KR20170072995A

KR20170072995A - Non-quenched and tempered wire rod having excellent strength and impact toughness and method for manufacturing same

Info

Publication number: KR20170072995A
Application number: KR1020150181019A
Authority: KR
Inventors: 문동준; 최염호; 이상윤; 손정수
Original assignee: 주식회사 포스코
Priority date: 2015-12-17
Filing date: 2015-12-17
Publication date: 2017-06-28
Also published as: KR101758490B1

Abstract

The steel sheet according to any one of claims 1 to 3, which comprises 0.15 to 0.35% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.5 to 2.0% of Cr, 0.02% or less of P, %, N: 0.01% or less, Nb: 0.005 to 0.05% and V: 0.05 to 0.5%, and the balance Fe and unavoidable impurities, including cargo, and an average circle equivalent diameter of the carbonitride 5 ~ 70nm, the unit of the number per unit area of a carbonitride than the average circle equivalent diameter of 80nm of the carbonitride 5 / μm ² or less non-adjustable quality wire and for making it A method is disclosed.

Description

TECHNICAL FIELD [0001] The present invention relates to a non-sintered wire having excellent strength and impact toughness and a method of manufacturing the same. [0002]

The present invention relates to a non-woven wire having excellent strength and impact toughness and a method for producing the same. More particularly, the present invention relates to a non-woven wire having excellent strength and impact resistance suitable for use as a material for automobiles or machine parts, &Lt; / RTI >

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. Is made of micro-alloy steel (Micro-Alloyed Steel).

Conventional wire products are manufactured by hot rolling, cold drawing, spheroidizing heat treatment, cold drawing, cold pressing, quenching and bake process, while non-tempering wire products are processed by hot rolling, cold drawing and cold pressing The final product is made through.

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.

As a method for solving the above problems, a bainite-based microstructure is formed by utilizing grain refinement using a precipitate, a high-cost precipitating alloy such as molybdenum (Mo) and boron (B) Technology has been proposed.

However, in the case of molybdenum (Mo) -added steel, there is a merit that it has a high incineration property, while a cost increase of the material is greatly increased as the amount of the expensive element increases. In the case of boron (B) Though it is low price, it is pointed out as a problem such as limit of ingenuity.

One of the objects of the present invention is to provide a non-cored wire which can secure excellent strength and impact toughness without further heat treatment and a method of manufacturing the same.

In order to achieve the above object, one aspect of the present invention provides a steel comprising 0.15 to 0.35% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.5% or less of Cr, 0.005 to 0.05% of Nb and 0.05 to 0.5% of V, and the balance of Fe (Al), Fe (Al) And a carbonitride containing Nb and / or V, wherein the carbonitride has an average circle equivalent diameter of 5 to 70 nm and a carbonitride having an average circle equivalent diameter of 80 nm or more among the carbonitride, Wherein the number of grains is 5 / μm ² or less.

In another aspect of the present invention, there is provided a ferritic steel comprising 0.15 to 0.35% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.5% or less of Cr, 0.02% or less of P, % Of Al, 0.01 to 0.05% of sol.Al, 0.01% or less of N, 0.005 to 0.05% of Nb and 0.05 to 0.5% of V, and the balance Fe and unavoidable impurities A step of reheating the billet to a reheating temperature (T _H ) satisfying the following relational expression 1, obtaining the wire rod by subjecting the reheated billet to a wire rolling under the condition of a finish rolling temperature Ae3 to (Ae3 + 50) DEG C, And cooling the steel sheet at a speed of 0.1 to 1 占 폚 / sec after winding the steel sheet.

[Relation 1] T _X ? T _H ? T _X + 80

(Case where, T ₁ ≥T ₂ _X is T ₁ and T il, T _2> T is _1, _X T is T _2, T ₁ is "833.5 + 447.9x {1-exp (- [Nb] [C ] /0.01799)} + 226.5x {1-exp (- [Nb] [C] /0.00163)} and T ₂ is 860.7 + 348.1x { } + 199.6x {1-exp (- [V] [N] /0.00064)})

According to the present invention, it is possible to provide a non-tempered wire having excellent strength and impact resistance even if the annealing for spheroidizing annealing is omitted.

