KR101665886B1 - Non-quenched and tempered steel having excellent cold workability and impact toughness and method for manufacturing same - Google Patents

Abstract

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.4% of C, 0.005 to 0.5% of Si, 0.2 to 2.0% of Mn, 0.03% or less of P, 0.007 to 0.06% of S, 0.0056 to 0.02% of B, %, Ca: 0.0005 to 0.01%, the balance Fe and unavoidable impurities, and comprising a single or complex precipitate of BN and MnS, and having a P _C of 60% or more, .
[Formula 1]
P _C (%) = {N _c / (N _s + N _c )} x 100
Where N _s is the number of BN-alone precipitates per 1 mm ² existing in the cross-section perpendicular to the longitudinal direction of the non-conditioned steel, and N _c is the cross-section of the non- Means the number of complex precipitates of BN and MnS per 1 mm ² )

Description

TECHNICAL FIELD [0001] The present invention relates to a non-tempered steel having excellent cold workability and impact toughness and a method of manufacturing the same. [0002]

The present invention relates to a non-treated steel excellent in cold workability and impact toughness and a method for producing the same.

Non - tempered steel, which can reduce costs by cutting the heat treatment process, has been developed for the first time in Europe in the early 1970s, and has been spreading rapidly all over the world because of its economic and practical effects. Currently, we are developing various steel grades in Europe and Japan and mass-producing them in Korea.

Unconditioned steel is a steel which is developed to omit the quenching-punching heat treatment to improve the strength and toughness of the final product, and can be roughly divided into thermal quenching, direct cutting, and cold pressing. Nonconditioned steels for direct cutting and non-tempering steels used for pin and bolt are produced by controlled rolling, controlled cooling and microalloying. They are used for automobile parts Non-tempered steels for heat treatment are manufactured using microalloying and component control. Recently, the development trend of untreated steel has been focused on the development of materials with excellent function in addition to the steel with process omission which omits the heat treatment and processing process. In particular, existing heat-resisting steel has a disadvantage in that productivity is very low in terms of heat treatment cost and process process, and thus efforts are being made to develop heat resisting steel for cold resisting. In order to develop a non-tempered steel for cold storage, omission of spheroidizing heat treatment is essential. Since the spheroidizing heat treatment requires a heat treatment for a long period of time of 20 hours or more, development of a material having a low deformation resistance and excellent toughness is required in view of energy saving and productivity improvement. Since the non-tempered steel for cold-tempering is secured through the drawing process, many electric potentials are generated inside the wire during the drawing process. These electric potentials react with the solid carbon (C) and solid nitrogen (N) The strength of the deformation resistance is increased to cause a reduction in die life at the time of cold forging and a problem that the toughness of the final product is disadvantageously lowered.

Therefore, the inventors of the present invention have conducted research to solve these problems, and have developed a non-tempered steel capable of omitting the spheroidizing heat treatment, having low strength of deformation resistance during cold forging and having excellent impact toughness in the final product .

Japanese Patent Application Laid-Open No. 2014-047356 Japanese Patent Application Laid-Open No. 2001-152289

An aspect of the present invention is to provide a non-treated steel capable of securing excellent cold workability and impact toughness without performing a spheroidizing annealing heat treatment and a method of manufacturing the same.

In order to achieve the above object, according to one aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: 0.05 to 0.4% of C, 0.005 to 0.5% of Si, 0.2 to 2.0% of Mn, And the balance of Fe and unavoidable impurities, and containing either single or complex precipitates of BN and MnS, and having a composition expressed by the following formula 1: And a P _C of 60% or more.

