JP2003129183A - High-strength steel slab and casting method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel slab and a casting method therefor. SOLUTION: The high-strength steel slab comprises, by mass%, 0.01-0.15% C, 0.005-0.5% Si, 0.1-2% Mn, 0.02% or less P, 0.01% or less S, 0.5-5.5% Ni, and 0.001-0.01% N, further one or more of 0.005-0.5% Nb, 0.003-0.05% Ti, 0.1-2% Mo, 0.01-1.5% Cu, 0.05-2.5% Cr, 0.01-0.5% V, and 0.0005-0.005% B, two or more of 0.001-0.1% Al, 0.001-0.5% Zr, 0.0005-0.02% Mg, 0.0005-0.02% Ca, and 0.001-0.5% REM (rare earth metals), and the balance Fe with unavoidable impurities, and has an isometric crystallization rate of 70% or more.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention has a tensile strength of 780.
The present invention relates to a high-strength steel cast slab that is used for all structures requiring a low temperature toughness of MPa class or higher and a casting method thereof.

[0002]

2. Description of the Related Art When a high strength steel having a tensile strength of 780 MPa grade or more is produced by a direct quenching method (DQ), it is essential to make a quenching structure fine in order to secure toughness. For that purpose, it is effective to refine the γ (austenite) grains before quenching by controlled rolling and to refine the transformation structure by introducing processing into the γ grains, but in the case of thick materials, the thickness reduction ratio is limited, There is a limit to the effect of rolling. In order to cover this and to promote the grain refinement of γ before DQ regardless of the plate thickness, it is most effective to make the reheat γ fine before rolling. The refinement of the reheating γ is generally achieved by lowering the heating temperature, but it is not always easy to achieve in high strength steel containing a relatively large amount of various alloying elements. That is, in the alloy steel, when the heating temperature becomes lower, the reheating γ reproduces the old γ of the pre-heating structure as it is (old γ memory), and this old γ is reproduced as coarse grains and some new generation. The mixed γ particles are mixed to form a significantly mixed particle. In addition, as a matter of course, if the heating temperature is raised, the heating γ grows, and it becomes difficult to miniaturize. Therefore, in the case of high strength steel and alloy steel, it is difficult to control the microstructure before rolling by controlling the heating temperature and thereby improve the toughness of the final steel sheet.

The fact that reheating γ is not refined even by low temperature heating but rather becomes coarse / mixed grain has been recognized even when high-strength steel is manufactured by reheating quenching / tempering, but in recent years it has become mainstream. In the case of the direct quenching method, which is being developed, the influence is more remarkable because the starting point is a slab. That is, in the case of normal continuous casting, the old γ grain size of the slab becomes coarse on the order of 1 mm, and if the coarse old γ grain is reproduced by reheating the slab, the reheated structure becomes This is because it results in a significantly mixed grain structure containing very coarse γ. The reheat γ of ordinary steel is 100 to 30.
Considering that the particle size is about 0 μm, the size of the disadvantage can be easily imagined. Even if this is controlled rolling,
It is very difficult to homogenize γ grains in the limited thickness reduction ratio,
Eventually, the mixed grains are inherited in the final steel sheet structure formed through the bainite or martensitic transformation, resulting in toughness deterioration.

The mechanism of mixed grain formation due to reproduction of old γ grains of the previous structure by this reheating is still unclear. For example, Watanabe et al. "Iron and Steel" No. 6 of the Iron and Steel Institute of Japan.
1st year No. 1 (1975) pp. In the paper of 96-106, when the initial structure is a martensite or bainite lath structure, the old γ is reproduced because the γ generated from between the laths during reheating is subject to variant regulation and only the azimuth of a certain orientation is generated. Matsuda et al., Iron and Steel, Japan Iron and Steel Institute, No. 60, No. 2 (1974).
pp. 226-238, needle-like γ generated in the initial lath structure is generated by martensitic transformation, and these coalesce to form coarse grains corresponding to old γ. Furthermore, S. T. Kimmins et al.
et alScience "Vol. 17, No. 11 (1983)
pp. 519-532, γ generated from the initial lath structure is supposed to grow from the residual γ as a starting point.

