KR101035767B1 - Hot-rolled steel sheet with good formability, and method for producing the same - Google Patents

Abstract

The present invention relates to a soft hot-rolled steel sheet and a manufacturing method thereof. (S) more than 0 and not more than 0.01 wt%, manganese (Mn) is more than 0 and not more than 0.15 wt%, phosphorus (P) is not more than 0.01 wt% (Cu), nickel (Ni), tin (Sn), chromium (Cr), and other impurity elements (Fe) and other inevitable impurities (Cr)), more than 0 to 0.08 wt%, molybdenum 0.01 to 0.02 wt%, titanium (Ti) 0.02 to 0.08 wt%, and niobium (Nb) 0.005 to 0.05 wt% Alloy composition.

The present invention relates to a method of manufacturing a hot-rolled steel sheet having a tensile strength of 300 MPa or higher, excellent elongation, excellent r-value, and excellent non-aging property by controlling a heating element and a hot rolling temperature and a cooling condition after hot- There is an advantage that it can be widely applied to parts requiring high forming by replacing existing cold rolled steel sheets.

Hot-rolled steel sheet, r-value, workability, non-aging property

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet and a method of manufacturing the same, and more particularly, to a hot rolled steel sheet having excellent workability satisfying the material of a conventional cold rolled steel sheet and a method of manufacturing the same.

Research interest in the automobile industry is focused on environmental pollution and light weight, and as automobile design becomes complicated and consumer needs are diversified, steel sheet with excellent processability and moldability is required.

Cold-rolled steel sheets such as cold-rolled annealed steel sheets or cold-rolled steel sheets have high r-value (hereinafter referred to as "r value"), and are used in automobile outer plates and various industrial fields.

However, in the case of a hot-rolled steel sheet, the r-value is very low as compared with a cold-rolled annealed steel sheet or a cold-rolled steel sheet, which is disadvantageous in that it can not be used in fields requiring deep-

In the case of extremely low carbon steels, carbon (C) and nitrogen (N), which are employed as the elements, are contained at a very low level. Most steel sheets requiring workability include carbonitride-forming elements such as titanium (Ti) and niobium (Nb) to remove dissolved elements present in the steel as precipitates, thereby improving workability.

In the case of a cold rolled steel sheet, since the hot rolling finish temperature is higher than Ar3 at the time of hot rolling, a ferrite transformation occurs during the cooling process, and a random texture is formed, (111) texture can be developed after cold-rolling annealing to improve the r-value.

Since the austenite reverse hot rolling method has a high rolling finishing temperature, the rolling condition itself is difficult, and in actual processes, due to various problems such as equipment, rolling is performed at a temperature lower than Ar3, There is a problem that the deep drawability of the steel sheet is deteriorated.

Therefore, when the product is produced in the previous hot rolling step rather than in the cold rolling step as the final product, it is necessary to secure a sufficient r value by securing the texture obtained in the cold rolling annealing in the hot rolling step.

For this, if the steel sheet is subjected to a sufficient cooling process during finish rolling using hot rolled hot rolling, recrystallized microstructure can be obtained in the ferrite region.

In order to secure the excellent formability of the steel sheet, a high average plastic strain ratio (r _m ) and a low planar anisotropy coefficient (? R) value must be ensured.

The average plastic deformation ratio (r _m ) and the plane anisotropy coefficient (? R) are as follows.

r _m = (r ₀ + 2 r ₄₅ + r ₉₀ ) / 4

? R = (r ₀ - 2r ₄₅ + r ₉₀ ) / 2

Where r ₀ represents the rolling direction, the diagonal direction of the r ₄₅ rolling, and the vertical direction of the r ₉₀ rolling.

A high average plastic deformation ratio ( _rm ) means that a deep shape can be easily processed because of its excellent deep-darwing property. The low planar anisotropy coefficient value (r) Which means that the strain distribution is uniform. That is, the high average plastic deformation ratio (r _m ) and the low planar anisotropy coefficient value (? R) mean that the post-molding deformation is uniform, so that it is easy to manufacture as desired parts and excellent in shape crystallization.

In addition, in order for the steel material to have sufficient formability, a high r (plastic deformation ratio) value must be secured. The r value is an index representing the formability. In order to increase the r value, the content of carbon (C) and nitrogen (N), which are the solid elements present in the steel, should be lowered. Carbon (C) and nitrogen (N) which are present in the steel are removed by the addition of titanium (Ti) and niobium (Nb). That is, titanium (Ti) and niobium (Nb) are added to the steel to precipitate dissolved elements such as carbon (C) and nitrogen (N) present in the steel as precipitates to secure a sufficient r value.

