrogen delayed fracture resistance will also decrease, so that it is necessary to maintain a suitable tempering temperature. In addition, Austenite Grain Size (AGS) after QT heat treatment has a large influence on physical properties. Generally, as the strength of a material increases, the ductility/toughness decreases instead, and grain refinement is the only known method that can ensure both strength and ductility/toughness.

Generally, grain refinement is performed by using precipitates such as Nb or by rolling in a non-recrystallized region. However, in the case of non-recrystallization region rolling, rolling is performed at a temperature just higher than Ar3, so the temperature range is not wide, and the effect of suppressing rapid grain growth by accumulated deformation, phase transformation heat, and the like is limited when cooling after finishing rolling.

Prior Art

Patent document 1: korean laid-open patent No. 2011-

Disclosure of Invention

Technical problem

A preferred aspect of the present invention provides a high-toughness wire rod having excellent toughness, and a method for manufacturing the same.

Another preferred aspect of the present invention provides a high-toughness steel wire having excellent toughness and a method for manufacturing the same.

Technical scheme

One preferred embodiment of the present invention provides a wire rod excellent in toughness, comprising, in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05%, and N: 0.004 to 0.02%, and the balance being Fe and other unavoidable impurities, the wire rod having a microstructure including ferrite having a grain size of 12 μm or less and including 20 to 50 area% of subgrains, and including AlN precipitates.

The wire may further comprise Nb: 0.001-0.03%, Ti: 0.01% -0.05%, V: 0.2% -0.5%, Mo: 0.15% -0.50% and B: more than one of 0.005-0.02%.

The wire may further include one or more precipitates of niobium (Nb) -based carbonitride, titanium (Ti) -based carbonitride, vanadium (V) -based carbonitride, and molybdenum (Mo) -based carbonitride.

The microstructure of the wire rod comprises 20-80% of ferrite and the balance of pearlite in terms of area%.

After the QT heat treatment, the grain size (AGS; prior austenite grain size) of the wire rod is 5 [ mu ] m or less.

Another preferred embodiment of the present invention provides a method for manufacturing a wire rod having excellent toughness, comprising the steps of:

a step of heating a steel slab at a temperature of 900 ℃ to 1100 ℃, the steel slab comprising in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05% and N: 0.004-0.02 percent of the total weight of the alloy, and the balance of Fe and other inevitable impurities;

a hot rolling step of rolling the slab heated in the above manner at an entry temperature of a Finishing Mill (FM) of 820 to 780 ℃ and an entry temperature of a Reducing Sizing Mill (RSM) of 760 to 730 ℃ to obtain a wire rod having a subgrain fraction in a ferrite phase of 20 to 50 area%; and

cooling after winding the wire rod.

The steel slab may further comprise Nb: 0.001-0.03%, Ti: 0.01% -0.05%, V: 0.2% -0.5%, Mo: 0.15 to 0.50 wt% and B: more than one of 0.005-0.02%.

In the hot rolling, the finish rolling process and the reducing diameter process are preferably performed in a ferrite and austenite two-phase region.

The wire may have a diameter of 18mm or less.

After the coiling, the cooling rate may be 5 ℃/sec or more.

The wire thus obtained can be directly subjected to a QT heat treatment.

After the QT heat treatment, the grain size of the wire rod (AGS; prior austenite grain size) may be 5 μm or less.

Still another preferred embodiment of the present invention provides a steel wire excellent in toughness, comprising in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05%, and N: 0.004-0.02%, and the balance Fe and other inevitable impurities, and has a microstructure of tempered martensite containing AlN precipitates and a grain size (AGS; prior austenite grain size) of 5 μm or less.

The steel wire may further comprise Nb: 0.001-0.03%, Ti: 0.01% -0.05%, V: 0.2% -0.5%, Mo: 0.15% -0.50% and B: more than one of 0.005-0.02%.