The present inventors have studied in various angles in order to provide a non-tempered wire having excellent strength and impact toughness even when the annealing annealing for spheroidizing is omitted. As a result, Nb and / or V is added to the precipitate- To thereby provide a non-tempered wire having excellent strength and impact toughness by appropriately controlling the average grain size of the precipitate and the number density of the coarse precipitates. The present invention has been accomplished based on this finding.

Hereinafter, an untwisted wire having excellent strength and impact toughness, which is one aspect of the present invention, will be described in detail.

First, the alloy composition and the composition range of the non-cored wire will be described in detail. It is to be noted that the content of each component described below is based on weight unless otherwise specified.

C: 0.15 to 0.35%

Carbon improves the strength of the wire. In order to exhibit such an effect in the present invention, the content is preferably 0.15% or more, more preferably 0.16% or more. However, when the content thereof is excessive, the deformation resistance of the steel increases rapidly, thereby deteriorating the cold workability. Therefore, the upper limit of the carbon content is preferably 0.35%, more preferably 0.34%.

Si: 0.05 to 0.3%

Silicon is a useful element as a deoxidizer. In order to exhibit such an effect in the present invention, the content is preferably 0.05% or more, more preferably 0.06% or more. However, if the content is excessive, resistance to steel deformation increases rapidly due to reinforcement of the solid solution, which results in deterioration of cold workability. Therefore, the upper limit of the silicon content is preferably 0.3%, more preferably 0.25%.

Mn: 1.0 to 2.0%

Manganese is a useful element as a deoxidizer and desulfurizer. In order to exhibit such effects in the present invention, the content is preferably 1.0% or more, more preferably 1.1% or more. However, if the content is excessive, the strength of the steel itself becomes excessively high, so that the deformation resistance of the steel increases rapidly, thereby deteriorating the cold workability. Therefore, the upper limit of the manganese content is preferably 2.0%, more preferably 1.8%.

Cr: not more than 0.5%

Chromium promotes ferrite and pearlite transformation during hot rolling. In addition, without increasing the strength of the steel itself more than necessary, it precipitates carbides in the steel to reduce the amount of solid carbon, which contributes to reduction of dynamic strain aging due to solid carbon. However, if the content is excessive, the strength of the steel itself becomes excessively high, so that the deformation resistance of the steel increases rapidly, thereby deteriorating the cold workability. The chromium content is preferably 0.5% or less, more preferably 0.05 to 0.4%.

P: not more than 0.02%

Phosphorus is an impurity which is inevitably contained and is an element which segregates in the grain boundaries to decrease the toughness of the steel and reduce the delayed fracture resistance. Therefore, it is desirable to control the content as low as possible. Theoretically, it is advantageous to control the phosphorus content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the phosphorus content is controlled to 0.02%.

S: not more than 0.02%

Sulfur is an inevitably contained impurity which segregates in the grain boundaries to greatly reduce ductility of steel and form an emulsion in the steel to deteriorate delayed fracture resistance and stress relaxation characteristics. Therefore, the content of sulfur is controlled to be as low as possible . Theoretically, it is advantageous to control the sulfur content to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the sulfur content is controlled to 0.02%.

sol.Al: 0.01 to 0.05%

Soluble aluminum is an element which functions as a deoxidizing agent and is added in an amount of 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. However, when the content exceeds 0.05%, the effect of making the austenite grain size by AlN formation becomes large, and the cold workability is deteriorated. Therefore, in the present invention, the upper limit of the soluble aluminum content is controlled to 0.05%.