As described above, reheating γ of alloy steel and high strength steel
Since the mechanism of the coarsening / mixing of grains is still unclear, and the pre-structures peculiar to alloy steel and high-strength steel are entangled, a solution to that problem could not be found. The only possible solution is to increase the heating rate of reheating because the reproduction of old γ becomes more apparent by gradual heating. It is difficult with the current technology to uniformly heat a large material to a high temperature of Ac ₃ or more at a rate several times to several tens times the current heating rate. Moreover, effective measures have not been found in the disclosure of past patents.

However, in recent years, in the fields of low-temperature storage tanks, low-temperature pressure vessels, marine structures, line pipes, etc., the need for high-strength steel has increased due to the increase in size of structures, and in addition to safety and storage.・ Further low temperature toughness is required from the viewpoint of efficient transportation and severe application environment. In order to deal with this, improvement of the above problems in alloy steel or improvement of low temperature toughness by overcoming the above problems has become essential.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a high strength steel slab capable of producing a steel sheet having a tensile strength of 780 MPa or more, which is unlikely to cause mixed grains during reheating and hot rolling, and which has excellent low temperature toughness. And a method for casting the same.

[0008]

[Means for Solving the Problems] The inventors of the present invention have studied various means experimentally by tracing back the means for solving the above problems up to the fabrication of a cast piece. As a result, the reproduction of the old γ of the γ grain structure and re-mixing due to the re-heating of the high strength steel slab cannot be fundamentally suppressed over a wide range of high strength steel alloy compositions, but the initial slab It was found that the solidification structure has a great influence on the uniformity and toughness of the structure of the steel sheet produced by rolling after reheating, and it has been found that the equiaxed crystallization rate of the initial solidification structure has a large influence. Then, in order to study this in detail, as a result of detailed examination of the structural change in the process of reheating the cast structure by various heat treatment experiments and processing experiments, the solidification structure of the slab that does not cause mixed grains during reheating The knowledge about optimization and its casting method was obtained.

The present invention has been completed based on such findings, and the gist thereof is as follows. (1) Mass%, C: 0.01 to 0.15%, Si:
0.005-0.5%, Mn: 0.1-2%, P: 0.
02% or less, S: 0.01% or less, Ni: 0.5-5.
5%, N: 0.001-0.01%, and
Nb: 0.005 to 0.5%, Ti: 0.003 to 0.
05%, Mo: 0.1 to 2%, Cu: 0.01 to 1.5
%, Cr: 0.05 to 2.5%, V: 0.01 to 0.5
%, B: 0.0005 to 0.005%, one or more kinds, and Al: 0.001 to 0.1%, Zr:
0.001-0.5%, Mg: 0.0005-0.02
%, Ca: 0.0005 to 0.02%, REM: 0.0
A high-strength steel slab containing 01 to 0.5% of two or more kinds, consisting of the balance Fe and unavoidable impurities, and having an equiaxed crystallization ratio of 70% or more. (2)% by mass, C: 0.01 to 0.15%, Si:
0.005-0.5%, Mn: 0.1-2%, P: 0.
02% or less, S: 0.01% or less, Ni: 0.5-5.
5%, N: 0.001-0.01%, and
Nb: 0.005 to 0.5%, Ti: 0.003 to 0.
05%, Mo: 0.1 to 2%, Cu: 0.01 to 1.5
%, Cr: 0.05 to 2.5%, V: 0.01 to 0.5
%, B: 0.0005 to 0.005% of 1 type or 2 types or more, Al: 0.001 to 0.1%, Zr: at the time of refining molten steel containing the balance Fe and unavoidable impurities.
0.001-0.5%, Mg: 0.0005-0.02
%, Ca: 0.0005 to 0.02%, REM: 0.0
Casting of high-strength steel slab characterized by continuously casting molten steel having a dissolved oxygen of 10 ppm or less immediately after adding two kinds or more of 01 to 0.5% simultaneously or sequentially at a molten steel superheat degree of 50 ° C. or less Method.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The present inventors have conducted a detailed examination of the microstructure change during reheating for each part of the solidified structure, and the microstructure change during rolling thereafter, showing that the solidified portion with columnar crystals during solidification has a coarse columnar shape during reheating. While γ is reproduced and becomes a significantly mixed grain,
It was found that the mixed grains were reduced in the portion solidified by the equiaxed crystal.