The steel wire may further include one or more precipitates of niobium (Nb) -based carbonitride, titanium (Ti) -based carbonitride, vanadium (V) -based carbonitride, and molybdenum (Mo) -based carbonitride.

Still another preferred embodiment of the present invention provides a method for manufacturing a steel wire having excellent toughness, comprising the steps of:

a hot rolling step of rolling the slab heated in the above manner at an entry temperature of a Finishing Mill (FM) of 820 to 780 ℃ and an entry temperature of a Reducing Sizing Mill (RSM) of 760 to 730 ℃ to obtain a wire rod having a subgrain fraction in a ferrite phase of 20 to 50 area%;

cooling after coiling the wire;

drawing the wire rod at a drawing rate of 5% to 40% to produce a steel wire; and

subjecting the steel wire to a QT heat treatment.

Effects of the invention

As described above, according to the present invention, a wire rod and a steel wire having excellent toughness can be provided.

Drawings

FIG. 1 is a structural photograph showing SEM-EBSD structures of rolled wire rods of invention example 1 and comparative example 1.

Fig. 2 is a graph showing the grain boundary angle values of 15 ° or less in inventive example 1 and comparative example 1.

Fig. 3 is a graph showing the QT heat-treated grain size (AGS) of inventive example 1 and comparative example 1.

Fig. 4 is a photograph showing the QT heat-treated microstructures of inventive example 1 and comparative example 1.

Detailed Description

The present invention will be explained below.

When a steel material is subjected to a QT heat treatment, minimization of the initial austenite grain size and a short heating time before the QT heat treatment are important to ensure that fine AGS (austenite grain size) is obtained.

The aim of the invention is to refine the initial austenite grains before the QT heat treatment.

For this reason, the steel composition, microstructure and precipitates should be controlled for the wire rod and steel wire.

1) In the case of the microstructure, the initial austenite grains can be refined by including ferrite and pearlite and increasing the sub-grain (subgrain) fraction in the ferrite so that austenite grains can be generated in the ferrite grains as well.

2) The precipitates are formed by adding carbon/nitrogen forming elements such as Al, Ti, Nb, V, and Mo to form carbon/nitrogen precipitates, and when QT heat treatment is performed, in order to achieve austenitization, growth of austenite in reverse transformation of ferrite/pearlite is suppressed when the steel material is heated, thereby refining initial austenite grains.

3) In the production of a wire rod, the precipitates inhibit the growth of ferrite and pearlite to refine the ferrite and pearlite, and the refined ferrite and pearlite structures serve as nucleation sites to increase the effective grain boundary fraction, thereby contributing to the refinement of the initial austenite grains.

In the present invention, the steel composition and production conditions are controlled so as to increase the sub-grain (subgrain) fraction in ferrite. In particular, the hot rolling process, in particular the finish rolling process and the reducing sizing process, is controlled.

Controlling the inlet temperature of a Finishing Mill (FM) in the finishing process, and controlling the inlet temperature of a Reducing Sizing Mill (RSM) in the reducing sizing process.

As described above, by refining the initial austenite grains before the QT heat treatment of the steel material, it is possible to ensure that fine grains are obtained when the final QT heat treatment is performed, and thereby improve the toughness of the steel material.

Next, a wire rod and a steel wire excellent in toughness according to a preferred embodiment of the present invention will be described.

Carbon (C): 0.1 to 0.6% by weight (hereinafter referred to as "%")

If the C content is more than 0.6%, almost all the structure is composed of pearlite and a sufficient amount of ferrite subgrain required cannot be secured, and if the C content is less than 0.1%, the ferrite fraction is too large to form a complete pearlite microstructure at the time of QT heat treatment and a sufficient strength cannot be secured in the martensite structure.

Therefore, the C content is preferably limited to 0.1% to 0.6%, more preferably 0.2% to 0.5%.