N: not more than 0.01%

Nitrogen is an impurity inevitably contained. When the content is excessive, the amount of solid solution nitrogen is increased, so that the deformation resistance of the steel is rapidly increased, thereby deteriorating the cold workability. Theoretically, it is advantageous to control the nitrogen content to 0%, but it is inevitably contained in the manufacturing process normally. Therefore, it is important to manage the upper limit. In the present invention, the upper limit of the nitrogen content is preferably controlled to 0.01%, more preferably 0.008%, and more preferably 0.007%.

At least one of Nb: 0.005 to 0.05% and V: 0.05 to 0.5%

Niobium is added as an element which plays a role in limiting the grain boundary movement of austenite and ferrite by forming carbonitride in an amount of 0.005% or more. However, since the above-mentioned carbonitride acts as a breaking point, it may lower the impact toughness, especially, the impact resistance at low temperature, so it is also preferable to keep the solubility limit. Furthermore, when the content is excessive, the solubility limit is exceeded and coarse precipitates are formed. Therefore, the content thereof is preferably limited to 0.05% or less, more preferably 0.04% or less.

On the other hand, vanadium, like niobium, forms a carbonitride and is added in an amount of 0.05% or more as an element that serves to limit the grain boundary movement of austenite and ferrite. However, since the carbonitride acts as a breaking point, it may lower the impact toughness, particularly, the impact resistance at low temperature, so it is preferable to keep the solubility limit. Therefore, the content thereof is preferably limited to 0.5% or less, more preferably 0.4% or less.

The balance other than the alloy composition is iron (Fe). In addition, the non-cored wire of the present invention may include other impurities that may normally be included in the industrial production process of steel. These impurities can be known to anyone with ordinary knowledge in the art to which the present invention belongs, so that the kind and content of the impurities are not specifically limited in the present invention.

However, since Ti corresponds to a typical impurity whose content is to be suppressed as much as possible in order to obtain the effect of the present invention, a brief description thereof will be given below.

Ti: 0.005% or less

Titanium is a carbonitride-forming element and forms carbonitride at temperatures higher than Nb and V. Therefore, if titanium is included in the steel, it may be advantageous to fix C and N, but Nb and / or V is precipitated using Ti carbonitride as nuclei, so that a large amount of coarse carbonitride is formed in the matrix, have. Therefore, it is important to manage the upper limit. In the present invention, the upper limit of the content of titanium is preferably controlled to 0.005%, more preferably 0.004%.

The non-cored wire of the present invention comprises carbonitride containing Nb and / or V, and the average circular equivalent diameter of the carbonitride is preferably 70 nm or less. If the average circle-equivalent diameter of the carbonitride exceeds 70 nm, there is a fear that these carbonitrides act as starting points of fracture in the center segregation part. Here, the carbonitride means a precipitate containing carbon and / or nitrogen.

It is also preferable that the number of carbonitrides having an average circle equivalent diameter of 80 nm or more in the carbonitride containing Nb and / or V is 5 / μm ² or less per unit area. If the number of carbonitrides having an average circle equivalent diameter of 80 nm or more per unit area is more than 5 pieces / μm ² , it may be difficult to secure the desired strength and impact toughness.

On the other hand, in the present invention, the method of measuring the average circle equivalent diameter of carbonitride containing Nb and / or V is not particularly limited, but the following method can be used. That is, after cutting the non-flattened wire in the direction perpendicular to the longitudinal direction, it is cut at a 1 / 4d position (where d is the diameter of the non-cored wire) using a scanning electron microscope (FE-SEM, Field Emission Scanning Electron Microscope) ), And the composition of each precipitate was analyzed by using an electron probe micro-analyzer (EPMA). The types of the precipitates were classified and analyzed to determine Nb and / or V And the number of coarse carbonitrides having an average circle equivalent diameter of 80 nm or more can be calculated.

According to one example, the non-cored wire of the present invention may include ferrite and pearlite as its microstructure, more preferably at least 30% (excluding 100%) of ferrite ferrite) and 70% or less (excluding 0%) pearlite. When such a structure as described above is ensured, there is an advantage that strength after impact drawing can be ensured along with ensuring excellent impact toughness.