[Formula 1]

P _C (%) = {N _c / (N _s + N _c )} x 100

Where N _s is the number of BN-alone precipitates per 1 mm ² existing in the cross-section perpendicular to the longitudinal direction of the non-conditioned steel, and N _c is the cross-section of the non- Means the number of complex precipitates of BN and MnS per 1 mm ² )

Another aspect of the present invention is to provide a method of manufacturing a semiconductor device, comprising: 0.05 to 0.4% of C, 0.005 to 0.5% of Si, 0.2 to 2.0% of Mn, 0.03% or less of P, 0.007 to 0.06% of S, 0.02% of N, 0.003 to 0.02% of N, 0.0005 to 0.01% of Ca, the balance Fe and unavoidable impurities, and satisfying relational expressions 1 to 3; Reheating the billet to 1000 to 1150 占 폚 and then subjecting the billet to a wire rolling to obtain a non-tempered steel; And cooling the non-tempered steel at a rate of 1 DEG C / sec or less after winding the non-tempered steel.

[Relation 1]

10? [Mn] / [S]? 100

[Relation 2]

0.1? [Ca] / [S]? 1.0

[Relation 3]

-0.0040? [N] - [B]? 0.0020

(Wherein each of [Mn], [S], [Ca], [N] and [B] means the content (% by weight)

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a non-tempered steel which is excellent in cold workability at the same time, and which is required for cutting and toughness, even when the spheroidizing annealing heat treatment is omitted.

Hereinafter, a non-tempered steel excellent in cold workability 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-tempered steel will be described in detail.

Carbon (C): 0.05 to 0.4 wt%

Carbon improves the strength of the steel. In order to exhibit such an effect in the present invention, the content is preferably 0.05% by weight or more, more preferably 0.1% by weight or more. However, when the content is excessive, the pearlite structure is excessively formed, so that the deformation resistance of the steel is rapidly increased, thereby deteriorating the cold workability. Therefore, the upper limit of the carbon content is preferably 0.4 wt%, more preferably 0.35 wt%.

Silicon (Si): 0.005-0.5 wt%

Silicon is a useful element as a deoxidizer. In order to exhibit such effects in the present invention, the content is preferably 0.005% by weight or more, more preferably 0.01% 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. Accordingly, the upper limit of the silicon content is preferably 0.5 wt%, more preferably 0.4 wt%, and even more preferably 0.3 wt%.

Manganese (Mn): 0.2 to 2.0 wt%

Manganese is a useful element as a deoxidizer and desulfurizer. In order to exhibit such an effect in the present invention, the content is preferably at least 0.2 wt%, more preferably at least 0.3 wt%, and even more preferably at least 0.4 wt%. 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 wt%, more preferably 1.5 wt%, and even more preferably 1.0 wt%.

Phosphorus (P): 0.03% by weight or less

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 preferable to control the phosphorus content to 0 wt%, 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 0.03% by weight.

Sulfur (S): 0.007-0.06 wt%

Sulfur is generally classified as an impurity inevitably contained in the steel, but in the present invention, it is an element intentionally added to form a MnS precipitate and serve as a nucleation site of a BN precipitate. In order to exhibit such an effect in the present invention, the content is preferably 0.007% by weight or more, more preferably 0.008% by weight or more. However, when the content is excessive, there is a problem of deteriorating the toughness of the steel segregated in the grain boundaries and forming a coarse emulsion to deteriorate the delayed fracture resistance and stress relaxation characteristics. Therefore, the upper limit of the sulfur content is preferably 0.06 wt%, more preferably 0.05 wt%, and even more preferably 0.04 wt%.

Boron (B): 0.0056-0.02 wt%

Boron is an element that plays a role in fixing solid nitrogen by BN formation. Particularly, in the present invention, by using MnS precipitates as nuclei for BN precipitation, it is a technical feature that the content of boron which can be used as a fixed element of dissolved nitrogen is greatly increased. In particular, in view of the fact that the average N content added to the steel is about 0.004% by weight, it is more preferable that the boron content is 0.0056% by weight or more for minimizing the content of solid N in the present invention. However, if the content is excessive, BN precipitates may form at the grain boundaries, which may adversely affect the ductility of the steel. Therefore, the upper limit of the boron content is preferably 0.02 wt%, more preferably 0.015 wt%, and even more preferably 0.01 wt%.