Silicon (Si): 0.1 to 2.0 percent

Si is a representative substitution element, and has a large influence on the strength of steel. If the Si content is less than 0.1%, it is difficult to secure sufficient steel strength and hardenability, and if the Si content is more than 2.0%, the formation of a decarburized structure is promoted during rolling of the wire rod, thereby requiring additional removal costs.

Therefore, the Si content is preferably limited to 0.1% to 2.0%, more preferably 0.15% to 2.0%, and still more preferably 0.2% to 2.0%.

Chromium (Cr): 0.1 to 1.0 percent

The chromium functions to promote ferrite and pearlite transformation during hot rolling.

In addition, without excessively increasing the strength of the steel itself, carbides in the steel are precipitated to reduce the amount of solid-solution carbon, thereby reducing the dynamic strain aging due to the solid-solution carbon. In order to provide the above effects of the present invention, chromium is preferably contained in an amount of 0.1% or more. However, if the chromium content is too high, the strength of the steel itself becomes too high, and the deformation resistance of the steel increases sharply, thereby deteriorating the cold forgeability. Therefore, the upper limit of the chromium content is preferably 1.0%.

Manganese (Mn): 0.2 to 2.0 percent

The manganese (Mn) forms a substitutional solid solution in the matrix structure, lowers the a1 transformation point to reduce the pearlite interlayer spacing, and increases the subgrain in the ferrite structure.

If the amount of manganese added is greater than 2.0%, the steel is adversely affected by the heterogeneity of the structure due to manganese segregation. Macrosegregation and microsegregation easily occur according to a segregation mechanism when steel is solidified, and manganese segregation promotes the formation of a segregation band due to a relatively lower diffusion coefficient than other elements, and thus improved hardenability is an important cause for the formation of a low temperature structure (core martensite) in the center portion. In addition, if the amount of manganese added is less than 0.2%, it is difficult to ensure sufficient hardenability for obtaining a martensitic structure after the QT heat treatment.

Therefore, the Mn content is preferably limited to 0.2% to 2.0%, more preferably 0.5% to 2.0%.

Aluminum (Al): 0.01 to 0.05 percent

Al is added as a deoxidizing agent and is a component forming precipitates such as AlN, and if the Al content is less than 0.01%, sufficient deoxidizing ability cannot be secured and precipitates are difficult to form, but if the Al content is more than 0.05%, Al is added₂O₃And the hard inclusions may increase, and particularly, the nozzle may be clogged by the inclusions during continuous casting.

The AlN precipitates effectively suppress grain growth by deformation-induced precipitation during rolling and contribute to the achievement of austenite grain refinement.

Therefore, the content of Al is preferably limited to 0.01% to 0.05%, more preferably 0.01% to 0.042%.

Nitrogen (N): 0.004-0.02 percent

If the content of N is less than 0.004%, it is difficult to secure sufficient nitrides so that the amount of precipitation of Ti, Nb, V, etc. is reduced, but if the content thereof is more than 0.02%, solid-solution nitrogen which does not combine with the precipitates causes a reduction in toughness/ductility of the material.

Therefore, the content of N is preferably limited to 0.004% to 0.02%.

Precipitates of Ti, Nb, V, AlN, etc. first form nitrides at high temperature, and thereafter unbound precipitates are bound with C to precipitate in the form of carbides at low temperature or to adhere to nitrides formed at high temperature to grow together therewith. Therefore, the nitrogen amount has a great influence on the distribution of precipitates, and the content thereof should be controlled.

Niobium (Nb): 0.001 to 0.03 percent

The Nb forms carbonitrides such as Nb (C, N), and contributes to refining the ferrite/pearlite wire structure when appropriately rolled. In addition, before precipitation, refinement of austenite grain boundaries during rolling is affected due to the effect of solute drags.

However, if the content of Nb is less than 0.001%, the precipitation effect is insufficient, whereas if the content is more than 0.03%, the precipitation effect is reduced by coarse precipitates.