According to an example, the average particle diameter of the ferrite may be 3 to 25 탆, and more preferably 5 to 15 탆. If the average particle diameter of the ferrite is less than 3 탆, the strength may increase due to grain refinement, which may reduce the cold workability. On the other hand, when the average grain size exceeds 25 탆, the strength may decrease. On the other hand, the average particle diameter of the pearlite formed together is not particularly limited because it is influenced by the average particle diameter of the ferrite. Here, the average particle diameter means an equivalent circular diameter of the particles detected by observing one longitudinal end face of the wire.

According to one example, the product of the tensile strength (MPa) and impact toughness (J) of the non-cored wire of the present invention is 120,000 MPa · J Or more.

The non-cored wire of the present invention described above can be manufactured by various methods, and the manufacturing method thereof is not particularly limited. However, as an embodiment, it can be produced by the following method.

Hereinafter, a method of producing a non-woven wire having excellent strength and impact toughness, which is another aspect of the present invention, will be described in detail.

First, a bloom satisfying the above-mentioned component system is heated and then rolled to obtain a billet.

The heating temperature of the bloom is preferably 1200 to 1300 deg. C, more preferably 1220 to 1280 deg. When the heating temperature of the bloom is less than 1200 ° C, there is a fear that the hot deformation resistance increases. On the other hand, when the heating temperature exceeds 1300 ° C, the ductility may deteriorate due to the coarsening of the austenite.

According to one example, during heating of the bloom, the holding time at the heating temperature may be 4 hours or more. If the holding time is less than 4 hours, the homogenization treatment may not be sufficient. On the other hand, the longer the holding time at the heating temperature is, the more advantageous it is for the homogenization and the reduction of the segregation is advantageous. In the present invention, the upper limit of the holding time is not particularly limited.

Next, after the billet is reheated, the billet is rolled to obtain a non-tempered wire rod.

When reheating the billet, it is preferable to control the reheating temperature so as to satisfy the following relational expression (1).

[Relation 1] T _X ? T _H ? T _X + 80

The reason for reheating the billets under the above conditions is to reuse the carbonitride formed by Nb and / or V in the component system in the base material. When the carbonitride formed by Nb and / or V remains in the non-dissolved state in the heating furnace, it is difficult to refine the grain in the subsequent wire rolling process due to continuous coarsening at high temperature, Lt; / RTI >

On the other hand, it is known that the reuse temperature of carbonitride depends on the content of C in the case of Nb-based carbonitrides and greatly depends on the content of N in the case of V-based carbonitrides. Therefore, in order to reuse the carbonitride formed by Nb and / or V in the reheating step of the billet in the base material and precipitate as fine precipitates in the process after the wire rolling, it is necessary to consider the limit of solidification of Nb and V- It is desirable to design the reheating temperature of the reheating apparatus. The present inventors have found through numerous experiments on the solubility limits of Nb and V-carbonitride that the Nb and / or V-carbonitride is completely reused under the condition of the above-mentioned relational expression 1 during reheating of the billets. When the reheating temperature (T _H ) of the billet of the above-mentioned formula (1) is less than T _X ° C, the Nb and V coarse precipitates are not completely reused, whereas when exceeding T _X + 80 ° C, the austenite structure is excessively grown The possibility is high and it is not preferable.

According to one example, the holding time at the reheating temperature during reheating of the billet may be 60 to 240 minutes or more. If the holding time is less than 60 minutes, the homogenization process may not be sufficient. On the other hand, the longer the holding time at the reheating temperature is, the more favorable the homogenization of the segregation promoting elements, but the austenite structure is excessively grown and the ductility is lowered, and the upper limit of the holding time can be limited to 240 minutes.

When the wire is rolled, the finishing rolling temperature is preferably Ae3 to (Ae3 + 50) deg. If the finish rolling temperature is lower than Ae3, there is a fear that the deformation resistance increases due to the increase in strength due to refinement of the ferrite grain size. On the other hand, when the finish rolling temperature exceeds Ae3 + 50 deg. C, the ferrite grains become too coarse and the toughness may decrease.