Nitrogen (N): 0.003 to 0.02 wt%

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. In theory, it is preferable to control the content of nitrogen to 0 wt%, but it is inevitably contained in the manufacturing process normally, and is usually contained in an amount of 0.003 wt% or more. Therefore, it is important to manage the upper limit. In the present invention, the upper limit of the nitrogen content is preferably controlled to 0.02 wt%, more preferably 0.015 wt%, more preferably 0.01 wt% desirable.

Calcium (Ca): 0.0005 to 0.01 wt%

In the present invention, in order to improve the cold simple composition, in order to fix the solute N, the content of B was increased and the shape and distribution of MnS were adjusted by calcium addition in order to improve impact toughness. Calcium is an element that controls the formation of inclusions in steel and is effective in improving toughness and ductility. In the present invention, B is used as an element for fixing employment N, and MnS is used as a nucleation site of BN in order to greatly increase the amount of boron used. However, when the content of S in the steel is increased to utilize MnS, a large amount of MnS inclusions are known to be detrimental to the toughness and ductility of the steel after being rolled in the rolling direction. Therefore, in the present invention, calcium is used to solve the problem of toughness deterioration due to MnS formation. The addition of calcium in the steel serves to make the shape of the MnS inclusions spherical, and further improvement of the impact toughness can be expected. In order to exhibit such an effect in the present invention, the content is preferably 0.0005% by weight or more, more preferably 0.001% by weight or more, and still more preferably 0.0015% by weight or more. However, if the content is excessive, the calcium inclusion may act as a crack initiation point, so that the upper limit of the calcium content is preferably 0.01 wt%, more preferably 0.008 wt%.

The balance other than the alloy composition is iron (Fe). In addition, the non-nitrided steel 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, Nb and V correspond to representative impurities whose content should be suppressed as much as possible in order to obtain the effect of the present invention, a brief description thereof will be given below.

Titanium (Ti): 0.01 wt% or less

Titanium is a carbonitride-forming element that forms carbonitride at temperatures higher than B. Therefore, if titanium is included in the steel, it may be advantageous to fix C and N, but Ti (C, N) may precipitate at a high temperature to deteriorate the cold toughness. Therefore, it is important to manage the upper limit. In the present invention, the upper limit of the titanium content is preferably controlled to 0.01 wt%, and more preferably 0.008 wt%.

Niobium (Nb) and vanadium (V): Not more than 0.05% in total

Niobium and vanadium also form a carbonitride-forming element, but carbonitride is formed at a temperature lower than B. In the present invention, the fixing effect of N is insufficient even when these elements are added. However, if these contents are excessive, the precipitation strengthening or fine grain refinement due to the formation of fine carbonitride during cooling may increase the strength of the steel more than necessary, thereby deteriorating the cold-rolled steel composition. Therefore, it is important to manage the upper limit, and in the present invention, it is preferable that the upper limit of the sum of the contents of niobium and vanadium is controlled to 0.05 wt%.

According to one embodiment of the present invention, when designing an alloy of a steel material having the above-described composition range, it is preferable to control the content of Mn and S so as to satisfy the following relational expression (1).

[Relation 1]

10? [Mn] / [S]? 100

The inventors of the present invention have found out that, in the present invention in which B and N are added in excess of the usual levels, when a proper amount of MnS precipitate is formed by appropriately controlling the contents of Mn and S, most of the BN precipitates are composed of MnS precipitates as nuclei Precipitates to form BN and MnS complex precipitates, and the complex precipitates as described above are formed in the mouth instead of in the grain boundary, and thus it is possible to secure a good cold step composition. If the value of [Mn] / [S] is too low, sufficient MnS precipitates can not be secured, and a large amount of BN precipitates are formed in the grain boundaries, thereby reducing the ductility. Therefore, the lower limit of the value of [Mn] / [S] is preferably 10, more preferably 15, even more preferably 20. On the other hand, when the value of [Mn] / [S] is excessively high, MnS precipitates are formed to a great extent and mechanical properties of the steel may be deteriorated. Therefore, the upper limit of the value of [Mn] / [S] is preferably 100, more preferably 95.

