JP3027912B2 - Manufacturing method of hot rolled steel sheet with excellent hole spreadability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to excellent workability, particularly for members subjected to severe hole expansion forming or stretch flange forming and overhang forming, such as wheel discs and automobile undercarriage parts. The present invention relates to a method for producing a high-strength hot-rolled steel sheet exhibiting hole expandability and micro-extension crack resistance.

[0002]

2. Description of the Related Art From the viewpoint of resource saving and energy saving, there is a demand for steel materials, for example, high-strength hot-rolled steel sheets, to have higher strength and improved formability. In particular, in order to reduce the weight of the vehicle, there is an increasing tendency to use high-tensile steel for wheel discs, automobile suspension parts, and the like, and it has been desired to improve the hole-expandability of high-tensile hot-rolled steel sheets.

Further, in recent wheel discs and vehicle suspension parts, not only the hole expandability but also the nut seat of the disk and the overhang portion of the suspension parts need to be sufficiently elongated. This is because in these places, minute overhang cracks due to surface inclusions are likely to occur.

[0004] The following are known techniques for improving the hole expandability of a hot-rolled steel sheet. Japanese Patent Application Laid-Open No. 55-107732: hole expandability due to decrease in C content
(Stretch flangeability), but 440 N / m
m ² or more classes of high strength is difficult.

JP-A-54-88827: REM, Ca or
Zr is added to spheroidize sulfides, all oxides are deoxidized with Al, and cementite in pearlite is spheroidized by addition of Ti and hot rolling and winding conditions control to expand pores (stretch flangeability). ) To improve. Although it is spherical, the sulfide and alumina remain in the steel, so that the micro overhang cracking resistance is insufficient.

Japanese Patent Application Laid-Open No. 61-84330: A ferrite + bainite structure is obtained by controlling the winding conditions to improve the hole expanding property (burring property). As in Japanese Patent Application Laid-Open No. 54-88827, the control of inclusions is insufficient, and the resistance to micro overhang cracking cannot be satisfied.

[0007]

As described above, in the prior art, since the emphasis was placed on the improvement of the hole expandability, the inclusions were not considered much, only to homogenize the structure. In other words, only the reduction of A-based inclusions (sulfide-based elongated inclusions) in the ASTM-A method was
No countermeasures have been taken to reduce (cluster inclusions), C-based inclusions (square inclusions), and D-based inclusions (spherical inclusions). For this reason, the possibility of forming cracks or fatigue cracks originating from foreign inclusions is becoming increasingly high, and in particular, micro-extension cracks (potch cracks) cause them,
There is an urgent need to improve micro overhang crack resistance.

[0008] As measures against inclusions for conventional hot-rolled steel sheets,
As shown below, the form control is almost the same, and there is no example of a drastic solution of reducing the amount of inclusions. JP-A-56-9317: This is a method of controlling the form of inclusions by injecting Ca and then deoxidizing and desulfurizing and then injecting Ca, but this does not reduce the amount of inclusions.

Japanese Patent Application Laid-Open No. 56-98415: (Steel Making) At the time of tapping, CaO is charged, Al is charged into a ladle, and desulfurization is ensured by bubbling Ar. An alloy is added, but adding a Ca alloy has to be expensive.

JP-A-64-75622: (Steel Making) A method of desulfurizing or spheroidizing inclusions by adding a Ca alloy to a ladle has been disclosed, but from the viewpoint of reducing inclusions. Is not enough. JP-A-4-301029: Form control of inclusions by CaO injection immediately after tapping. Although B-based inclusions decrease, D-based inclusions still cannot be reduced.

The morphological control of inclusions alone cannot improve the resistance to micro-extension cracking. Therefore,
To simultaneously improve hole expansion and micro overhang crack resistance,
In order to reduce the amount of inclusions themselves, it is required to establish a new method of manufacturing hot-rolled steel sheets, including a comprehensive review from the steelmaking stage.

An object of the present invention is to provide a method capable of inexpensively and efficiently producing a hot-rolled steel sheet having excellent hole expandability and micro-extension crack resistance by reducing the amount of inclusions.