However, the above-mentioned method causes the aging phenomenon when the carbon (C) and nitrogen (N) present in the steel are not sufficiently removed and exist as a solid element. This aging phenomenon causes a defect called Strecher Strain. Therefore, it is practically difficult to produce a cold-rolled steel sheet which requires a high-quality steel sheet.

When tin (Sn), copper (Cu), nickel (Ni), chromium (Cr) or the like is added as an impurity element, it is difficult to secure sufficient elongation and r value.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and it is an object of the present invention to provide a method of manufacturing a steel sheet, which is capable of achieving high elongation and r- And a method of manufacturing the same.

According to an aspect of the present invention for achieving the above object, the present invention provides an exhaust gas purifying catalyst comprising at least one member selected from the group consisting of carbon (C) 0 to 0.01 wt% (B) more than 0 and not more than 0.001 wt%, and the other impurity elements (copper (Al) and copper (B) are contained in an amount of not more than 0.15 wt% (Ti), 0.02 to 0.08 wt.% Of niobium (Nb), 0.005 to 0.05 wt.% Of niobium (Nb), and 0.005 to 0.05 wt.% Of molybdenum (Cu), nickel (Ni), tin wt%, and has an alloy composition of the balance iron (Fe) and other unavoidable impurities.
Wherein the content of titanium (Ti), carbon (C), nitrogen (N) and sulfur (S) satisfies the formula Ti? 4? C + 3.4N + 1.5S and the content of niobium (Nb) Satisfies the expression Nb? 0.5C * (93/12).
(S) more than 0 and not more than 0.01 wt%, manganese (Mn) more than 0 and not more than 0.15 wt%, (P) more than 0 (P) (Cu), nickel (Ni), tin (Sn), and chromium (Cr)) in an amount of 0.01 to 0.06 wt% (Fe) and other unavoidable impurities, the alloy being composed of at least 0.08 wt% of Ti, at least 0.08 wt% of molybdenum, at least 0.02 wt% of molybdenum, 0.02 to 0.08 wt% of titanium and 0.005 to 0.05 wt% of niobium , The content of titanium (Ti), carbon (C), nitrogen (N) and sulfur (S) satisfies the formula Ti ≥4C + 3.4N + 1.5S and the content of niobium (Nb) The steel slab satisfying the formula Nb? 0.5C * (93/12) is reheated to a temperature equal to or higher than the Ac3 point and held for 1 to 2 hours, and the hot rolling is finished in the temperature range of 800 to 850 deg. Wind at 500 to 650 ° C and air-cool.

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After the hot rolling, the steel sheet is cooled to a temperature range of 710 to 750 ° C at a cooling rate of 20 to 50 ° C / sec, maintained for 5 to 10 seconds, and then cooled to a coiling temperature at a cooling rate of 20 to 50 ° C / Cooling is performed.

The present invention relates to a method of controlling a heating element in a steel by adding titanium (Ti) and niobium (Nb), which are carbonitride-forming elements, using an extremely low carbon steel, and controlling a cooling condition after the hot rolling finish temperature and hot rolling. Reverse hot rolling is carried out to produce a hot-rolled steel sheet free from phase transformation and having a satisfactory r-value similarly to the annealing process of cold rolling.

Such a hot-rolled steel sheet has a tensile strength of 300 MPa or more, elongation of 45% or more, excellent r-value and non-aging property. Accordingly, since it is possible to replace the conventional cold-rolled steel sheet, there is a useful effect that can be widely applied to parts requiring high-quality formation at a lower cost.

Hereinafter, preferred embodiments of the soft hot-rolled steel sheet and the manufacturing method thereof according to the present invention will be described in detail.

(S) more than 0 and not more than 0.01 wt%, manganese (Mn) is more than 0 and not more than 0.15 wt%, phosphorus (P) is not more than 0.01 wt% (Cu), nickel (Ni), tin (Sn), chromium (Cr), and other impurity elements (Fe) and other unavoidable impurities (Cr)), more than 0 to 0.08 wt%, molybdenum 0.01 to 0.02 wt%, titanium (Ti) 0.02 to 0.08 wt%, and niobium (Nb) 0.005 to 0.05 wt% Alloy composition.
The content of titanium (Ti), carbon (C), nitrogen (N) and sulfur (S) satisfies the formula Ti? 4? C + 3.4N + 1.5S and the content of niobium (Nb) Nb? 0.5C * (93/12).