Therefore, the content of Nb is preferably limited to 0.001% to 0.03%, more preferably 0.01% to 0.025%.

Titanium (Ti): 0.01 to 0.05 percent

The Ti, which is the most active element for forming carbonitride, contributes to grain refinement in the heating furnace. However, if the Ti content is less than 0.01%, the effect is reduced because the precipitation amount is too small, but if the Ti content is more than 0.05%, coarse precipitates may become crack initiation sites (crack initiation sites), and the toughness/ductility may be reduced.

Therefore, the content of Ti is preferably limited to 0.01% to 0.05%, more preferably 0.01% to 0.04%.

Vanadium (V): 0.2 to 0.5 percent

V forms VC, VN, V (C, N), etc., and when rolling is appropriately performed, the microstructure of the ferrite/pearlite wire rod is promoted to be finer. If the content of V is less than 0.2%, the distribution of vanadium precipitates in the matrix is reduced, so that the function of fixing ferrite grain boundaries is not exerted, and the influence on toughness is small, but if the content of V is more than 0.5%, coarse vanadium carbonitrides are formed within the range of carbon and nitrogen defined in the present invention, resulting in deterioration of toughness.

Therefore, the content of V is preferably limited to 0.2% to 0.5%, more preferably to 0.2% to 0.4%.

Molybdenum (Mo): 0.15 to 0.50 percent

Mo can form Mo during tempering in the QT heat treatment process₂C is a precipitate, thereby can haveEffectively suppressing the phenomenon of strength reduction (temper softening) during tempering. However, if the Mo content is less than 0.15%, it is difficult to obtain a sufficient temper softening effect, and if the Mo content is more than 0.50%, a low temperature structure is generated in a wire rod state, so that additional heat treatment cost for removing the low temperature structure is required.

Molybdenum (Mo) carbide is mainly precipitated during tempering in the QT heat treatment process as a secondary precipitation strengthening element. In particular, in order to improve the hydrogen-induced delayed fracture resistance, the material does not soften during high-temperature tempering, but rather the strength is increased. In this case, the Mo content is less than 0.15%, since the secondary precipitation strengthening effect is hardly exhibited, it is preferable that the content is limited to the above range.

Therefore, the content of Mo is preferably limited to 0.15% to 0.50%, more preferably 0.2% to 0.4%.

Boron (B): 0.005 to 0.02 percent

The B functions to densify the rust formed on the surface and improve corrosion resistance, and also to improve hardenability to improve grain boundary strength. If the B content is less than 0.005%, sufficient hardenability cannot be secured and the strength required for the high-strength wire rod cannot be obtained, but if the B content is more than 0.02%, the tensile properties and fatigue properties are deteriorated due to coarse carbonitride-based precipitates. Therefore, the content of B is preferably limited to 0.005% to 0.02%, more preferably 0.005% to 0.015%.

Another preferred embodiment of the present invention provides a high-toughness wire rod having a microstructure including ferrite and pearlite, the ferrite having a grain size of 12 μm or less and including 20 to 50 area% of subgrains, and including aluminum (Al) -based precipitates.

The precipitates serve to suppress grain growth to the maximum extent during rolling and cooling. In order to achieve such pinning (pinning) effect of the precipitates, the size of the precipitates is preferably made finer to 10nm to 500nm, and if the precipitates are made coarse in size, the pinning (pinning) effect is rapidly reduced.

In addition, the density of precipitates may be 2/um²Above, preferably 10/um²Above, more preferably 2/um²100 pieces/um²The above.

When the density distribution of the precipitates is secured, grain boundary migration (grain boundary migration) can be effectively suppressed. If the precipitate density is less than 2/um²It is difficult to suppress the grain growth, and the precipitate density is more effective as it is, but more than 100 precipitates/um²In this case, the effect is also saturated.