[0013] Here, the "hole spreading property" in the present invention means that after punching a steel plate to a diameter of 12 mm, the punching hole is pushed and expanded with a 60 ° conical punch, and cracks generated on the periphery of the hole penetrate the plate thickness. Defined as the rate of change of pore size when stopped at the time,
The goal is to get more than 70% of this.

[0014] The "micro-extension cracking resistance" refers to 10R
It is defined by whether or not there is a local dent (scratch on the steel sheet surface) in the bulging part when bulging with a ball-head punch is performed.
If the dent occurrence is 2 or less after forming individually, it is considered as acceptable, and for ultra-hard machining applications such as wheel disc nut seats,
200 pieces were molded, and the case where the number of dents was 3 or less was determined to be acceptable.

[0015]

Means for Solving the Problems The present inventors have conducted various studies to achieve the above-mentioned object, and have obtained the following findings.

1) By introducing a CaO + Al ₂ O ₃ system flux at the time of tapping, a high desulfurization ability is realized at the tapping stage, so desulfurization can be achieved only by vigorous stirring of Ar bubbling. 2) Conventionally, A-based inclusions, that is, sulfide
(MnS, FeS) was only controlled, so Al ₂ O ₃ sometimes remained.
₂ O ₃ ), D-type (CaO)
The hole expandability and the resistance to micro-extension cracking can be simultaneously improved.

3) As the low melting point flux, CaO + Al ₂ O ₃
By using a system flux and injecting only Ar into the ladle following it and optimizing its amount,
A system, B system, and D system inclusions can be reduced. 4) Furthermore, for wheel disc applications, when the amount of Si is high, problems in appearance are likely to occur, so it is more effective to combine the removal of Si. 5) By establishing a steelmaking method that can simultaneously achieve low S and low Si, a method for stably manufacturing wheel disks can be realized.

[0018]

Means for Solving the Problems The present inventors aim to reduce inclusions in order to improve hole expandability and crack resistance to inclusions, and at the same time, to improve the beauty of the steel sheet surface if necessary. In order to secure (suppress the stripe scale), slabs manufactured using a steelmaking method with a low Si content are wound up under hot rolling conditions that are optimal for hole expandability and elongation. The summary is as follows.

(1) At the time of tapping the converter, it is based on CaO and Al
₂ O ₃ , CaF ₂ , a low melting point flux containing at least one of SiO ₂ and a slag modifier based on Al and CaCO ₃ are charged into the molten metal, and an inert gas is introduced during ladle refining. By blowing, the chemical composition is as follows: C: 0.03 to 0.20 wt%, Si: 2.0 wt% or less, Mn: 0.3 to 2.
0 wt%, S: 0.002 wt% or less, a step of smelting molten steel consisting of a balance of Fe and unavoidable impurities, a step of continuously casting the obtained molten steel into a slab, and a finishing temperature of the slab: 800 to 950. ℃, winding temperature: 400
A process of hot rolling at a temperature of up to 550 ° C., comprising the steps of:

(2) After a medium solvent having a basicity of 8 or more is charged at the time of baking out of the converter, the above (1) is repeated, and the flow rate of the inert gas blown during ladle refining is adjusted to 10 to 60 Nm ³ per 250 tons of molten steel.
(0.04 to 0.24Nm ³ / Ton), a method for producing a hot-rolled steel sheet having excellent hole expandability and resistance to micro overhang cracking, characterized in that the amount of Si in the chemical composition of molten steel is reduced to 0.2% or less by regulating it. .

(3) The smelted steel further contains one or more elements of Cr: 1.0 wt% or less, Ti: 0.1 wt% or less, and Nb: 0.1 wt% or less. The method for producing a hot-rolled steel sheet according to the above (1) or (2).

(4) The smelted steel further contains one or more elements of P: 0.1 wt% or less, Cu: 0.5 wt% or less, and Ni: 0.5 wt% or less. The method for producing a hot-rolled steel sheet according to any one of the above (1) to (3).

[0023]

Next, the operation of each step in the present invention will be specifically described. The present invention comprises a steel making process, a continuous casting process, and a hot rolling process as described above.

