KR20150074968A - Hot rolled steel sheet having a good low temperature toughness and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a hot-rolled steel sheet for a steel pipe comprising: 0.03-0.1 wt% of C, 0.01-0.50 wt% of Si, 1.2-2.5 wt% of Mn, 0.002 wt% or less of S, 0.03 wt% or less of P, 0.01-0.10 wt% of Ti, 0.01 wt% or less of N, 0.01-0.13 wt% of Nb, 0.1-0.5 wt% of Ni, 0.1-0.5 wt% of Mo, 0.01-0.5 wt% of Cr, 0.002 wt% or less of B, and the remainder consisting of Fe and inevitable impurities; and to a manufacturing method thereof. The elements of the hot-rolled steel sheet for a steel pipe satisfy a formula (1) to a formula (3). Fine structure thereof comprises 60-80% of acicular ferrite and 20-40% of granular bainite with respect to the total area.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet for steel pipes having excellent low temperature toughness,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for steel pipes for use in applications such as construction, line pipes and offshore structures, and a method for manufacturing the same. More particularly, the present invention relates to a hot-rolled steel sheet for ultra-

The present invention relates to a hot-rolled high-strength steel sheet having high strength and toughness for use in pipelines and marine structures. Such a steel plate for steel pipe is often exposed to high pressure. For example, the transport pressure is gradually increasing to increase the transport efficiency in transporting crude oil or gas, and recently the transport pressure has reached 120 atm. In order to withstand such transport pressure, the thickness of the pipe must be increased. However, as the thickness increases, the amount of reduction during rolling is insufficient and it is difficult to ensure a sufficient cooling rate. As a result, the ferrite grains become coarse, There is a problem that the toughness is deteriorated. In addition, there is a problem that the pearlite fraction increases due to the low cooling rate, and the strength decreases after the spiral gouging.

As a technique for solving such a problem, Japanese Laid-Open Patent Publication No. 2003-328080 (hereinafter referred to as Patent Document 1) discloses a low-C-Nb-Ti system as a steel pipe base material, And a weld metal portion of a low C-Mn-B type base material portion containing a composite of oxide and sulfide and a fine carbonitride containing an oxide composed of Mg and Al. According to Patent Document 1, it is said that the hardness of the welded metal portion is excellent in the toughness and deformability of the welded heat affected portion by 0.95 to 1.15 times the hardness of the base portion.

However, in the case of the steel pipe of Patent Document 1, the manufacturing cost is high because it requires expensive V and Mg. Therefore, there is a need to develop a steel sheet for a super high strength steel pipe having excellent low temperature toughness and strength and low manufacturing cost.

Disclosure of Invention Technical Problem [8] Accordingly, the present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a steel sheet for a post material high strength steel pipe excellent in strength and low temperature toughness, and a manufacturing method thereof.

In one aspect, the present invention provides a steel sheet comprising, by weight%, 0.03-0.1% of C, 0.01-0.50% of Si, 1.2-2.5% of Mn, 0.002% or less of S, , N: 0.01% or less, Nb: 0.01-0.13%, Ni: 0.1-0.5%, Mo: 0.1-0.5%, Cr: 0.01-0.5%, B: 0.002 or less and the balance Fe and other impurities, Wherein the elements are included so as to satisfy the following formulas (1) to (3), and the microstructure includes 60% to 80% of acicular ferrite and 20% to 40% of granular bainite in an area fraction The hot-rolled steel sheet for a steel pipe.

(1) 0.19? {(Ti * / 48) + (Nb * / 93)} / (C / 12)? 0.28

(2) 1.3? Cr + 2Mo + Cu + Ni + 2B? 1.6

(Mo / 96) / (P / 31)? 30

In the above formulas (1) to (3)

Ti * = Ti - 0.8 × (48/14) N, Nb * = Nb - 0.8 × (93/14) N, and the symbol of each component represents the concentration (weight%) of the component.

At this time, the hot-rolled steel sheet for a steel pipe may have a yield strength of 680 MPa or more and an impact energy of 160 J or more at -20 ° C.