According to one embodiment of the present invention, in designing an alloy of a steel material having the above-described composition range, it is preferable to control the content of Ca and S so as to satisfy the following relational expression (2).

[Relation 2]

0.1? [Ca] / [S]? 1.0

(Where each of [Ca] and [S] means the content (% by weight) of the corresponding element)

In the present invention, MnS was used as a nucleation site of BN, and most of the solid solution N was fixed and a simple cooling composition could be secured. However, when MnS is formed by using the content of S more than the normal level, a large number of MnS elongated in the rolling direction is generated, thereby reducing impact toughness. Therefore, Ca is used as an element to control the shape of MnS in order to ensure excellent impact toughness in addition to the cold end composition. Ca is an element which makes highly stable compounds at high temperatures such as CaO and CaS in the steel, and acts as a nucleation site of MnS to make the shape of the drawn MnS spherical, which can lead to improvement of impact toughness. If the value of [Ca] / [S] is too low, there is no improvement in the distribution of MnS and if the value of [Ca] / [S] is excessively high, the toughness due to the inclusion increase may be drastically lowered. Therefore, the lower limit of the value of [Ca] / [S] is preferably 0.1, and the upper limit of the value of [Ca] / [S] is preferably 1.0.

According to one embodiment of the present invention, when designing an alloy of a steel material having the above-described composition range, it is preferable to control the content of N and B so as to satisfy the following relational expression (3).

[Relation 3]

-0.0040? [N] - [B]? 0.0020

(Where each of [N] and [B] represents the content (weight%) of the corresponding element)

If, when the nitrogen content of the steel than the boron content is too excessive, there is a danger of the remaining boron after forming BN precipitates degrade the mechanical properties by forming a precipitate ₆ Fe ₂₃ (C, B) in the grain boundary. Therefore, the lower limit of the value of [N] - [B] is preferably -0.0040, more preferably -0.0030, even more preferably -0.0020. On the other hand, if the nitrogen content in the steel is too high compared to the boron content in the steel, the nitrogen fixation effect by boron is insufficient, which may result in deterioration of the cold simple composition. Therefore, the upper limit of the value of [N] - [B] is preferably 0.0020, more preferably 0.0010.

According to an embodiment of the present invention, the non-tempered steel may include BN and MnS alone or a complex precipitate, and the P _C defined by the following formula 1 may be 60% or more. If the P _C is less than 60%, the ductility of the steel may be deteriorated due to the grain boundary formation of the excessive BN precipitates. On the other hand, the larger P is advantageous for obtaining the effect of the present invention, the upper limit of P _C is not particularly limited.

[Formula 1]

P _C (%) = {N _c / (N _s + N _c )} x 100

Where N _s is the number of BN-alone precipitates per 1 mm ² existing in the cross-section perpendicular to the longitudinal direction of the non-conditioned steel, and N _c is the cross-section of the non- Quot; means the number of complex precipitates of BN and MnS per 1 mm < ² >).

In the present invention, the method of measuring the number of BN or MnS alone or the number of complex precipitates is not particularly limited, but the following method can be used, for example. That is, after cutting the non-tempered steel in a direction perpendicular to the longitudinal direction, it is cut at a R / 2 position (where R means the radius of the non-tempered steel) using a scanning electron microscope (FE-SEM, Field Emission Scanning Electron Microscope) Sectional photographs are taken at a magnification of 1,000, and the number of the total precipitates can be calculated by analyzing them. The composition of each precipitate is analyzed by using an electron probe micro-analyzer (EPMA) .

According to an embodiment of the present invention, the average aspect ratio of the MnS precipitates may be 10 or less.

If the average aspect ratio of the MnS precipitates is excessively high, a large number of MnS elongated in the rolling direction may be generated to deteriorate the impact toughness of the non-tempered steel. Therefore, the average aspect ratio of the MnS precipitates is preferably 10 or less, more preferably 9 or less. Here, the aspect ratio means the ratio of the major axis length to the minor axis length (long axis length / minor axis length) of the MnS precipitate.