Steelmaking Step During the tapping of the converter, the reforming flux and the slag modifier are charged into the molten metal, and an inert gas is blown into the molten steel in the ladle. The reforming flux used in the present invention is to change the slag in the ladle into a low melting point and a slag composition in the direction of low viscosity and at the same time to exert a desulfurizing action. Although there is no, the present invention is based on the CaO Al ₂ O _3, those containing CaF _2, at least one of SiO _{2, preferably, CaO + Al 2 O 3,} CaO + CaF 2 + Al
_At least one member selected from the group consisting of ₂ O ₃ .

The slag modifier is added to reduce lower oxides in the slag, and is not limited to a specific one as long as it is, for example, a slag modifier based on Al and CaCO _3. And preferably, Al + CaCO ₃ , Al + CaCO
₃ + Al ₂ O ₃ .

In a preferred embodiment of the present invention, a high basicity solvent having a basicity of 8 or more is introduced during the blowing of the converter to reduce SiO ₂ in the slag composition at the time of ladle outflow. And
During tapping CaO-Al ₂ O ₃ based low-melting flux and Al and CaCO ₃
Slag modifier based on With this,
It has a high basicity slag composition and ensures slag fluidity that can exhibit the de-S ability.

Subsequently, in the ladle, the S gas and the Si content are reduced by limiting the amount of Ar gas to preferably 0.04 to 0.24 Nm ³ per ton of molten steel.
Balance of re-dissolution. Also, at this time, the D-type inclusions that cause inclusion cracking can be reduced at the same time due to the synergistic effect of shortening the ladle processing time and lowering the slag viscosity.

That is, in the above-mentioned preferred embodiment of the present invention, the slag composition is shifted to the low SiO ₂ side to improve the de-S property and prevent the re-dissolution of Si. It is characterized by fundamentally preventing the inclusion of inclusions.

Here, the high basicity solvent is, for example, CaO
+ SiO ₂ + CaF ₂ or CaO + SiO ₂ + MgO, basicity = CaO / SiO ₂ : 8 to 40 is desirable. The inert gas may be an inert gas of Ar, Ne, or He in order to promote an interfacial desulfurization reaction of slag = molten steel. Economical
Ar is preferred.

Therefore, according to the present invention, CaO—Al ₂ O ₃
By introducing a low melting point flux and a slag modifier, the slag is promoted in the ladle, and then the Ar gas is blown into the ladle to perform desulfurization at the slag-molten steel interface. .

In the conventional Ca treatment, desulfurization from the inside of molten steel is attempted by blowing CaO in a ladle, whereas in the present invention, desulfurization mechanism only at the slag-molten steel interface is performed by blowing inert gas. Use

In the Ca treatment, the CaO-based reforming flux is introduced after tapping, which causes a significant drop in the temperature of the molten metal, and accordingly, in order to secure the temperature during CaO injection, the RH degassing tank is used.
Since O ₂ blowing is performed, a large number of Al ₂ O ₃ inclusions (B-based and D-based) are generated. In addition, by blowing CaO on the ladle,
The spheroidized sulfide does not float up to the interface between the slag and the molten steel, remains in the steel, and becomes a D-based inclusion.

Therefore, in the present invention, a low melting point flux and a slag modifying material are introduced during the tapping of the converter so as to obtain a slag composition having a small temperature drop of the molten metal surface and a low melting point and low viscosity. It is characterized in that an interfacial desulfurization reaction is performed only by blowing in an inert gas.

The blowing of the inert gas may be performed by using an injection lance immersed in the molten steel in the ladle, or by providing a blowing plug as appropriate. For example
The amount of the inert gas such as Ar gas to be blown is not particularly limited, but may be sufficient to cause the flow of the molten steel. About 0.02 to 0.4 Nm ³ is sufficient.

However, in a preferred embodiment of the present invention, it is preferable that the blowing amount of the inert gas such as Ar gas is 0.04 to 0.24 Nm ³ per ton of molten steel. Thus, according to the present invention, not only the system A defined by the ASTM-A method but also the system B
And D-based inclusions can be greatly reduced, and the resistance to micro-extension cracking is dramatically improved.