Thereafter, the non-cored wire is wound and cooled.

The winding temperature of the non-flattened wire may be 750 to 900 캜, more preferably 800 to 850 캜. If the coiling temperature is less than 750 占 폚, the martensite at the surface layer during cooling may not be recovered by the double refraction, and burnt martensite may be generated to form a hard and soft steel, which may lower the cold workability. On the other hand, when the coiling temperature exceeds 900 ° C, a thick scale is formed on the surface of the coater so that troubles on descaling may easily occur, and the cooling time may be prolonged, which may lower productivity.

The cooling rate during cooling of the non-flattened wire can be 0.1 to 1 占 폚 / sec, preferably 0.3 to 0.8 占 폚 / sec or less. If the cooling rate is less than 0.1 ° C / sec, the lamellar spacing of the pearlite structure may widen and the ductility may be insufficient. If the cooling rate exceeds 1 ° C / sec, the low temperature structure So that the strength of the steel is excessively increased and the toughness may be rapidly lowered.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.

( Example )

A bloom having an alloy composition as shown in the following Table 1 was heated at 1250 占 폚 for 4 hours and then subjected to rolling at a finish rolling temperature of 1150 占 폚 to obtain a billet. Thereafter, the billet was reheated at the reheating temperature shown in Table 2 for 3 hours, and then subjected to wire rolling under the finish rolling conditions shown in Table 2 below to produce a non-tempered wire (20 mm in diameter). Thereafter, the film was wound at a temperature of 800 DEG C and then cooled at a rate of 0.5 DEG C / sec.

Then, the types and fractions of microstructures of the wire rods cooled using a scanning electron microscope, the types and sizes of the precipitates were analyzed and measured. The results are shown in Table 3 below.

Then, the tensile strength at room temperature and the impact at room temperature of the cooled wire rod were measured, and the results are shown in Table 3 below. The tensile strength at room temperature was measured at 25 ° C at the center of the non-tempered steel specimen. The impact at room temperature was evaluated by Charpy impact energy obtained by Charpy impact test of the specimen having U notch at 25 ° C.

Steel grade Alloy composition (% by weight) C Si Mn P S Cr Al Nb V N Ti Inventive Steel 1 0.16 0.21 1.56 0.010 0.0035 - 0.02 0.020 0.07 0.0052 0.003 Invention river 2 0.18 0.18 1.33 0.011 0.0045 - 0.03 0.006 0.21 0.0047 0.001 Invention steel 3 0.21 0.22 1.64 0.012 0.0055 0.35 0.03 0.015 - 0.0040 0.002 Inventive Steel 4 0.24 0.15 1.41 0.009 0.0052 0.15 0.04 0.009 0.10 0.0054 0.003 Invention steel 5 0.26 0.20 1.37 0.011 0.0047 - 0.02 0.011 0.09 0.0056 0.002 Invention steel 6 0.29 0.17 1.20 0.010 0.0042 0.11 0.03 - 0.15 0.0042 0.001 Invention steel 7 0.32 0.21 1.31 0.011 0.0064 - 0.03 0.010 0.06 0.0037 0.003 Inventive Steel 8 0.34 0.19 1.15 0.010 0.0053 - 0.02 - 0.12 0.0058 0.001 Comparative River 1 0.15 0.12 1.54 0.011 0.0045 - 0.03 0.035 0.05 0.0054 0.004 Comparative River 2 0.17 0.15 1.33 0.011 0.0053 - 0.04 0.005 0.24 0.0058 0.005 Comparative Steel 3 0.19 0.18 1.45 0.010 0.0055 0.42 0.02 0.031 - 0.0043 0.009 Comparative Steel 4 0.22 0.16 1.28 0.010 0.0044 0.17 0.03 0.007 0.19 0.0048 0.011 Comparative Steel 5 0.25 0.20 1.34 0.011 0.0041 - 0.03 0.025 0.06 0.0062 0.009 Comparative Steel 6 0.28 0.13 1.01 0.012 0.0052 0.11 0.02 - 0.21 0.0042 0.007 Comparative Steel 7 0.31 0.19 1.15 0.009 0.0064 - 0.03 0.022 0.06 0.0051 0.015 Comparative Steel 8 0.35 0.11 1.05 0.010 0.0048 - 0.03 - 0.15 0.0057 0.012