The wire rod can be subjected to QT heat treatment, the microstructure of the wire rod subjected to the QT heat treatment is tempered martensite, and the grain size (AGS; prior austenite grain size) can be less than 5 mu m.

After the wire rod is made into the steel wire by the drawing process, QT heat treatment can be carried out, the microstructure of the steel wire after the QT heat treatment is tempered martensite, and the grain size (AGS; prior austenite grain size) can be less than 5 mu m.

Next, a method for producing a wire rod and a steel wire excellent in toughness according to another preferred embodiment of the present invention will be described.

Another preferred embodiment of the present invention provides a method for manufacturing a wire rod having excellent toughness, comprising the steps of: a step of heating a steel slab at a temperature of 900 ℃ to 1100 ℃, the steel slab comprising in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05% and N: 0.004-0.02 percent of the total weight of the alloy, and the balance of Fe and other inevitable impurities; a hot rolling step of rolling the slab heated in the above manner at an entry temperature of a Finishing Mill (FM) of 820 to 780 ℃ and an entry temperature of a Reducing Sizing Mill (RSM) of 760 to 730 ℃ to obtain a wire rod having a subgrain fraction in a ferrite phase of 20 to 50 area%; and cooling the wire rod after the wire rod is coiled.

Billet heating step

Heating the steel billet at 900-1100 ℃.

After the billet is heated to 900 to 1100 ℃, it is preferably kept at that temperature for 10 to 30 minutes, for example.

The total time for heating the billet to 900 to 1100 ℃ and holding is preferably 90 to 120 minutes, for example.

The heating of the billet is intended for hot rolling of wire rods, and if the heating temperature is less than 900 ℃, the rolling load is rapidly increased, and problems such as roll chipping, steel stacking failure (cobble), and surface defects may occur during rolling, and if the heating temperature is more than 1100 ℃, the decarburization reaction on the billet surface is rapidly increased, and the surface deteriorated structure may be increased.

Therefore, the heating temperature of the billet is preferably limited to 900 to 1100 ℃.

If the holding time at the temperature of 900 to 1100 ℃ is less than 10 minutes, the center portion of the material may not reach the target temperature sufficiently, but if the holding time exceeds 30 minutes, the scale, surface decarburization, or the like may increase, and the surface deteriorated structure may increase.

Step of Hot Rolling

The rolling method preferred in the present invention is roughly classified into a process of rough rolling a heated billet, a finish rolling process, and a reducing diameter process.

The billet heated in the above-described manner is hot-rolled to obtain a wire rod.

The finish rolling process and the reduction sizing process are preferably performed in a ferrite and austenite two-phase region.

The hot rolling is preferably carried out at an inlet temperature of a Finishing Mill (FM) of 820 ℃ to 780 ℃ and at an inlet temperature of a Reducing Sizing Mill (RSM) of 760 ℃ to 730 ℃.

If the Finishing Mill (FM) inlet temperature is lower than 780 ℃, the supercooling/quenching may cause a decarburized structure such as ferrite and surface defects to be rapidly formed on the surface during rolling, and if the temperature is higher than 820 ℃, the fraction of sub-grains (deformed ferrite) is reduced, and thus a desired grain size cannot be secured.

Therefore, the Finishing Mill (FM) inlet temperature is preferably defined to be 820 ℃ to 780 ℃.

If the inlet temperature of the Reducing Sizing Mill (RSM) is lower than 730 ℃, supercooling is too low to the Ar1 transformation point, resulting in an increase in roll load and difficulty in ensuring obtainment of a microstructure composed of ferrite/pearlite, and if the temperature is higher than 760 ℃, the fraction of subgrain (deformed ferrite) is reduced, failing to ensure a desired grain size.

The hot rolling is carried out under conditions such that the sub-grain fraction in the ferrite phase of the wire rod obtained by rolling is 20 to 50 area%.