Continuous Casting Step When desulfurization in a ladle reaches the target S level and Si level, deslagging is performed, and then a slab is obtained by, for example, conventional continuous casting.

Hot Rolling Step In order to improve the hole expandability, it is necessary to homogenize the structure, but the bainite single phase has insufficient ductility and the ferrite single phase has insufficient strength. In addition, to prevent the occurrence of cracks due to residual inclusions,
Larger crystal grains are more effective.

For this reason, the finishing temperature of the hot rolling is set to a high temperature finish in order to increase the size of polygonal ferrite grains.
Cross the ferrite nose in the processed CCT diagram to complete in the bainite area.

Finishing temperature : 800 ° C. in order to make the ferrite polygonal and coarse.
The above is necessary. If it is higher than 950 ° C, the threadability will decrease,
This leads to reduced economics.

For forming bainite at the winding temperature , a continuous cooling curve for processing (processing CCT)
(Fig.)
~ 550 ° C. The reasons for limiting the composition of molten steel in the present invention are as follows. In this specification, "%"
"%" Is "% by weight" unless otherwise specified.

C: C is set to 0.03% or more for solid solution strengthening of ferrite and formation of bainite. If it is more than 0.20%, the hole expandability is rapidly deteriorated. Preferably, 0.05 to 0.15
%.

Si: added for strengthening solid solution of ferrite and suppressing generation of pearlite. However, if it is more than 2.0%,
This leads to embrittlement of the weld. Preferably, it is set to 1.50% or less. According to a preferred embodiment of the present invention, if the content of Si is more than 0.2%, striped scale frequently occurs on the surface of the steel sheet, so that the content is set to 0.2% or less in order to maintain beautiful appearance.

Mn: 0.3% or more is indispensable for compensating the strength for lowering the C content and for generating bainite. But 2.
If it is more than 0%, ductility is deteriorated due to an increase in hardenability. Preferably, it is 0.70 to 1.60%.

S: Since it greatly affects the formation of inclusions,
002% or less. Preferably it is 0.0015% or less, more preferably 0.0005% or less.

In the present invention, if desired,
(I) Cr: 1.0 wt% or less, Ti: 0.1 wt% or less, Nb: 0.1
one or more of wt% or less, and / or (II)
One or more of P: 0.1 wt% or less, Cu: 0.5 wt% or less, and Ni: 0.5 wt% or less may be contained.

Cr, Ti, Nb: These are all effective elements for increasing the strength, and may contain at least one element as desired. Cr is a quench hardening element effective for increasing the strength to 590 N / mm ² or more. If it exceeds 1.0%, ductility is deteriorated.

Ti is a precipitation strengthening element effective for increasing the strength to 540 N / mm ² or more. However, if it exceeds 0.2%, YR
Becomes too high, and the shape freezing property is reduced. Nb, like Ti, is a precipitation strengthening element effective for increasing the strength to 540 N / mm ² or more, but if it exceeds 0.1%, the YR becomes too high and Ti
Similarly, the shape freezing property is reduced.

P, Cu, Ni: At least one of these elements is also added to improve corrosion resistance. Simultaneous addition of P and Cu is effective for solid solution strengthening and improvement of base metal corrosion resistance. Ni
Is preferably added in the same amount as Cu to prevent Cu checking. However, if the content of P is more than 0.1%, grain boundary embrittlement is caused. If the content of Cu is more than 0.5% and the content of Ni is more than 0.5%, recyclability and
Economic efficiency decreases. Preferably, P and Cu are added simultaneously, or P, Cu and Ni are added simultaneously. Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0049]

EXAMPLES (Example 1) Molten steel having a steel composition shown in Table 1 was smelted in a converter, and a modified flux and a slag modifier were introduced into the molten steel at the time of exiting the converter according to the following operation. Ladle desulfurization was performed.

In the example of the present invention, at the time of tapping of a 250 ton converter,
After 1.0 ton of the low melting point flux shown in Table 3 was introduced into the ladle molten steel as a flux, 300 kg of the self-decomposable flux shown in Table 3 was immediately introduced.