The steel slab having such a composition is reheated to a temperature equal to or higher than the Ac3 point and maintained for 1 to 2 hours, and the hot rolling is finished at an Ar3 point or less. After hot rolling, the steel sheet is held in a temperature range of 710 to 750 ° C. for 5 to 10 seconds so as to have a ferrite structure, and then rolled at a temperature range of 500 to 650 ° C.

The present invention is characterized in that the steel slab containing the above-mentioned weight% of the alloying element and inevitably contained impurities is reheated and hot-rolled to obtain a tensile strength of 300 MPa or higher and an excellent aging property and an excellent r-value (plastic deformation ratio) .

More specifically, titanium (Ti) and niobium (Nb), which are carbonitride forming elements, are added to control the solid elements in the steel, and a stepwise ferrite reverse rolling process is performed to control the cooling conditions after the hot rolling finish temperature and hot rolling So that there is no phase transformation and an excellent r value is secured as in the annealing process of cold rolling.

The strength to be raised by the impurity elements (Cu, Sn, Ni, Cr) inevitably included in the electric furnace process is the maximum value of the content of manganese (Mn) and phosphorus (P) So that a high r value is secured.

On the other hand, in the design of the alloy, the content of titanium (Ti) is designed in the range of the capability of titanium (Ti) precipitated depending on the temperature to remove the solid element. The content of titanium (Ti), carbon (C), nitrogen (N) and sulfur (S) satisfies the formula Ti≥4C + 3.4N + 1.5S.

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Titanium (Ti) precipitates in the order of TiN, TiS and TiC depending on the temperature. If the amount of addition of titanium (Ti) is smaller than the value of the above formula, the solid solution is present in the steel, and the r value is lowered, thereby decreasing the formability. When the addition amount of titanium (Ti) satisfies the above-described calculation formula or is larger than the above-mentioned calculation formula, it is possible to secure the r-value, so that excellent formability can be secured.

In the alloy design, the content of niobium (Nb) is designed to precipitate NbC precipitates depending on the temperature of niobium (Nb). For this purpose, the contents of niobium (Nb) and carbon (C) * (93/12).

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The above formula indicates the ability of niobium (Nb) to precipitate as precipitates of NbC according to the temperature as in the case of titanium (Ti). When the addition amount of niobium (Nb) is less than the value of the above formula, The r value is lowered and the moldability is lowered. When the addition amount of niobium (Nb) is larger than the calculation formula, the r value for securing excellent moldability can be secured.

The function and content of the alloying elements of the present invention are as follows.

Carbon (C): more than 0 and not more than 0.01 wt%

Carbon (C) is present as a solid element in the steel sheet and inhibits the formation of (111) aggregate structure favorable to the workability in the process of forming the aggregate structure of the steel sheet during cold rolling and annealing, thereby lowering the workability and formability of steel. In addition, when carbon is present as a solid element in the steel, it causes an aging problem and causes a defect called a stretcher strain. .

Therefore, if the carbon content increases, the addition amount of titanium and niobium, which are carbide forming elements, must be increased to precipitate carbon. In this case, not only the manufacturing cost of the steel is increased but also the steel material and surface physical properties may be lowered by adding a large amount of niobium and titanium. For this reason, carbon is advantageous as the content is lower, but it is limited to 0.01 wt% or less, because there is a limitation of steelmaking technology in which steel plating characteristics are weakened and strength is lowered and grain boundary embrittlement phenomenon may occur.

Nitrogen (N): more than 0 and not more than 0.01 wt%

Nitrogen (N), like carbon (C), exists as a solute element in steel and decreases the elongation and r value to lower the workability and formability of the steel sheet. As the content of nitrogen (N) increases, the content of titanium and niobium is increased to increase the cost. The smaller the content, the more favorable the formability. However, it is difficult to consider the steel level and cost. Therefore, nitrogen (N) is limited to 0.01 wt% or less considering steel making level and cost.

Sulfur (S): more than 0 and not more than 0.01 wt%

Sulfur (S) forms inclusions in the emulsion system (such as MnS) when the content is increased, forms FeS, and generates edge cracks. Therefore, it is regulated to be 0.01 wt% or less. In particular, when sulfur is denitrified in steelmaking, sulfur is placed on the surface of the steel, which deteriorates the denitrification effect, so the upper limit should be regulated.