In another aspect of the present invention, there is provided a ferritic stainless steel comprising: 0.03-0.1% C, 0.01-0.50% Si, 1.2-2.5% Mn, 0.002% , N: 0.01% or less, Nb: 0.01-0.13%, Ni: 0.1-0.5%, Mo: 0.1-0.5%, Cr: 0.01-0.5%, B: 0.002 or less, and the balance Fe and other impurities , Heating the steel slab containing the elements to satisfy the above-mentioned formulas (1) to (3) at a temperature of 1100 캜 to 1350 캜; Rolling the heated slab at a temperature not lower than the non-recrystallization temperature to 60% or more; Cooling the rolled slab at a rate of 10 ° C / sec or more; And winding the water-cooled slab at a temperature of 200 ° C to 400 ° C. The present invention also provides a method for manufacturing a hot-rolled steel sheet for a steel pipe.

At this time, the rolling step includes: rough rolling the heated slab at 950 ° C to 1100 ° C; And finishing hot-rolling the rough-rolled slab at 780 캜 to 880 캜.

The water-cooling step may be performed at a rate of 10 ° C / sec to 50 ° C / sec.

The hot-rolled steel sheet for a steel pipe according to the present invention is characterized in that needle-shaped ferrite having excellent strength and pulling strength is formed into a matrix structure to maximize the formation of precipitates and to suppress the grain boundary segregation of P to promote grain boundary fracture at low temperature So that it is possible to have excellent low temperature toughness and strength while having a thick thickness.

Further, the method of manufacturing a hot-rolled steel sheet for a steel pipe of the present invention can control the heating temperature, the rolling temperature, the water-cooling rate and the coiling temperature under specific conditions to produce a hot-rolled steel sheet having an excellent low temperature toughness and strength Respectively.

The hot-rolled steel sheet of the present invention is excellent in strength, low-temperature toughness and yield strength, and can be usefully used in buildings, pipelines, and marine structures requiring high strength and toughness.

Hereinafter, the present invention will be described in detail.

The hot-rolled steel sheet for a steel pipe according to the present invention comprises 0.03-0.1% of C, 0.01-0.50% of Si, 1.2-2.5% of Mn, 0.002% or less of S, 0.1 to 0.5% of Cr, 0.01 to 0.5% of Cr, 0.002 or less of B, and the balance of Fe and other impurities are contained in an amount of 0.10 to 0.10%, N: 0.01 to 0.1%, Nb: 0.01 to 0.13% , Wherein the elements are included so as to satisfy the following formulas (1) to (3), wherein the microstructure comprises 60% to 80% of acicular ferrite in an area fraction and 20% to 40% of granular bainite .

(1) 0.19? {(Ti * / 48) + (Nb * / 93)} / (C / 12)? 0.28

(2) 1.3? Cr + 2Mo + Cu + Ni + 2B? 1.6

(Mo / 96) / (P / 31)? 30

In the above formulas (1) to (3)

Ti * = Ti - 0.8 × (48/14) N, Nb * = Nb - 0.8 × (93/14) N, and the symbol of each component represents the concentration (weight%) of the component.

First, constituent components of the steel sheet of the present invention will be described.

In the present invention, the content of carbon (C) is preferably about 0.03 to 0.1 wt%.

C is the most economical and effective element for strengthening the steel, but the weldability, formability and toughness may be lowered when added in large amounts. In the present invention, the addition amount of carbon is limited to 0.03-0.10 wt% in consideration of this. If the addition amount is less than 0.03 wt%, it is not economical because other alloying elements must be added in a relatively large amount in order to exhibit the same strength. Addition of more than 0.10 wt% leads to deterioration of weldability, formability and toughness.

The content of silicon (Si) is preferably about 0.01 to 0.5 wt%.

Si is also required for deoxidizing molten steel and is also effective as a solid solution strengthening element, so it is preferable that Si is added in the range of 0.01-0.50 wt%. If the addition amount is less than 0.01 wt%, it is difficult to obtain a clear steel because it does not sufficiently deoxidize the molten steel. If the addition amount exceeds 0.5 wt%, a red scale due to Si is formed by hot rolling, Is also undesirable.

The content of manganese (Mn) is preferably about 1.2 to 2.5 wt%.

Mn should be added as an effective element to strengthen the solid solution of steel by 1.2 wt% or more so that high strength can be exhibited in addition to the effect of increasing the incombustibility. However, if the addition amount exceeds 2.5 wt%, it is not preferable because the segregation part is developed at the center of the thickness during casting of the slab in the steelmaking process and the weldability of the final product is deteriorated.