Further, in the hot rolling after reheating, the progress of recrystallization is slow from the mixed grain γ structure in the columnar crystal solidification portion, whereas the equiaxed crystal solidification portion is in the mixed grain γ structure solidification. From this, it was found that recrystallization is relatively fast and grain size control is promoted by a relatively low processing rate. Therefore, by improving the equiaxed crystallization of the slab, it is possible to solve the problems of the generation of mixed grain γ structure during reheating peculiar to high-strength steel and alloy steel, and the accompanying inhomogeneity of the rolling structure and material deterioration. There was found. Here, equiaxed crystals are solidified crystals that are generated and grown by fragmentation / release of growing columnar dendrites or new nucleation in molten steel, and even though columnar crystals extend from the casting wall in the temperature gradient direction. On the other hand, it is a crystal that grows relatively isotropically.

Further, generally, the equiaxed crystal is formed in the central portion of the slab during solidification, and the width of the equiaxed solidification region with respect to the slab thickness (this is defined as the equiaxed crystallization rate) is 70% or more. Then, it was also found that the columnar crystals extending from the surface layer were also relatively fine, and the γ-grains after rolling were almost uniform over the entire thickness. On the other hand, if the equiaxed crystallization rate is lower than this, the columnar γ that develops from the surface layer becomes relatively long, resulting in nonuniform reheating γ and nonuniform recrystallization during rolling. Therefore, an equiaxed crystallization rate of 70% or more is required to make the γ grains during rolling finer and promote higher toughness of the final steel sheet. The equiaxed crystallization rate is preferably as high as 100% as the upper limit.

The inventors of the present invention have earnestly studied how to make such a slab and obtained new knowledge. Conventionally, in order to improve the equiaxed crystallization rate, electromagnetic stirring is applied to molten steel inside a slab during casting, as disclosed in, for example, JP-A-53-45627 and JP-A-5-115948. Although it has been proposed to add it, this requires additional equipment to the continuous casting equipment, and the optimum position of electromagnetic stirring application changes because the solidification temperature changes depending on the steel composition, and the effect is There is a problem that it greatly varies depending on the situation.

Particularly, it is remarkable in high strength steel and alloy steel containing a relatively large amount of alloying elements. In order to solve the problem, as a result of searching a technique for promoting equiaxed crystal with better controllability, molten steel contains two or more deoxidizing elements of Al, Zr, Mg, REM, and Ca, and immediately after the addition of the deoxidizing element. It was found that the formation acceleration of equiaxed crystals is remarkable when the dissolved oxygen content of the above is less than 10 ppm. This was because, as a result of detailed analysis, the composite oxide formed by adding the deoxidizing element in a complex manner greatly promotes equiaxed crystallization.

As a result of various studies, the effect of adding two or more kinds of deoxidizing elements at the same time or sequentially does not change,
If the dissolved oxygen in the molten steel after the addition of the deoxidizing element exceeds 10 ppm, the coarsening of the complex oxide will be severe and the number effective for equiaxed crystals will decrease, or it will rise during continuous casting and also decrease. It was found that the effect of the present invention was reduced. Further, it was found that even when the degree of superheat of molten steel is large, the formation of the complex oxide is not stable, the effect is reduced, and it becomes difficult to set the proportion of equiaxed crystals to 70% or more. Therefore, in order to ensure the effectiveness of equiaxed crystal formation by complex deoxidation, the dissolved oxygen in the molten steel immediately after the addition of the deoxidizing element is 10 ppm.
It is necessary to set the superheated degree of molten steel at the time of casting to 50 ° C. or less. It is preferable that the dissolved oxygen and molten steel superheat are lower,
If the molten steel superheat degree is less than 0 ° C., problems such as molten steel fluidity occur, which is not preferable.

The term “immediately after the addition of the deoxidizing element” means the time after the effect of the deoxidizing element has spread, and
Although it depends on the addition process, 5 to 15 minutes after the addition is a standard. If the length is longer than this, the possibility of reoxidation of molten steel also appears. In addition, the deoxidizing element can be added from the secondary refining stage to the pot after secondary refining, the tundish during casting, or the mold, and the addition method is injection of powder or lumps of pure metal or alloy or addition by wire. Can be applied.