Manganese (Mn): more than 0 and not more than 0.15 wt%

Manganese (Mn) improves the strength and precipitates sulfur (S) dissolved in the steel into MnS to prevent cracks caused by sulfur (S). When the content of manganese (Mn) exceeds 0.15%, the workability and formability of the manganese (Mn) are lowered due to intergranular segregation. In the present invention, the upper limit value is limited to 0.15 wt% or less since it is intended to improve workability rather than the effect of increasing the strength.

Phosphorus (P): more than 0 and not more than 0.01 wt%

Phosphorus (P) is an element with a high solubility enhancement effect and a small decrease in r-value (plastic deformation ratio). Phosphorus is added much in the IF river for the purpose of increasing the strength. Phosphorus (P) is segregated at the grain boundaries when the content is increased, resulting in secondary processing brittleness. The induction of second-order brittleness by phosphorus can be suppressed by the addition of boron. However, in the present invention, the content of phosphorus is regulated to 0.01 wt% or less for the purpose of improving the workability rather than the effect of increasing the strength.

Aluminum (Al): 0.01 to 0.06 wt%

Aluminum (Al) is a component acting as a deoxidizer and maintains the dissolved oxygen amount in the steel sufficiently low. Further, aluminum reacts with nitrogen, which is a solid element, to generate precipitates of AlN to remove the solid element.

As for the role of deoxidizing agent, aluminum may cause problems in performance when added above the upper limit, and when it is added in an amount of less than 0.01%, the precipitation amount of AlN is decreased and sufficient strength can not be secured. Therefore, the content of aluminum is preferably in the range of 0.01 to 0.06 wt%.

Boron (B): more than 0 and not more than 0.001 wt%

The most representative role of boron (B) is the inhibition of secondary process brittle induction. In the IF river, boron plays a role to prevent grain segregation of phosphorus and inhibits secondary process embrittlement induced by scaly stones. When the content of boron is high, the increase in strength is caused, so that the upper limit is set at 0.001 wt%.

Other impurity element (Cu, Sn, Ni, Cr: more than 0 and 0.08 wt% or less)

Tramp element is the cause of deteriorating the mechanical properties of steel, so it is regulated to the minimum possible. Copper (Cu) and tin (Sn) are mainly elements that adversely affect the processability. In case of copper, it is necessary not only to lower the r value but also to minimize the content of the steel in order to lower the surface quality. In the case of annotation, it should be minimized because it is fatal to r value than other Tramp elements. In particular, since the steel of the present invention has an object of improving the formability rather than the purpose of increasing the strength, the impurity element causing the increase in strength is regulated to 0.08 wt% or less.

Titanium (Ti): 0.02 to 0.08 wt%

Titanium (Ti) increases the r value and exhibits non-vitrification as the amount of Ti is increased. Titanium improves the r value by precipitating carbon and nitrogen present as solute elements in steel in the form of precipitates such as TiN and TiC. In addition, when carbon remains in the steel, the aging phenomenon is observed, in which the electric potential is restrained by moving with time.

The lower limit was set to a stoichiometrically precipitable amount of the solid solution element. Normally, as the content of titanium (Ti) increases, the r value increases. However, as the content of titanium (Ti) increases, the cost is increased, so the upper limit is regulated. Therefore, the preferable content of titanium (Ti) is 0.02 to 0.08 wt%.

Niobium (Nb): 0.005 to 0.05 wt%

Niobium (Nb) Similar to titanium (Ti), carbon and nitrogen present as solid elements in steel are precipitated in the form of precipitates of NbC and NbN to remove dissolved elements in the steel.

Niobium lowers the r value and increases the yield strength to weaken the formability when the content of the niobium exceeds the amount capable of stoichiometrically depositing the solid element in the steel. In addition, the manufacturing cost is increased by the cost increase. Therefore, the content of niobium added to the steel is preferably 0.005 to 0.05%. However, the addition of niobium in the steel ensures better plating properties.

The present invention includes the components of the alloy steel, the rest being substantially iron (Fe) and inevitable elements, and the inevitable elements such as silicon (Si), oxygen (O) and the like as elements contained depending on the conditions of raw materials, materials, Minor incorporation of impurities is also permitted.

The slab having the composition as described above is produced through ingot or continuous casting process after obtaining molten steel through a steelmaking process. Here, the steel sheet is subjected to the following steps, which are performed in the form of a steel plate through a hot rolling process.