The content of sulfur (S) is preferably 0.02 wt% or less.

S is an impurity element existing in the steel and forms a non-metallic inclusion by binding with Mn or the like, thereby greatly impairing the toughness and strength of the steel. Therefore, it is preferable to reduce the content as much as possible, so that the content is preferably limited to 0.02 wt% or less .

The content of phosphorus (P) is preferably 0.03 wt% or less.

P is an impurity element present in the steel, which promotes center segregation along with Mn and the like at the time of playing to deteriorate the impact toughness and stress resistance of the emulsion stress crack, as well as reduce the weldability. .

The content of Niobium (Nb) is preferably 0.01 to 0.13 wt% or less.

At least 0.01 wt% of Nb should be added because it is a very useful element for finely graining the grain and it also improves the strength of steel significantly. If it exceeds 0.13 wt%, excessive Nb carbonitride precipitates and toughness of steel It is harmful.

The content of titanium (Ti) is preferably about 0.01 to 0.1 wt%.

Ti is an element which is very useful for refining the crystal grains and exists as TiN in the steel and has an effect of inhibiting the growth of grains during the heating process for hot rolling. Also, Ti reacted with nitrogen and solidified in the steel, Precipitates are formed and the formation of TiC is very fine, which greatly improves the strength of the steel. Therefore, at least 0.01 wt% or more of Ti should be added in order to obtain an effect of inhibiting the growth of austenite grain growth by TiN precipitation and an increase in strength by TiC formation. If the content exceeds 0.1 wt%, the steel sheet is welded, , The toughness of the weld heat affected zone may deteriorate as the TiN is reused.

The content of nitrogen (N) is preferably about 0.01 wt%.

The component limitation of N is due to the addition of Ti described above. In general, N is dissolved in the steel and precipitates to increase the strength of the steel, which is much greater than carbon. On the other hand, it is known that the presence of nitrogen in the steel significantly degrades the toughness, and it is a general tendency to reduce the nitrogen content as much as possible. However, in the present invention, a proper amount of nitrogen is present and reacted with Ti to form TiN and to inhibit crystal growth during reheating. However, since a part of Ti does not react with N and reacts with carbon in a subsequent step, it is preferable that the content of Ti is about 0.01 wt%.

The content of molybdenum (Mo) is preferably about 0.1 to 0.5 wt%.

Mo is very effective in increasing the strength of the material and serves to lower the yield ratio by promoting the formation of acicular ferrite, which is a low-temperature transformed structure. In addition, the cementite and carbide are aggregated, exhibiting deteriorated impact properties, and suppressing the formation of pearlite structure which contributes to lowering the yield strength after the cementation, thereby reducing the impact toughness and the drop in yield strength after the cementation. For this purpose, Mo should be added in an amount of 0.1 wt% or more, but it is preferable that the Mo content is less than 0.5 wt% in order to inhibit welding low-temperature cracking and to prevent low-temperature transformation phase from being generated in the base material.

The content of nickel (Ni) is preferably about 0.1 to 0.5 wt%.

Ni is an austenite stabilizing element and suppresses pearlite formation. It is preferable to add 0.1 wt% or more as an element which facilitates the formation of acicular ferrite which is a low temperature transformation texture. However, since it is an expensive element and inhibits toughness of a welded portion, % Or less.

The content of chromium (Cr) is preferably about 0.01 to 0.5 wt%.

Cr generally increases the hardenability of the steel during direct quenching. It also generally improves corrosion resistance and hydrogen cracking resistance. Similarly to Mo, generation of pearlite structure is suppressed, and good impact toughness can be obtained. For this, Cr should be added in an amount of 0.01 wt% or more, but it tends to cause cracking after field welding when added in excess, and it tends to deteriorate the steel and its HAZ toughness. .

The content of boron (B) is preferably 0.002 wt% or less.

B is useful for increasing the hardenability of steel to form acicular ferrite of milder phase than polygonal ferrite. However, when it is added in an amount of 0.002 wt% or more, the hardenability is so increased that martensite or the like which is harmful to the impact is formed, and is preferably contained in an amount of 0.002 wt% or less.

On the other hand, the steel sheet of the present invention includes the above-mentioned components so as to satisfy the following conditions (1) to (3).