In the present invention, the method of measuring the average aspect ratio of the MnS precipitates is not particularly limited, but the following method can be used, for example. That is, the non-treated steel was cut in the direction parallel to the longitudinal direction through the center of the untreated steel, and then D / 8 and D / 4 positions were measured using a scanning electron microscope (FE-SEM, Field Emission Scanning Electron Microscope) Sectional photographs were taken at one D / 2 position (where D is the diameter of the non-precipitated steel) at two magnifications and at a magnification of 1,000, and the aspect ratios of the MnS precipitates were obtained. The aspect ratio can be used.

According to an embodiment of the present invention, the non-tempered steel of the present invention may include ferrite and pearlite as its microstructure, more preferably at least 30% (excluding 100%) in area fraction, Of ferrite and 70% or less (excluding 0%) of pearlite. When the above-described structure is secured, excellent cold workability can be ensured, and strength and toughness can be secured after proper drawing processing.

Also, according to one embodiment of the present invention, the average particle diameter of the ferrite may be 8 to 30 탆, and more preferably 15 to 25 탆. If the average particle diameter of the ferrite is less than 8 탆, the strength of the ferrite may be increased due to grain refinement, which may cause a decrease in cold-rolled steel composition. 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 end face of the steel sheet.

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

Hereinafter, a method for producing a non-refined steel of the present invention, which is another aspect of the present invention, will be described in detail.

First, a billet satisfying the component system is prepared.

In the present invention, the step of preparing a billet is not particularly limited, but a bloom satisfying the above-mentioned component system is heated to 1150 to 1300 ° C, then rolled into a billet, Can be prepared. This is to reduce the size of coarsened MnS in the casting process by rolling some of the MnS by heating and then rolling the steel.

The heating temperature of the bloom is preferably 1150 to 1300 ° C, and more preferably 1200 to 1250 ° C. If the heating temperature of the bloom is less than 1150 ° C, the coarse MnS produced during the casting process may not decompose and may not be sufficient to serve as a nucleation site for a large number of BNs, The toughness may deteriorate due to coarsening of the austenite.

When rolling a billet of a heated bloom, the finish rolling temperature may be 1000 to 1200 캜, and preferably 1050 to 1150 캜. If the finish rolling temperature is less than 1000 ° C, there is a fear that the thermal deformation resistance is increased and the productivity is lowered. On the other hand, when the finish rolling temperature is higher than 1200 ° C, the ductility may decrease due to the coarsening of the ferrite grains.

Next, the billet is heated again to obtain non-tempered steel. This step is carried out to inhibit the decomposition of BN formed using MnS as nuclei.

The reheating temperature of the billet is preferably 1000 to 1150 캜, and more preferably 1050 to 1100 캜. If the reheating temperature of the billet is less than 1000 ° C, there is a fear that the resistance to hot deformation increases. On the other hand, if the billet temperature exceeds 1150 ° C, the BN produced using MnS as nuclei may be decomposed.

In billet rolling, the finishing rolling temperature may be 900 to 1000 캜, preferably 920 to 980 캜.

The present invention is characterized in that the finishing rolling temperature is controlled to be somewhat higher at the time of rolling the wire, in order to overcome the deterioration of the hot rolling property due to the formation of a large amount of MnS. For this purpose, it is necessary to control the finishing rolling temperature to 900 DEG C or higher. However, if the finishing rolling temperature is too high, the ferrite grains may become excessively coarse and the ductility may deteriorate. The finishing rolling temperature needs to be controlled to 1000 캜 or lower.

Thereafter, the uncorrugated steel is wound and cooled.

The coiling temperature of the non-refined steel may be 750 to 950 占 폚, more preferably 780 to 900 占 폚, and still more preferably 780 to 850 占 폚. 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 950 deg. C, a thick scale is formed on the surface of the coiling, so that troubles on descaling may easily occur, and the cooling time may be prolonged, thereby lowering the productivity.