Next, the reasons for limiting the chemical composition in the present invention will be described. C is contained as an effective component for improving the strength of steel, and if it is less than 0.01%, it is difficult to secure the strength required for structural steel, but if it exceeds 0.15%, it is a mother component. Since the toughness of the material and the welded portion and the weld cracking resistance are reduced, the range is set to 0.01 to 0.15%.

Next, Si is an element effective as a deoxidizing element and for securing the strength of the base material. However, if the content of Si is less than 0.005%, the deoxidation becomes insufficient and it is disadvantageous for securing the strength. is there. On the contrary, if the content exceeds 0.5%, a coarse oxide is formed and ductility and toughness are deteriorated. Therefore, the range of Si is set to 0.005 to 0.5%.

Further, Mn is an element necessary for securing the strength and toughness of the base metal, and it is necessary to contain at least 0.1% or more, but if it is contained excessively, hardenability becomes excessive and weld cracking property is caused. In order to cause deterioration, the upper limit was set to 2% within the allowable range of the material.

P and S are impurity elements that deteriorate ductility and toughness and are preferably reduced as much as possible.
The upper limit of P is set to 0.02% and the upper limit of S is set to 0.01% as the allowable amounts without material deterioration.

Ni is the most effective element for ensuring the toughness, and at least 0.5 is included in the case where the guaranteed temperature of the toughness is -40 ° C. or lower in the high strength steel having a tensile strength of 780 MPa or more. %, Preferably 1% or more. On the other hand, Ni is an expensive alloy element, and if its content is further increased, the hardenability in the case of producing a steel material by thermomechanical treatment becomes excessive, so the upper limit is made 5.5%.

N is harmful to the steel plate toughness when it is in a single solid solution state in steel, but it is impossible to completely remove N in steel industrially, and reducing N more than necessary in the manufacturing process. It is not preferable because it applies an excessive load. Therefore, the lower limit is set to 0.001% as a range in which industrial control is possible and the load on the manufacturing process is allowable. However, if it is contained excessively, the amount of solute N increases and the ductility and toughness are adversely affected, so the upper limit is made 0.01% as an allowable range.

Further, in the component system of the present invention, Nb and Ti which are effective in controlling the recrystallization of austenite in thermomechanical treatment and Mo which is effective in controlling the hardenability are used for controlling the structure for securing strength and toughness. Cu, Cr, V, B
1 type or 2 types or more of are included. The addition range of each element is limited as follows.

Nb is a solid solution and a precipitation state in the austenite phase in order to suppress recrystallization of austenite,
Further, by forming Nb (C, N) during transformation or tempering, it is an element effective for improving strength, but if it is contained in excess, the toughness deteriorates due to precipitation embrittlement. Therefore, as a range in which the effect can be exhibited without degrading the toughness, 0.
It is limited to the range of 005 to 0.5%.

Ti is an element that contributes to the improvement of the strength of the base material by precipitation strengthening and is also effective for refining the heated austenite grain size by forming TiN that is stable even at high temperatures. In the present invention based on thermomechanical processing, It is an essential element. In order to exert the effect, the content of 0.003% or more is required. On the other hand, if it exceeds 0.05%, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so the upper limit is made 0.05%.

Mo is also an element effective in improving hardenability, strength, temper embrittlement resistance, and SR embrittlement resistance, and in order to exert its effect, it is necessary to add 0.1% or more. On the contrary, if the content exceeds 2%, the toughness and weldability deteriorate, so the content is limited to 0.1 to 2%.

Cu has the effect of improving hardenability, strengthening solid solution, and strengthening precipitation by adding 0.01% or more.
%, The hot workability will be problematic. Therefore, in the present invention, the range is 0.01 to 1.5% as a range in which the effect is exhibited and the hot workability or the like does not occur. .

Cr is an element effective for improving the strength of the base material by improving the hardenability and precipitation strengthening, but 0.05% or more is necessary for producing a clear effect, while 2.5% is required.
If added in excess of 0.1%, the toughness and weldability tend to deteriorate, so the content is made 0.05 to 2.5%.