(1) 0.19? {(Ti * / 48) + (Nb * / 93)} / (C / 12)? 0.28

In the above formula (1), Ti * = Ti - 0.8 × (48/14) N, Nb * = Nb - 0.8 × (93/14) N, .

According to the research conducted by the present inventors, when Ti, Nb, C and N are included so as to satisfy the above formula (1), it is found that fine (Ti, Nb) C precipitates are increased and excellent strength can be obtained. (Ti, Nb) C and NbC precipitates are obtained when the value of the above formula (1) is not less than 0.19, and when it exceeds 0.28, It was found that the effect of improving the strength was insignificant. On the other hand, Ti * and Nb * are contents which react with N and react with C at the total content of Ti and Nb, respectively.

(2) 1.3? Cr + 2Mo + Cu + Ni + 2B? 1.6

According to the studies of the present inventors, when Cr, Mo, Cu, Ni and B are included so as to satisfy the above formula (2), the percentage of acicular ferrite structure in microstructure is increased and the strength is improved. On the other hand, when the value of the formula (2) is out of the above range, that is, when the value is less than 1.3, the effect of improving the strength is insignificant. When the value exceeds 1.6, separation occurs and the impact toughness is decreased.

(Mo / 96) / (P / 31)? 30

On the other hand, according to the study of the present inventors, when Mo and P are included so as to satisfy the above formula (3), the grain boundary segregation of P is effectively controlled. When the value of the formula (3) is out of the above range, that is, less than 11, the effect of controlling the grain boundary segregation is insignificant. When the value exceeds 30, the impact energy decreases .

On the other hand, the microstructure of the hot-rolled steel sheet for a steel pipe of the present invention includes acicular ferrite having an area fraction of 60% to 80% and granular bainite of 20% to 40%. As described above, since the steel sheet of the present invention contains the needle-like ferrite excellent in toughness and strength as the matrix, it has an advantage of being excellent in low temperature toughness and strength.

The hot-rolled steel sheet for steel pipe according to the present invention, whose composition and structure are controlled as described above, has an ultrahigh strength property with a yield strength of 680 MPa or more, preferably 680 MPa to 850 MPa.

In addition, the hot-rolled steel sheet for a steel pipe of the present invention has an impact energy of 160 J or more, preferably 160 J to 250 J at -20 캜.

Next, a method for manufacturing a hot-rolled steel sheet for a steel pipe of the present invention will be described.

The production method of the present invention is characterized in that the steel sheet contains 0.03-0.1% of C, 0.01-0.50% of Si, 1.2-2.5% of Mn, 0.002% or less of S, 0.03% or less of P, And the balance of Fe and other impurities, wherein the N content is 0.01% or less, the Nb content is 0.01-0.13%, the Ni content is 0.1-0.5%, the Mo content is 0.1-0.5%, the Cr content is 0.01-0.5% Heating the steel slab containing the elements satisfying the above-mentioned equations (1) to (3) at a temperature of from 1100 DEG C to 1350 DEG C; Rolling the heated slab at a temperature not lower than the non-recrystallization temperature to 60% or more; Cooling the rolled slab at a rate of 10 ° C / sec or more; And winding the water-cooled slab at a temperature of 200 ° C to 400 ° C.

First, 0.003% or less of S, 0.03% or less of P, 0.01-0.10% or less of Ti, 0.01% or less of N, 0.01 to 0.13% of Nb, 0.1 to 0.5% of Ni, 0.1 to 0.5% of Mo, 0.01 to 0.5% of Cr, 0.002 or less of B, and the balance Fe and other impurities, (1) to (3) are satisfied. Since the composition of the steel slab is the same as that described above, a detailed description thereof will be omitted.

The step of heating the steel slab having the controlled composition as described above at 1100 ° C to 1350 ° C is performed. When the heating temperature of the steel slab is less than 1100 ° C, the precipitated additive alloying elements are not sufficiently reused, and precipitates such as (Ti, Nb) C and NbC are reduced in the process after the hot rolling. Therefore, by maintaining the heating temperature of the steel slab at 1100 DEG C or higher, the precipitation can be reused and the austenite grain size of the appropriate size can be maintained, thereby improving the strength level of the material and obtaining a uniform microstructure in the longitudinal direction of the coil . On the other hand, if the heating temperature is too high, the strength is lowered due to abnormal grain growth of the austenite grains. Therefore, the heating temperature is preferably 1350 占 폚 or lower.