The sub-grain fraction in the ferrite phase of the wire rod affects the heat treatment process performed by customers in the future, and if the sub-grain fraction is less than 20 area%, grain refinement may not be ensured at the time of QT heat treatment, and if the sub-grain fraction is greater than 50 area%, the strength of the wire rod is greatly increased, good linearity of the wire rod may not be ensured, and the like.

Therefore, the sub-grain fraction in the ferrite phase of the wire rod is preferably limited to 20 to 50 area%.

The diameter of the wire is preferably limited to 18mm or less, for example.

Coiling and cooling step

The wire rod obtained after hot rolling in the above manner was wound up and then cooled.

The cooling rate during cooling is preferably set to, for example, 5 ℃/sec or more.

If the cooling rate is less than 5 ℃/sec, the crystal grains may be coarsened due to deformation and phase transformation heat, and therefore, the cooling rate is preferably 5 ℃/sec or more, more preferably 5 to 20 ℃/sec.

The cooling termination temperature is preferably defined to be, for example, 200 ℃ to 600 ℃.

The cooling termination temperature is limited to the range of 200 ℃ to 600 ℃ in order to terminate the phase transition at the cooling stage and obtain the desired microstructure.

QT Heat treatment (quenching and tempering heat treatment)

The wire rod thus obtained can be directly subjected to quenching and tempering heat treatment (QT heat treatment).

The heating temperature during the quenching heat treatment is preferably 800 ℃ to 1000 ℃, and the cooling speed is preferably 20 ℃/s.

If the quenching temperature is less than 800 ℃, it is difficult to obtain sufficient strength after QT heat treatment because the heat treatment temperature is low and it is not ensured that an austenite structure is obtained, and if the quenching temperature is more than 1000 ℃, coarsening of crystal grains may cause deterioration of physical properties while surface decarburization occurs. In the case where the cooling rate is less than 20 ℃/sec, a complete martensitic structure cannot be obtained.

The heating temperature in the tempering heat treatment is preferably 300 to 600 ℃.

If the tempering temperature is less than 300 ℃, a thin film (film) -like plate-like cementite may be formed along grain boundaries, which may cause problems of hydrogen embrittlement, etc., and if the tempering temperature is more than 600 ℃, excessive tempering may cause difficulty in obtaining sufficient strength.

The microstructure of the wire rod subjected to the QT heat treatment is tempered martensite, and the grain size (AGS) may be, for example, 5 μm or less.

Drawing and QT Heat treatment

After the wire rod thus produced is drawn, quenching and tempering heat treatment (QT) may be performed.

The drawing ratio at the time of drawing is preferably limited to, for example, 5% to 40%.

If the draw ratio is less than 5%, the deformed structure cannot sufficiently progress to the central portion of the material, and austenitization is difficult to achieve in a short time at the QT heat treatment. Therefore, when the heat treatment is performed for a long time, the crystal grains may be coarsened, and if the drawing ratio is more than 40%, the 1-pass (pass) drawing is impossible, which makes the manufacturing process longer and complicated.

The microstructure of the QT heat treated steel wire is tempered martensite, and the grain size (AGS) may be, for example, 5 μm or less.

The present invention will be described in more detail by way of examples.

(examples)

A steel slab having the composition shown in table 1 below was heated and hot-rolled under the conditions shown in table 2 below, to thereby obtain a wire rod having a diameter of 13 mm.

After the wire rod thus obtained was subjected to QT heat treatment, AGS (prior austenite grain size) was measured, and the results thereof are shown in table 2. At this time, the QT heat Treatment employs Induction Quenching and Tempering (IQT) heat Treatment with a short heating time to minimize AGS deviation according to a heating temperature, which is 850 to 950 ℃.

In the following table 2, FM represents the finish mill inlet temperature, RSM represents the reducing mill inlet temperature, FGS represents the ferrite grain size, and AGS represents the prior austenite grain size.