V is an element effective for improving the strength by forming VN as well as improving the hardenability, but if contained in excess, the toughness deteriorates due to precipitation embrittlement. Therefore, as a range in which the effect can be exhibited without significantly deteriorating the toughness, 0.01 to
It is limited to the range of 0.5%.

B is an element capable of enhancing the hardenability in a small amount by segregating to the austenite grain boundaries in a solid solution state, but in the state segregated to the grain boundaries, it is also effective in suppressing recrystallization of austenite. Is. In order to exert the effect of hardenability and suppression of recrystallization, it is necessary to add 0.0005% or more, while if added in excess of 0.005%, BN, Fe ₂₃ (C, B) 0.0005 to 0.005% because coarse precipitates such as ₆ are generated and the toughness deteriorates.
Limited to

Further, Al, Zr, Mg, Ca and R are used as deoxidizing elements for promoting equiaxed crystal formation which is a feature of the present invention.
Two or more kinds of EM are contained. The addition range of each element is limited as follows.

Al is an element which has a strong deoxidizing power and is effective for oxide formation. 0.001 to form oxides in molten steel
% Or more must be contained. On the other hand, when the content exceeds 0.1% and is excessive, the oxide becomes coarse and the ductility is extremely deteriorated. Therefore, it is necessary to limit the content to 0.001% to 0.1%.

Zr has a strong deoxidizing power similar to Al and forms a complex oxide having a relatively high specific gravity, and is therefore an effective element for promoting equiaxed crystallization. To form an oxide in molten steel, it is necessary to contain 0.001% or more, but 0.5
If it is contained excessively in excess of%, the oxide becomes coarse and the ductility is extremely deteriorated, so the content is limited to 0.001% to 0.5%.

Mg is a strong deoxidizing element and is effective because it forms a complex oxide with various elements. In order to form an oxide in molten steel, it is necessary to contain 0.0005% or more, but if it is stably contained in excess of 0.02%, the vaporized vapor pressure is high, which is dangerous in the steelmaking process. Therefore, it is set to 0.0005% to 0.02%.

REM also has a strong deoxidizing power and promotes the formation of complex oxides. In order to form an oxide in molten steel, it is necessary to contain 0.001% or more, but since it is expensive, the addition of more than 0.5% significantly raises the cost of the steel sheet and has the effect as a deoxidizing element. Since it is saturated, the range is made 0.001% to 0.5%.

Ca is an element having the maximum deoxidizing power in steel and can be complex deoxidized with any of the above elements. In order to form an oxide in molten steel, it is necessary to contain 0.0005% or more, but if added in excess of 0.02%, liquefaction of the oxide is promoted and coarsening is caused by coalescence. Is 0.0005% to 0.02%.

[0037]

The above is a description of the requirements of the present invention. Furthermore, the effects of the present invention will be shown based on Examples. The chemical compositions of 6 to 6 shown in Table 1 were examined. Of these, A to D are compositions within the scope of the present invention, and two kinds of E and F are compositions outside the invention. As a deoxidizing element, A is Al-Zr-
Mg, B are Al-Ca, C is Mg-Ca, D is Zr-R
Includes EM. Further, E and F of the comparative steels are Al deoxidized. All were melted in an actual converter, the deoxidizing element was added in the RH of secondary refining, and 5 minutes after the addition, the dissolved oxygen in the molten steel was measured. The results are shown in Table 2. The molten steel temperature during oxygen measurement was constant at approximately 1600 ° C in all cases. This molten steel was cast to a thickness of 240 mm by an actual continuous casting machine. Table 2 also shows the degree of superheat of molten steel during casting. Regarding alloys B and C, alloys having different superheats of molten steel were also cast as comparative examples. After casting, 1/2 of the casting width is cut in the longitudinal direction, and the cross-section is etched printed and over Hoffer liquid (edited by the Iron and Steel Institute of Japan, Iron and Steel Handbook Vol. 1,
p. The equiaxed crystallization rate was determined from the macrostructure corroded in 206). The results are also shown in Table 2.