Steel grade T ₁ T ₂ Reheat temperature
(° C) Ae3
(° C) Wrap-up
Rolling temperature
(° C) Remarks Inventive Steel 1 1131.2 959.8 1153 818.8 832 Inventory 1 Invention river 2 999.3 1050.5 1087 828.5 853 Inventory 2 Invention steel 3 1129.2 860.7 1155 799.9 827 Inventory 3 Inventive Steel 4 1080.5 992.8 1102 803.2 824 Honorable 4 Invention steel 5 1116.7 986.7 1117 800.4 825 Inventory 5 Invention steel 6 863.5 1007.1 1047 799.6 821 Inventory 6 Invention steel 7 1131.2 926.9 1146 789.2 814 Honorable 7 Inventive Steel 8 863.5 1016.5 1051 788.7 816 Honors 8 Comparative River 1 1194.3 938.7 1102 820.1 935 Comparative Example 1 Comparative River 2 976.2 1083.1 1043 833.1 897 Comparative Example 2 Comparative Steel 3 1209.0 860.7 1115 806.3 842 Comparative Example 3 Comparative Steel 4 1038.7 1042.8 1048 814.9 875 Comparative Example 4 Comparative Steel 5 1216.6 961.5 1124 803.5 902 Comparative Example 5 Comparative Steel 6 1100.3 1034.8 1043 807.5 886 Comparative Example 6 Comparative Steel 7 1227.9 947.1 1131 794.2 835 Comparative Example 7 Comparative Steel 8 863.5 1036.5 1055 788.8 863 Comparative Example 8

Steel grade Microstructure type and fraction
(area%) Ferrite average particle diameter
(μm) Precipitates Average circle equivalent diameter
(μm) Number of coarse carbonates per unit area
(Pieces / μm ² ) TS
(MPa) vE 25 ° C
(J) TS · vE 25 ° C
(MPa · J) Remarks Inventive Steel 1 F55.4 + P44.6 5.69 0.21 1.4 689.6 227 156,539 Inventory 1 Invention river 2 F52.7 + P47.3 5.81 0.24 1.7 749.8 198 148,460 Inventory 2 Invention steel 3 F50.3 + P49.7 6.25 0.37 2.3 684.3 208 142,334 Inventory 3 Inventive Steel 4 F47.2 + P52.8 5.94 0.28 2.5 730 185 135,050 Honorable 4 Invention steel 5 F44.1 + P55.9 7.29 0.63 2.8 736.1 169 124,401 Inventory 5 Invention steel 6 F40.5 + P59.5 6.37 0.45 3.6 758.8 165 125,202 Inventory 6 Invention steel 7 F36.8 + P63.2 6.12 0.42 4.2 750.5 171 128,336 Honorable 7 Inventive Steel 8 F33.3 + P66.7 6.45 0.48 4.7 765.4 159 121,699 Honors 8 Comparative River 1 F56.7 + P43.3 8.64 0.71 6.2 694.7 164 113,931 Comparative Example 1 Comparative River 2 F52.2 + P47.8 8.72 0.74 6.5 761.1 144 109,598 Comparative Example 2 Comparative Steel 3 F51.5 + P48.5 9.25 0.81 7.3 683.4 153 104,560 Comparative Example 3 Comparative Steel 4 F49.7 + P50.3 7.34 0.46 7.6 765.9 133 101,865 Comparative Example 4 Comparative Steel 5 F44.6 + P55.4 9.68 0.87 8.1 737.1 118 86,978 Comparative Example 5 Comparative Steel 6 F41.8 + P58.2 10.62 0.98 8.5 786.9 98 77,116 Comparative Example 6 Comparative Steel 7 F36.4 + P63.6 11.74 1.06 9.2 750.4 111 83,294 Comparative Example 7 Comparative Steel 8 F32.1 + P67.9 7.59 0.54 10.3 777.2 116 90,155 Comparative Example 8