The cooling rate during cooling of the non-tempered steel may be 1 ° C / sec or less, preferably 0.8 ° C / sec or less, and more preferably 0.5 ° C / sec or less. In order to stably form ferrite and pearlite composite structure, if the cooling rate exceeds 1 캜 / sec, a low-temperature structure may be formed to deteriorate the cold-rolled steel composition. On the other hand, in the present invention, the lower limit of the cooling rate during the cooling is not particularly limited. However, considering productivity, it may be limited to 0.1 占 폚 / sec or more.

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 Table 1 below was heated at 1200 DEG C for 4 hours and then subjected to rolling at a finishing rolling temperature of 1050 DEG C to obtain a billet. Thereafter, the billet was heated at 1150 占 폚 for 3 hours, and hot-rolled at a wire diameter of 20 mm to prepare a non-tempered steel. At this time, the finish rolling temperature was 950 占 폚, followed by winding at a temperature of 850 占 폚 and cooling at a rate of 0.5 占 폚 / sec.

Then, types and fractions of microstructures, types and numbers of precipitates and the like were analyzed and measured using an electron microscope, and the results are shown in Table 2 below.

Then, the tensile strength at room temperature and the impact at room temperature were measured, and the results are shown in Table 2 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.

Then, the cold workability was evaluated, and the results are shown in Table 2 below. For the cold workability, the notched specimens were subjected to a compressive test of true deformation 0.7 to evaluate the presence of cracks. The results were evaluated as "GO" when cracks did not occur and "NG" when cracks occurred.

Steel grade Alloy composition (% by weight) ① ② ③ C Si Mn P S Ca B N Inventive Steel 1 0.14 0.16 0.76 0.010 0.008 0.0018 0.0065 0.0052 91 0.22 -0.0013 Invention river 2 0.15 0.20 0.67 0.011 0.024 0.0026 0.0060 0.0054 28 0.11 -0.0006 Invention steel 3 0.16 0.20 0.74 0.010 0.016 0.0045 0.0062 0.0058 46 0.28 -0.0004 Inventive Steel 4 0.14 0.18 0.74 0.009 0.012 0.0074 0.0064 0.0065 62 0.62 0.0001 Invention steel 5 0.24 0.20 0.69 0.010 0.018 0.0042 0.0061 0.0062 38 0.23 0.0001 Invention steel 6 0.33 0.18 0.64 0.010 0.020 0.0038 0.0065 0.0056 32 0.19 -0.0009 Comparative River 1 0.17 0.21 0.75 0.009 0.002 0.0001 0.0064 0.0055 375 0.05 -0.0009 Comparative River 2 0.25 0.19 0.68 0.010 0.004 0.0002 0.0066 0.0053 170 0.05 -0.0013 Comparative Steel 3 0.33 0.19 0.63 0.009 0.003 0.0002 0.0067 0.0064 210 0.07 -0.0003 Comparative Steel 4 0.15 0.17 0.74 0.009 0.017 0.0001 0.0063 0.0060 44 0.01 -0.0003 Comparative Steel 5 0.23 0.19 0.67 0.011 0.016 0.0001 0.0068 0.0058 42 0.01 -0.0010 Comparative Steel 6 0.34 0.17 0.62 0.010 0.017 0.0001 0.0069 0.0054 37 0.01 -0.0015 [Mn], [S], [N], and [B], respectively, where 1 = [Mn] / [S], 2 = [Ca] / [S] Means the content (weight%) of the element.