Subsequently, the injection lance was immersed in the molten steel so that the discharge hole was located near the bottom of the ladle.
Argon gas was blown at 4 Nm ³ / min for 15 minutes. Next, the molten steel thus produced was continuously cast to obtain a slab. See steel types A1 to A14 in Table 1.

In Comparative Example B1 of Table 1, after tapping from the 250 ton converter, 1.0 ton of the modified flux (CaO) was introduced into the ladle molten steel, and immediately the slag modifier (Al 50% + Al ₂ O ₃ 50%) to 50
0 kg was introduced.

Subsequently, oxygen gas is injected by injection.
After blowing at 2.5 Nm ³ / min for 10 minutes and further 750 kg of a desulfurizing agent (CaO powder) from an injection lance, molten steel was continuously cast to obtain a slab.

In Comparative Example B2, during tapping of a 250 ton converter,
The injection lance was immersed in the ladle molten steel, and 450 kg of the slag modifier (Al 50% + Al ₂ O ₃ 50%) was injected, followed by 500 kg of the desulfurizing agent (CaO 80% + CaF ₂ 20%). . And slab was obtained by continuously casting molten steel.

In Comparative Example B3, after tapping from a 250 ton converter,
Add slag modifier (50% Al + 50% Al ₂ O ₃ ) to molten steel in ladle
0 kg was introduced. Then, the injection lance was immersed in the ladle molten steel, and argon gas was blown at 4 Nm ³ / min for 10 minutes to continuously cast the molten steel to obtain a slab.

The slabs thus produced in each steelmaking continuous casting process were heated to a heating temperature of 1270 ° C.
Under the hot rolling conditions of each finishing temperature and winding temperature shown in
Hot rolling was performed to 6 mm. And after pickling each coil,
The plate was cut on the cutting line. A JIS No. 5 test piece was used for the tensile test.

In the hole expansion test, after a steel plate was punched with a diameter of 12 mm, the hole was pushed and expanded with a 60 ° conical punch, stopped when the crack penetrated the plate thickness, and changed at a rate of change from the punched hole (diameter 12 mm). Expressed expandability. The content of B type and D type inclusions is ASTM-A
It was measured by the method and expressed by the sum of the number of inclusions of 3.0 ranks or less in each of the Heavy and Thin series according to the ASTM method regulations.

In the micro-extension cracking test, extension molding was performed using a 10R ball-head punch, and the number of concave portions in the deformed portion was measured.
The case where the sum of the number of concave portions in 100 test pieces was 2 or less was regarded as good in micro-extension crack resistance, and the case where the total was 3 or more was defective.

[0059]

[Table 1]

[0060]

[Table 2]

[0061]

[Table 3]

According to the present invention, a hot-rolled steel sheet having a tensile strength of 440 N / mm ² or more and having excellent hole expandability and micro-extension cracking resistance can be obtained. In Comparative Example 1, although the Ca treatment was performed in the steelmaking process in order to secure the hole expandability, the micro-extension crack resistance was impaired because a large number of B-based inclusions were present in the steel.

In Comparative Example 2, as a result of blowing the desulfurizing agent in the steel making process, a large number of D-based inclusions were generated, and the resistance to micro overhang cracking was impaired. In Comparative Examples 3 to 5, the hot rolling conditions were removed, so that the hole expandability was reduced. In Comparative Example 6, the hole expandability (and elongation) was reduced due to an excessive amount of C. In Comparative Example 7, since the desulfurization treatment was not performed in the steel making process and the inclusion reduction treatment was not sufficiently performed, the hole expanding property and the resistance to micro-overhang cracking were also impaired.

Example 2 In this example, a low melting point flux and a slag modifier were charged into molten steel at the time of converter tapping according to the following operation, and ladle desulfurization was performed.

In the example of the present invention, at the time of tapping of the 250 ton converter,
After adding the high basicity solvent shown in Table 4, the low melting point flux was 1.0 ton and the slag modifier was 300 kg at the time of tapping.
I put it in.

Subsequently, the injection lance was immersed in the molten steel so that the discharge hole reached the vicinity of the bottom of the ladle.
Argon gas was blown in the amounts shown in Examples Nos. 1 to 14, and molten steel was continuously cast to obtain slabs.