SEM-EBSD structure photographs of the rolled wire rods of invention example 1 and comparative example 1 in table 2 below were observed and shown in fig. 1, and the results of examining the grain boundary angle value of 15 ° or less are shown in fig. 2.

In addition, AGS (prior austenite grain size) of the wire rods subjected to the QT heat treatment of invention example 1 and comparative example 1 was measured, and the results are shown in fig. 3, and the microstructure of the wire rods subjected to the QT heat treatment was observed, and the results are shown in fig. 4.

[ TABLE 1 ]

Steel grade	C	Si	Cr	Mn	Al	N	Nb	Ti	V	Mo	B
												Invention steel 1	0.25	0.30	0.35	1.30	0.042	0.015	0.015	-	-	-	-
Invention steel 2	0.35	1.20	0.20	1.30	0.010	0.004	0.015	0.02	-	-	-
												Invention steel 3	0.40	0.80	0.30	1.20	0.042	0.013	0.020	0.02	-	0.2	-
Comparative Steel 1	0.72	0.30	0.15	0.80	0.035	0.010	-	-	-	-	-
												Invention steel 4	0.35	0.20	0.50	0.70	0.035	0.010	-	-	-	-	-
Invention steel 5	0.20	0.25	0.35	0.80	0.030	0.015	0.015	-	0.3	0.2	-
												Invention steel 6	0.25	0.30	0.45	1.20	0.040	0.020	0.015	-	-	-	-
Invention steel 7	0.45	0.18	0.25	1.20	0.036	0.016	-	0.03	-	0.3	-
												Invention steel 8	0.50	0.15	0.20	1.50	0.032	0.012	-	0.02	0.3	0.2	-
Invention steel 9	0.45	0.18	0.65	1.20	0.036	0.016	-	0.03	-	0.3	0.01

[ TABLE 2 ]

In table 2, F represents ferrite, and P represents pearlite.

As shown in table 2 above and fig. 1 to 4, the wire rods according to the present invention (inventive examples 1 to 6) had A Grain Size (AGS) of 5 μm or less after the QT heat treatment.

The wire rods outside the scope of the present invention (comparative examples 1 to 4) had A Grain Size (AGS) of more than 5 μm after being subjected to QT heat treatment.

As shown in table 2 and fig. 2 above, in the invention examples (1 to 6), 30% or more of high-ratio subgrains (grains having a grain boundary angle value of 15 ° or less) which serve as nucleation points of austenite grains of reverse transformation at the time of heating in the final QT heat treatment were generated, and as is clear from table 2, fig. 3, and fig. 4, ultrafine grains of 5 μm or less were obtained at the time of final QT heat treatment. This result indicates that securing a sub-grain ratio of an appropriate amount or more in the initial wire state has a great influence on the final AGS.

When precipitates were observed in the wire rods according to the present invention (invention examples 1 to 6), aluminum (Al) -based precipitates were distributed. The size of the precipitates is 500nm or less, and the density of all precipitates is 10 precipitates/um²The above.

In addition, the AGS of the wire rod after QT heat treatment in the above formula was measured after the wire rods obtained according to invention example 1 of tables 1 and 2 were drawn at a draw ratio of 10% to 30%, and as a result, the AGS was similar to that of invention example 1 without drawing.

Claims

1. A wire rod having excellent toughness, characterized in that,

the wire comprises, in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05%, and N: 0.004 to 0.02%, and the balance being Fe and other unavoidable impurities, the wire rod having a microstructure including ferrite having a grain size of 12 μm or less and including 20 to 50 area% of subgrains, and including AlN precipitates.

2. The wire rod excellent in toughness according to claim 1,

the wire further comprises Nb: 0.001-0.03%, Ti: 0.01% -0.05%, V: 0.2% -0.5%, Mo: 0.15% -0.50% and B: more than one of 0.005-0.02%.