Steel grade Microstructure Ferrite fraction (%) Ferrite average particle diameter
(탆) P _C (%) The average aspect ratio of the MnS precipitates The tensile strength
(MPa) Impact value
(J) Cold
Processability Remarks Inventive Steel 1 F + P 75% 22 63 8.8 426 150 G0 Inventory 1 Invention river 2 F + P 74% 18 84 7.2 432 153 G0 Inventory 2 Invention steel 3 F + P 72% 19 69 5.7 441 156 G0 Inventory 3 Inventive Steel 4 F + P 75% 21 66 4.3 425 160 G0 Honorable 4 Invention steel 5 F + P 56% 15 74 6.0 490 149 G0 Inventory 5 Invention steel 6 F + P 34% 13 76 6.4 559 138 G0 Inventory 6 Comparative River 1 F + P 72% 19 12 6.1 450 102 NG Comparative Example 1 Comparative River 2 F + P 57% 16 10 5.8 495 86 NG Comparative Example 2 Comparative Steel 3 F + P 34% 13 15 6.5 544 72 NG Comparative Example 3 Comparative Steel 4 F + P 74% 18 70 11.3 431 118 G0 Comparative Example 4 Comparative Steel 5 F + P 60% 15 67 12.7 480 106 G0 Comparative Example 5 Comparative Steel 6 F + P 36% 12 72 12.2 549 95 G0 Comparative Example 6 Of the microstructures, F means ferrite and P means pearlite.

Thereafter, tensile strength at room temperature (25 ° C) and impact at room temperature were measured for each of the manufactured steel wires by applying 10%, 20%, and 30% The formability was evaluated and is shown in Table 3 below. At this time, the method of measuring the impact at room temperature and the cold workability evaluation method are as described above.

Steel grade Tensile Strength (MPa) Impact value (J) Cold workability Remarks 10% 20% 30% 10% 20% 30% 10% 20% 30% Inventive Steel 1 504 523 529 130 126 125 G0 G0 G0 Inventory 1 Invention river 2 511 532 538 132 124 127 G0 G0 G0 Inventory 2 Invention steel 3 520 538 544 135 129 123 G0 G0 G0 Inventory 3 Inventive Steel 4 504 522 529 142 136 132 G0 G0 G0 Honorable 4 Invention steel 5 568 588 594 129 121 122 G0 G0 G0 Inventory 5 Invention steel 6 638 656 663 120 113 115 G0 G0 G0 Inventory 6 Comparative River 1 - - - - - - NG NG NG Comparative Example 1 Comparative River 2 - - - - - - NG NG NG Comparative Example 2 Comparative Steel 3 - - - - - - NG NG NG Comparative Example 3 Comparative Steel 4 511 530 536 101 92 90 G0 G0 G0 Comparative Example 4 Comparative Steel 5 562 584 589 96 85 83 G0 G0 G0 Comparative Example 5 Comparative Steel 6 627 653 662 84 78 72 G0 G0 G0 Comparative Example 6

Inventive steels 1 to 6 in Table 1 were intentionally doped with a large amount of S in order to form MnS precipitates and act as nucleation sites of BN precipitates, Ca was added for controlling the shape of MnS, and N content was adjusted to a normal level , Or B is added at a level higher than the normal level while being added at a level higher than the normal level. In addition, comparative steels 1 to 3 are Ca-untreated steels in which S and N are added at a normal level of about 50 ppm and B is added at a level higher than the normal level. The comparative steels 4 to 6 are those in which a large amount of S is added to the Ca-free steel and the content of B is higher than the normal level while N is added to the normal level.

In the case of Invention steels 1 to 6, the alloy composition and the manufacturing conditions proposed in the present invention are all satisfied, and the composite precipitates of MnS and BN are used to form solid solution N It can be seen that the excellent cooling and simple composition is shown even after the drawing process due to the fixing effect. Also, it can be seen that excellent impact toughness is exhibited by controlling the average aspect ratio of MnS precipitates through Ca addition.

On the other hand, comparative steels 1 to 3 are obtained by adding B and N higher than normal levels while controlling S to the invention steel at a low level. MnS as a nucleation site of BN is insufficient and BN is precipitated at grain boundaries, A number of cracks were generated, and the additional property evaluation was meaningless. Comparative steels 4 to 6 show that when a large amount of S is added and B and N are added at a level higher than the normal level, sufficient MnS functions as a nucleation site of BN, and N is fixed without internal crack, Able to know. However, it can be seen that impact toughness is inferior due to the addition of Ca.