Next, the heated slab is rolled by 60% or more at a temperature not lower than the non-recrystallization temperature. When the steel slab is rolled at 60% or more at a temperature not higher than the non-recrystallization zone as described above, acicular ferrite is contained in an area fraction of 60% to 80%, and granular bainite is contained in an area fraction of 20% A steel sheet including the steel sheet can be obtained.

Meanwhile, the rolling step may include: rough rolling the heated slab at 950 ° C to 1100 ° C; And a step of finishing hot-rolling the rough-rolled slab at 780 캜 to 880 캜. When the temperatures of the rough rolling and the finish hot rolling satisfy the above numerical range, it is possible to prevent the final structure from being coarse and to prevent a load from being generated in the rolling mill equipment.

After completion of the rolling step as described above, water cooling is performed on the run-out table. The water cooling is a step for forming fine needle-like ferrite and precipitates, and may be performed at a rate of 10 ° C / sec or more, preferably 10 ° C / sec to 50 ° C / sec. If the water cooling rate is less than 10 占 폚 / sec, the average size of the precipitates becomes as large as 0.2 占 퐉 or more, and the effect of improving the strength becomes insignificant. The higher the cooling rate, the more nuclei are formed and the precipitates become finer. On the other hand, the upper limit of the cooling rate is not particularly limited. However, when the cooling rate exceeds 50 DEG C / sec, there is no additional fine grain refining effect, so that the cooling rate is about 10 DEG C / sec to 50 DEG C / sec desirable.

Next, the water-cooled slab is wound. If the coiling temperature is less than 200 DEG C, the second phase fraction may increase and the impact characteristics may deteriorate. If the coiling temperature is more than 500 DEG C, And it is difficult to secure strength.

Hereinafter, the present invention will be described in more detail with reference to preferred embodiments.

Inventive steels A1 to A2 and comparative materials B1 to B2 having a chemical composition as shown in Table 1 were produced by slab casting by the continuous casting method and then heated, rolled, cooled and rolled under the conditions described in [Table 3] Steel sheets of Examples 1 to 2 and Comparative Examples 1 to 4 having a thickness of 16 mm were produced.

On the other hand, the values of the formulas (1) to (3) calculated from the compositions of the inventive steels A1 to A2 and the comparative materials B1 to B4 are shown in Table 2 below.

division C Si Mn Ni Cu Cr Mo Nb V Ti B P N Invention steel A1 0.06 0.3 2.1 0.3 0.2 0.3 0.3 0.1 0 0.015 0 0.009 0.0031 Invention steel A2 0.06 0.3 2.1 0.3 0.2 0.5 0.3 0.1 0 0.015 0.001 0.008 0.0033 Comparative material B1 0.06 0.3 2.1 0.3 0.2 0.3 0.45 0.1 0 0.015 0 0.013 0.0047 Comparative material B2 0.06 0.3 2.1 0.3 0.2 0.3 0.3 0.085 0.045 0.015 0 0.011 0.0053 Comparative material B3 0.06 0.3 2.1 0.3 0.2 0.3 0.2 0.1 0 0.015 0.001 0.015 0.0046 Comparative material B4 0.06 0.3 2.1 0.3 0.2 0.6 0.3 0.085 0.045 0.015 0 0.014 0.0037

Ti * Nb * {(Ti * / 48) + (Nb * / 93)} / (C / 12) (Mo / 93) / (P / 31) Cr + 2Mo + Cu + Ni + 2B Invention steel A1 0.006497 0.083526 0.20 11.1 1.4 Invention steel A2 0.005949 0.082463 0.20 12.5 1.6 Comparative material B1 0.002109 0.075023 0.17 11.5 1.7 Comparative material B2 0.000463 0.056834 0.12 9.1 1.4 Comparative material B3 0.002383 0.075554 0.17 4.4 1.2 Comparative material B4 0.004851 0.065337 0.16 7.1 1.7

division Steel grade Reheating temperature (℃) Unrecrystallized reverse descent
(%) Finish rolling
Temperature (℃) Cooling rate
(° C / s) Coiling temperature
(° C) Example 1 A1 1223 75 763 21 311 Example 2 A2 1256 62 772 19 295 Comparative Example 1 B1 1182 54 784 19 254 Comparative Example 2 B2 1195 63 769 21 301 Comparative Example 3 B3 1212 61 768 18 286 Comparative Example 4 B4 1210 64 773 20 332

The structure, yield strength and impact energy at -20 캜 of the steel sheets of Examples 1 to 2 and Comparative Examples 1 to 4 were measured and shown in Table 4 below.