[Reheating Process]

The steel slab having the above composition is heated at a temperature of 1100 to 1300 ° C for 1 to 2 hours at a temperature of Ac3 or higher in a heating furnace in order to reuse the segregated components during casting.

The lower the reheating temperature is, the more the Ti-based precipitation is promoted. In the case of TiN and TiC precipitates, the precipitation tendency does not vary greatly depending on the reheating temperature, but the Ti4C2S2 and TiS precipitates vary greatly depending on the reheating temperature. For this reason, as the heating temperature is lowered, the precipitate fraction is increased, and the content of C and N present in the solid element is lowered, which is an advantage in the production of the soft hot-rolled steel sheet.

[Hot rolling process]

The steel slabs reheated in the heating furnace process are subjected to hot rolling at an Ar3 point temperature or lower, and after hot rolling, they are cooled to 710 to 750 ° C at a cooling rate of 20 to 50 ° C / s and maintained for 5 to 10 seconds. And then cooled to a coiling temperature at a cooling rate of 20 to 50 DEG C / s.

A major difference from the conventional steel of the present invention is the hot rolling method. Typical extreme low carbon steels are hot-rolled at temperatures above Ar3, cold-rolled and annealed to (111) fully develop the texture and increase the r-value.

On the other hand, in the present invention, hot rolled is performed at a temperature of 800 to 850 deg. C, which is a temperature below Ar3, in order to secure all the material values obtained through the subsequent process in the hot rolling step.

It is the ferrite back rolling method to finish the hot rolling at the Ar3 temperature or less. Unlike the conventional ferrite rolling method, the hot rolled steel sheet is maintained at a temperature ranging from 710 ° C to 750 ° C for a set period of time, and the crystal structure of the rolled material is developed to increase the r value.

Since the ferrite reverse rolling method of the present invention is excellent in the formation of Ti and Nb precipitates in ferrite, it is advantageous to lower the content of dissolved carbon and dissolved nitrogen as compared with that of austenite reverse hot rolling. Thus, excellent workability can be ensured.

If the cooling rate is slower than 20 ° C / s, the grain size rapidly increases. If the cooling rate is higher than 50 ° C / s, the grain size is too fine to increase the strength and decrease the elongation, It is in the range of 50 ℃ / s.

After the hot rolling finish, the rolled recrystallized structure is obtained by holding for 5 to 10 seconds at a temperature range of 710 to 750 ° C. In general, the IF steel uses a stepped ferrite hot rolling method in which the excessively high temperature causes the ferrite recrystallization to be delayed and adversely affects the increase in the r value, so that the temperature is maintained at 710 to 750 ° C for 5 to 10 seconds.

[Coiling temperature]

Coiling is performed at a temperature range of 500 to 650 ° C, and air cooling is performed after winding. It exhibits non-aging properties by controlling carbon sufficiently.

Table 1 shows component ratios of Inventive Examples and Comparative Examples of the present invention, and Table 2 shows the results of measurement of mechanical properties of the specimens prepared by Inventive Examples and Comparative Examples of Table 1.

 (Unit: wt%, balance Fe) division Chemical composition (wt%) Hot rolling condition
C Mn P S Al Ti Nb Mo N B Comparative Example 1 0.0030 0.20 0.015 0.008 0.040 - - - 0.003 - Hot rolling at a temperature above Ar3 Comparative Example 2 0.0033 0.20 0.015 0.008 0.040 - - - 0.003 - Comparative Example 3 0.0030 0.20 0.015 0.008 0.040 0.050 - - 0.003 - Hot rolling at a temperature below Ar3 Comparative Example 4 0.0040 0.20 0.015 0.008 0.040 0.020 0.040 - 0.003 0.0005 Example 1 0.0015 0.10 0.010 0.008 0.040 0.030 0.005 0.02 0.003 0.0005 After hot rolling at a temperature of Ar3 or lower, Example 2 0.0021 0.10 0.010 0.008 0.040 0.030 0.005 0.02 0.003 0.0005 Example 3 0.0029 0.10 0.010 0.008 0.040 0.045 0.005 0.02 0.003 0.0005 Example 4 0.0015 0.10 0.010 0.008 0.040 0.040 0.005 0.02 0.003 0.0005

In Comparative Example 1 and Comparative Example 2 of Table 2, the ingots corresponding to the compositions of Comparative Example 1 and Comparative Example 2 were heated at 1220 占 폚 for 2 hours in Table 1, finished hot-rolled at a temperature higher than Ar3, And the like,