Claims (15)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.4% of C, 0.005 to 0.5% of Si, 0.2 to 2.0% of Mn, 0.03% or less of P, 0.007 to 0.06% of S, 0.0056 to 0.02% of B, % Of Ca, 0.0005 to 0.01% of Ca, the balance Fe and unavoidable impurities, and including Pb or MnS alone or complex precipitates, and P _C of 60% or more,
[Formula 1]
P _C (%) = {N _c / (N _s + N _c )} x 100
Where N _s is the number of BN-alone precipitates per 1 mm ² existing in the cross-section perpendicular to the longitudinal direction of the non-conditioned steel, and N _c is the cross-section of the non- Means the number of complex precipitates of BN and MnS per 1 mm ² )
The method according to claim 1,
And the average aspect ratio of the MnS precipitates is 10 or less.
The method according to claim 1,
The content of Mn and S satisfies the following relational expression (1).
[Relation 1]
10? [Mn] / [S]? 100
(Where each of [Mn] and [S] means the content (weight%) of the corresponding element)
The method according to claim 1,
The content of Ca and S satisfies the following relational expression (2).
[Relation 2]
0.1? [Ca] / [S]? 1.0
(Where each of [Ca] and [S] means the content (% by weight) of the corresponding element)
The method according to claim 1,
Wherein the content of N and B satisfies the following relational expression (3).
[Relation 3]
-0.0040? [N] - [B]? 0.0020
(Where each of [N] and [B] represents the content (weight%) of the corresponding element)
The method according to claim 1,
The unavoidable impurities include Ti, Nb and V, and are suppressed to 0.01% or less of Ti, 0.05% or less of Nb and V in total by weight%.
The method according to claim 1,
Non - tempered steels containing microstructure, including ferrite and pearlite.
The method according to claim 1,
Microstructure, non-tempered steels containing 30% or more (excluding 100% area%) ferrite and 70% or less (excluding 0% area%) pearlite.
9. The method according to claim 7 or 8,
Wherein said ferrite has an average grain size of 8 to 30 占 퐉.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.4% of C, 0.005 to 0.5% of Si, 0.2 to 2.0% of Mn, 0.03% or less of P, 0.007 to 0.06% of S, 0.0056 to 0.02% of B, %, Ca: 0.0005 to 0.01%; preparing a billet containing the remainder Fe and unavoidable impurities and satisfying relational expressions 1 to 3;
Reheating the billet to 1000 to 1150 占 폚 and then subjecting the billet to a wire rolling to obtain a non-tempered steel; And
Cooling the non-tempered steel at a rate of 1 DEG C / sec or less;
≪ / RTI >
[Relation 1]
10? [Mn] / [S]? 100
[Relation 2]
0.1? [Ca] / [S]? 1.0
[Relation 3]
-0.0040? [N] - [B]? 0.0020
(Wherein each of [Mn], [S], [Ca], [N] and [B] means the content (% by weight)
11. The method of claim 10,
The step of preparing the billet comprises:
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.05 to 0.4% of C, 0.005 to 0.5% of Si, 0.2 to 2.0% of Mn, 0.03% or less of P, 0.007 to 0.06% of S, 0.0056 to 0.02% of B, %, Ca: 0.0005 to 0.01%, the balance Fe and unavoidable impurities, and heating the bloom satisfying the relational expressions 1 to 3 to 1150 to 1300 캜; And
Rolling the heated bloom to obtain a billet;
≪ / RTI >
11. The method of claim 10,
Wherein said inevitable impurities include Ti, Nb and V, and are suppressed to 0.01% or less of Ti, or 0.05% or less of Nb and V in total by weight%.
12. The method of claim 11,
And the finish rolling temperature is 1000 to 1200 占 폚 at the time of rolling the steel strip.
11. The method of claim 10,
Wherein the finish rolling temperature during the wire rolling is 900 to 1000 占 폚.
11. The method of claim 10,
Wherein the coiling temperature is 750 to 950 占 폚.