3. The excellent-toughness wire rod according to claim 2,

the wire further contains one or more precipitates selected from the group consisting of niobium (Nb) -based carbonitride, titanium (Ti) -based carbonitride, vanadium (V) -based carbonitride, molybdenum (Mo) -based carbonitride, and boron (B) -based carbonitride.

4. The wire rod excellent in toughness according to claim 1 or 2,

5. The wire rod excellent in toughness according to claim 1 or 3,

the size of the precipitate is 10nm to 500nm, and the precipitate density is 2/um²100 pieces/um²。

6. The wire rod excellent in toughness according to claim 1,

the wire rod has A Grain Size (AGS) of less than 5 [ mu ] m after QT heat treatment.

7. A method for producing a wire rod having excellent toughness, characterized by comprising the steps of:

a step of heating a steel slab at a temperature of 900 ℃ to 1100 ℃, said steel slab comprising in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05% and N: 0.004-0.02 percent of the total weight of the alloy, and the balance of Fe and other inevitable impurities;

cooling after winding the wire rod.

8. The method for producing a wire rod excellent in toughness according to claim 7,

the steel slab further comprises Nb: 0.001-0.03%, Ti: 0.01% -0.05%, V: 0.2% -0.5%, Mo: 0.15 to 0.50 wt% and B: more than one of 0.005-0.02%.

9. The method for producing a wire rod excellent in toughness according to claim 7,

in the hot rolling process, a finish rolling process and a reducing sizing process are performed in a ferrite and austenite two-phase region.

10. The method for producing a wire rod excellent in toughness according to claim 7,

the diameter of the wire is 18mm or less.

11. The method for producing a wire rod excellent in toughness according to claim 7,

in the hot rolling step, the sub-grain fraction in the ferrite phase of the hot-rolled wire rod is 20% to 50%.

12. The method for producing a wire rod excellent in toughness according to claim 7,

in the step of cooling after the coiling, the cooling rate is 5 ℃/sec or more.

13. The method for producing a wire rod excellent in toughness according to claim 7,

in the cooling step after the coiling, the cooling termination temperature is 200 to 600 ℃.

14. The method for producing a wire rod excellent in toughness according to claim 7,

the manufacturing method further includes a step of subjecting the wire rod to QT heat treatment.

15. The method for producing a wire rod excellent in toughness according to claim 14,

the grain size (AGS) of the wire rod subjected to the QT heat treatment is 5 [ mu ] m or less.

16. A steel wire having excellent toughness, characterized in that,

the steel wire comprises, in weight%: c: 0.1% -0.6%, Si: 0.1-2.0%, Cr: 0.1-1.0%, Mn: 0.2% -2.0%, Al: 0.01% -0.05%, and N: 0.004-0.02%, and the balance Fe and other inevitable impurities, and has a microstructure of tempered martensite containing AlN precipitates and a grain size (AGS; prior austenite grain size) of 5 μm or less.

17. The steel wire excellent in toughness according to claim 16,

the steel wire further comprises Nb: 0.001-0.03%, Ti: 0.01% -0.05%, V: 0.2% -0.5%, Mo: 0.15% -0.50% and B: more than one of 0.005-0.02%.

18. The steel wire excellent in toughness according to claim 17,

the steel wire further includes one or more precipitates of niobium (Nb) -based carbonitride, titanium (Ti) -based carbonitride, vanadium (V) -based carbonitride, molybdenum (Mo) -based carbonitride, boron (B) -based carbonitride.

19. A method for producing a steel wire having excellent toughness, characterized by comprising the steps of:

cooling after coiling the wire;

subjecting the steel wire to a QT heat treatment.

20. The method for producing a steel wire excellent in toughness according to claim 19,

21. The method for producing a steel wire excellent in toughness according to claim 19,

22. The method for producing a steel wire excellent in toughness according to claim 19,

23. The method for producing a steel wire excellent in toughness according to claim 19,

the grain size (AGS) of the steel wire subjected to the QT heat treatment is 5 μm or less.