In Comparative Example 3 and Comparative Example 4 in Table 2, ingots corresponding to the compositions of Comparative Example 3 and Comparative Example 4 were heated at 1220 占 폚 for 2 hours, finished hot-rolled at Ar3 or lower, And the like,

In the examples of Table 2, the ingots corresponding to the composition of the examples in Table 1 were heated at 1220 占 폚 for 2 hours, finishing hot-rolled at a temperature not higher than Ar3, maintained at 750 占 폚 for 5 seconds or longer, Of the material.

division FC
(MPa) TS
(MPa) EL
(%) r-value Aging index
(Al) Comparative Example 1 239 331 39 1.10 20 Comparative Example 2 228 334 38 1.09 23 Comparative Example 3 220 324 42 1.20 10 Comparative Example 4 224 320 41 1.29 9 Example 1 179 319 45 1.59 0 Example 2 170 304 46 1.55 0 Example 3 167 301 45 1.58 0 Example 4 160 306 48 1.68 0

Elongation (%): elongation, r-value: plastic anisotropy] TS (MPa): tensile strength, YS

As shown in Tables 1 and 2, r values of Comparative Example 1 and Comparative Example 2 which were subjected to finish hot rolling at a temperature higher than Ar3 were low. It can be seen that Comparative Example 3 and Comparative Example 4 which were subjected to finish hot rolling at Ar 3 or lower exhibited a relatively high r value but a low r value as compared with Examples 1 to 4.

On the other hand, in Examples 1 to 4, it can be seen that a high r value of 1.5 or more is secured and the non-aging property is also excellent.

That is, from the comparative examples and the examples, the steel according to the present invention has the effect of adding Ti, Nb and Mo, controlling the content of P and Mn, and controlling the cooling conditions after the hot rolling finish temperature and the hot rolling, even if the steel contains other unavoidable impurities. It is confirmed that excellent workability and non-aging properties can be secured by the reverse hot rolling. Accordingly, it can be seen that a soft hot-rolled steel sheet having excellent workability and non-aging properties can be produced.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. will be.

1 is a cycle diagram showing a hot rolling method for producing a soft hot-rolled steel sheet according to the present invention.

Claims (4)

(S) more than 0 and not more than 0.01 wt%, manganese (Mn) more than 0 and not more than 0.15 wt%, (P) more than 0 (P) (Cu), nickel (Ni), tin (Sn), and chromium (Cr)) in an amount of 0.01 to 0.06 wt% (Fe) and other unavoidable impurities, the alloy being characterized in that it contains an alloy composition of more than 0 to 0.08 wt%, molybdenum of 0.01 to 0.02 wt%, titanium (Ti) of 0.02 to 0.08 wt%, and niobium (Nb) of 0.005 to 0.05 wt% Wherein the hot rolled steel sheet is a soft rolled steel sheet. The method according to claim 1, Wherein the content of titanium (Ti), carbon (C), nitrogen (N) and sulfur (S) satisfies the formula Ti? 4? C + 3.4N + 1.5S and the content of niobium (Nb) Satisfies the following expression: Nb? 0.5C * (93/12). (S) more than 0 and not more than 0.01 wt%, manganese (Mn) more than 0 and not more than 0.15 wt%, (P) more than 0 (P) (Cu), nickel (Ni), tin (Sn), and chromium (Cr)) in an amount of 0.01 to 0.06 wt% (Fe) and other unavoidable impurities, the alloy being composed of at least 0.08 wt% of Ti, at least 0.08 wt% of molybdenum, at least 0.02 wt% of molybdenum, 0.02 to 0.08 wt% of titanium and 0.005 to 0.05 wt% of niobium , Wherein the content of titanium (Ti), carbon (C), nitrogen (N) and sulfur (S) satisfies the formula Ti? 4? C + 3.4N + 1.5S and the content of niobium (Nb) A steel slab satisfying the formula Nb? 0.5C * (93/12) Ac3 point or more and maintained for 1-2 hours, and the hot rolling is completed in a temperature range of 800 to 850 deg. C, which is a temperature lower than the Ar3 point, at a temperature of 500 to 650 DEG C, followed by air cooling. ≪ / RTI > The method of claim 3, After the hot rolling, the steel sheet is cooled to a temperature range of 710 to 750 ° C at a cooling rate of 20 to 50 ° C / sec, maintained for 5 to 10 seconds, and then cooled to a coiling temperature at a cooling rate of 20 to 50 ° C / And cooling the steel sheet.

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