As is clear from Table 2, N which is the present invention
o. 1 to 4 showed a high equiaxed crystallization rate of 70% or more, whereas No. 1 containing only one type of comparative deoxidizing element. 5,
The equiaxed crystallization rate of 6 was low. Moreover, even if two or more kinds of deoxidizing elements were contained, No. 1 which had a high degree of superheat of molten steel during casting. 7 and 8
No. of the same component The equiaxed crystallization rate was significantly reduced as compared with 2 and 3.

From these, the effect of the present invention is clear,
Furthermore, a full-thickness test piece was extracted from this cast slab, the slab reheating was reproduced using a heat treatment furnace, and the slab was rapidly cooled to observe reheated γ grains. The rate of temperature rise is 6 per minute, which is similar to that of an actual heating furnace.
The heating temperature is 1000 ° C. As a result, the No. The slabs 1 to 4 had a substantially uniform grain size over the entire thickness, whereas the comparative steels all had a mixed grain structure containing coarse grains. Further, using each cast, 100
A steel plate having a plate thickness of 25 mm was produced by a normal DQT process of heating at 0 ° C. When the toughness of the steel sheet was evaluated, higher toughness was obtained when the slab of the present invention was used than when the comparative slab was used, and it is judged that this is due to the grain size regulating effect during reheating and rolling. Table 2 also shows the γ grain size measurement result in the reheating experiment, the toughness evaluation result (Charpy vTrs) and the tensile strength of the 25 mm steel plate.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

Industrial Applicability According to the present invention, there is provided a slab of high-strength steel which has a tensile strength of 780 MPa or more, is unlikely to cause mixed grains during reheating and hot rolling, and enables production of a steel sheet excellent in low temperature toughness. It will be possible. As a result, it becomes possible to manufacture high-strength steel sheets with excellent toughness, and low-temperature storage tanks, low-temperature pressure vessels,
It is possible to provide structural materials with extremely high safety to marine structures, ships, bridges, line pipes, etc., and the industrial effect is extremely large.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoichi Tanaka             5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Corporation             Ceremony Company Nagoya Steel Works (72) Inventor Hiroyuki Kinoshita             2-6-3 Otemachi, Chiyoda-ku, Tokyo New Japan             Steelmaking Co., Ltd. (72) Inventor Takuya Hara             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division F-term (reference) 4E004 MB20 NB01 NC01

Claims (2)

[Claims] 1. In mass%, C: 0.01 to 0.15%, Si: 0.005 to 0.5%, Mn: 0.1 to 2%, P: 0.02% or less, S: 0.01% or less, Ni: 0.5 to 5.5%, N: 0.001 to 0.01%, and Nb: 0.005 to 0.5%, Ti: 0.003 to 0.05%, Mo: 0.1 to 2%, Cu: 0.01 to 1.5%, Cr: 0.05 to 2.5%, V: 0.01 to 0.5%, B: 0 0.0005 to 0.005%, one or more, and Al: 0.001 to 0.1%, Zr: 0.001 to 0.5%, Mg: 0.0005 to 0.02%, Ca: 0.0005 to 0.02%, REM: 0.001 to 0.5%, two or more kinds are contained, the balance is Fe and inevitable impurities, and the equiaxed crystallization rate is 7.
A high-strength steel slab that is 0% or more. 2. In mass%, C: 0.01 to 0.15%, Si: 0.005 to 0.5%, Mn: 0.1 to 2%, P: 0.02% or less, S: 0.01% or less, Ni: 0.5 to 5.5%, N: 0.001 to 0.01%, and Nb: 0.005 to 0.5%, Ti: 0.003 to 0.05%, Mo: 0.1 to 2%, Cu: 0.01 to 1.5%, Cr: 0.05 to 2.5%, V: 0.01 to 0.5%, B: 0 0.005 to 0.005% of one kind or two or more kinds of Al, 0.001 to 0.1%, Zr: 0.001 to 0.1% during refining of molten steel containing the balance Fe and inevitable impurities. 2% or more of 5%, Mg: 0.0005 to 0.02%, Ca: 0.0005 to 0.02%, REM: 0.001 to 0.5% are added simultaneously or sequentially. Casting method of high strength steel cast piece, characterized in that the dissolved oxygen immediately after the continuous casting below molten steel molten steel superheat 50 ° C. to be 10ppm or less.