As can be seen from Table 3, in Inventive Examples 1 to 8, which satisfy the alloy composition and manufacturing conditions proposed in the present invention, the tensile strength and the impact toughness multiplied by 120,000 (MPa · J) Is very excellent. On the other hand, in Comparative Examples 1 to 8, the tensile strength and / or the impact toughness of the alloy composition and / or the manufacturing conditions are deviated from the range suggested by the present invention.

Claims

The steel sheet according to any one of claims 1 to 3, which comprises 0.15 to 0.35% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.5 to 2.0% of Cr, 0.02% or less of P, 0.005 to 0.05% of Nb and 0.05 to 0.5% of V, the balance Fe and unavoidable impurities,
Nb and / or V, wherein the carbonitride has an average circle equivalent diameter of 5 to 70 nm,
Wherein the number of carbonitride in the carbonitride having an average circle equivalent diameter of 80 nm or more per unit area is 5 / μm ² or less.

The method according to claim 1,
Wherein the unavoidable impurities include Ti and are suppressed to Ti of 0.005% or less by weight.

The method according to claim 1,
Noncorrosive wire containing microstructures of ferrite and pearlite.

The method according to claim 1,
A non-temperate wire comprising microstructures comprising ferrite of 30% or more (excluding 100% area%) and pearlite of 70% or less (excluding 0% area%).

The method according to claim 3 or 4,
Wherein the ferrite has an average grain size of 3 to 25 占 퐉.

The steel sheet according to any one of claims 1 to 3, which comprises 0.15 to 0.35% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.5 to 2.0% of Cr, 0.02% or less of P, %, N: 0.01% or less, comprising at least one of Nb: 0.005 to 0.05% and V: 0.05 to 0.5%, and the balance Fe and unavoidable impurities;
Reheating the billet to a reheating temperature (T _H ) satisfying the following relational expression 1;
Subjecting the reheated billet to a wire rolling process at a finishing rolling temperature of Ae3 to (Ae3 + 50) DEG C to obtain a wire rod; And
Winding the wire rod and cooling the wire rod at a rate of 0.1 to 1 占 폚 / sec;
Of the non-flattened wire.
[Relation 1] T _X ? T _H ? T _X + 80
(Case where, T ₁ ≥T ₂ _X is T ₁ and T il, T _2> T is _1, _X T is T _2, T ₁ is "833.5 + 447.9x {1-exp (- [Nb] [C ] /0.01799)} + 226.5x {1-exp (- [Nb] [C] /0.00163)} and T ₂ is 860.7 + 348.1x { } + 199.6x {1-exp (- [V] [N] /0.00064)})

The method according to claim 6,
Wherein the unavoidable impurities include Ti and are suppressed to Ti of 0.005% or less by weight.

The method according to claim 6,
Wherein preparing the billet comprises:
The steel sheet according to any one of claims 1 to 3, which comprises 0.15 to 0.35% of C, 0.05 to 0.3% of Si, 1.0 to 2.0% of Mn, 0.5 to 2.0% of Cr, 0.02% or less of P, % Of N and 0.01-0.0% of N, 0.005 to 0.05% of Nb and 0.05 to 0.5% of V, and the balance Fe and inevitable impurities at 1200 to 1300 占 폚 Heating; And
And rolling the heated bloom to obtain a billet.

9. The method of claim 8,
Wherein the bloom is heated at a heating temperature of not lower than 4 hours.

The method according to claim 6,
Wherein the holding time at the reheating temperature during reheating of the billet is 60 to 240 minutes.

The method according to claim 6,
Wherein the coiling temperature is 750 to 900 占 폚 at the time of winding.