The yield strength was measured using tensile test specimens taken from the hot-rolled sheets of Examples 1 to 2 and Comparative Examples 1 to 4, and the tensile test was carried out at a crosshead speed of 10 mm / min . At this time, the tensile test specimens were collected in a clockwise direction at 30 degrees with respect to the rolling direction, which is a direction corresponding to the circumferential direction of the pipe during spiral pipe gutting. API 5L specimens were used as tensile test specimens.

The impact energy was tested in accordance with generally accepted ASTM E23.

division Acicular type ferrite fraction (%) Granular bainite fraction (%) Yield strength
(MPa) Impact energy (J)
@ -20 ℃ thickness
(mm) Example 1 80 20 686 206 16 Example 2 75 25 760 163 16 Comparative Example 1 60 40 737 80 16 Comparative Example 2 70 30 743 130 16 Comparative Example 3 80 20 789 100 16 Comparative Example 4 85 15 751 130 16

As shown in Table 4, the steel sheets of Examples 1 and 2 produced according to the production method of the present invention using the inventive steel satisfying the composition of the present invention had an ultrahigh strength with a yield strength of 680 MPa or more, Which is shown in Fig. On the other hand, the steel sheets of Comparative Examples 1 to 4 have low temperature toughness as compared with the present invention.

Claims (6)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains 0.03-0.1% of C, 0.01-0.50% of Si, 1.2-2.5% of Mn, 0.002% or less of S, 0.03% or less of P, 0.01-0.10% Wherein said elements include at least one element selected from the group consisting of Nb: 0.01-0.13%, Ni: 0.1-0.5%, Mo: 0.1-0.5%, Cr: 0.01-0.5% ) To (3), < / RTI >
Wherein the microstructure comprises 60% to 80% of acicular ferrite in an area fraction and 20% to 40% of granular bainite.

(1) 0.19? {(Ti * / 48) + (Nb * / 93)} / (C / 12)? 0.28
(2) 1.3? Cr + 2Mo + Cu + Ni + 2B? 1.6
(Mo / 96) / (P / 31)? 30

In the above formulas (1) to (3)
Ti * = Ti - 0.8 × (48/14) N, Nb * = Nb - 0.8 × (93/14) N, and the symbol of each component represents the concentration (weight%) of the component.
The method according to claim 1,
The hot-rolled steel sheet for a steel pipe has a yield strength of 680 MPa or more.
The method according to claim 1,
The hot-rolled steel sheet for a steel pipe has an impact energy of 160 J or more at -20 캜.
The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains, by weight%, 0.03-0.1% of C, 0.01-0.50% of Si, 1.2-2.5% of Mn, 0.002% or less of S, : 0.1 to 0.13% of Ni, 0.1 to 0.5% of Ni, 0.1 to 0.5% of Mo, 0.01 to 0.5% of Cr, 0.002 or less of B, and the balance Fe and other impurities, ) To (3) at a temperature of 1100 캜 to 1350 캜;
Rolling the heated slab at a temperature not lower than the non-recrystallization temperature to 60% or more;
Cooling the rolled slab at a rate of 10 ° C / sec or more; And
And winding the water-cooled slab at a temperature of 200 ° C to 400 ° C.
(1) 0.19? {(Ti * / 48) + (Nb * / 93)} / (C / 12)? 0.28
(2) 1.3? Cr + 2Mo + Cu + Ni + 2B? 1.6
(Mo / 96) / (P / 31)? 30

In the above formulas (1) to (3)
Ti * = Ti - 0.8 × (48/14) N, Nb * = Nb - 0.8 × (93/14) N, and the symbol of each component represents the concentration (weight%) of the component.
5. The method of claim 4,
Wherein the rolling comprises:
Subjecting the heated slab to rough rolling at 950 캜 to 1100 캜; And
And subjecting the rough-rolled slab to finish hot-rolling at 780 캜 to 880 캜.
5. The method of claim 4,
Wherein the water-cooling step is performed at a rate of 10 ° C / sec to 50 ° C / sec.