In Comparative Example B1, the solvent shown in Table 4 was added during blowing of a 250 ton converter and 1.0 ton of a low melting point flux was introduced into a ladle after tapping, and 500 kg of a slag modifier was immediately added. did.

Subsequently, oxygen gas is injected by injection.
After injecting only 50 Nm ³ and further injecting 750 kg of desulfurizing agent (CaO powder) from the injection lance, molten steel was continuously cast to obtain a slab.

In Comparative Example B2, the low basicity solvent shown in Table 4 was used instead of the high basicity solvent in the present invention. In Comparative Examples B3 and B4, the Ar gas blowing rates were 4 and 72 Nm ³ .

Comparative Example B5 shows the results of Table 5 when spraying with a 250 ton converter.
After the tapping, 400 kg of a slag modifier was introduced into the ladle molten steel. Then, the injection lance was immersed in the ladle molten steel, and argon gas was blown in by 4 Nm ³ to continuously cast the molten steel to obtain a slab.

The slab produced in each steelmaking continuous casting process was heated to a heating temperature of 1270 ° C.
Under the hot rolling conditions of each finishing temperature and winding temperature shown in
Hot rolling was performed to 6 mm. And after pickling each coil,
The plate was cut on the cutting line.

The tensile test used a JIS No. 5 test piece. In the hole expansion test, after punching a steel plate with a diameter of 12 mm, the hole is pushed and expanded with a 60 ° conical punch, stopped when the crack penetrates the plate thickness, and the hole expansion property is determined by the rate of change from the punched hole (diameter 12 mm). expressed.

The amounts of B- and D-system inclusions were measured by the ASTM-A method, and represented by the sum of the number of inclusions of 3.0 ranks or less in each of the Heavy and Thin series according to the ASTM method. The micro overhang cracking test is performed by overhanging with a 10R ball-head punch, and the number of recesses in the deformed portion is measured. If the total number of recesses in 200 test pieces is 3 or less, the micro overhang cracking resistance is considered to be good. , 4 or more was regarded as defective.

[0074] The surface properties were determined as defective if the stripped scale having a width of 2 mm x length of 5 mm or more at the coil top / 10 m or more was generated on the steel sheet surface after pickling, and good if the number was 9 or less. did. The results of these tests are summarized in Table 6.

[0075]

[Table 4]

[0076]

[Table 5]

[0077]

[Table 6]

According to the present invention, a hot-rolled steel sheet having a tensile strength of 440 N / mm ² or more and having excellent hole expanding properties, micro-extension cracking resistance, and excellent surface properties can be obtained. In Comparative Example No. 15, the CaO treatment was performed in the steelmaking process to ensure the hole expandability.
Since a large number of B-based inclusions were present in the steel, the resistance to micro overhang cracking was impaired.

In Comparative Example No. 16, since a high SiO ₂ system was used as the low basicity solvent, a large number of D-based inclusions were generated.
The resistance to micro overhang cracking was impaired. In Comparative Example No. 17, Ar
Since the blowing amount was small, the S amount was high, and the hole expandability was impaired.

In Comparative Example No. 18, since the Ar blowing amount was large,
The Si content was high and the surface properties were impaired. In Comparative Examples Nos. 19 to 21, the hot rolling conditions were out of the range of the present invention, so that the hole expandability was reduced.

In Comparative Example No. 22, since the amount of carbon was excessive, the hole expandability and elongation were reduced. In Comparative Example No. 23, since the desulfurization treatment was not performed in the steel making process and the inclusion reduction treatment was not sufficiently performed, the hole expansion property and the resistance to micro-overhang cracking were also impaired.

[0082]

As described above, according to the present invention,
By simply performing ladle desulfurization, desulfurization as well as removal of inclusions is effectively performed, and the practical significance of the present invention is significant because of the simplicity of the means.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigeki Nomura 3, Kashima-cho, Kashima-gun, Ibaraki Pref., Sumitomo Metal Industries, Ltd. Inside the Kashima Steel Works of Metal Industry Co., Ltd. (72) Kenji Saka, 3rd light section, Kashima-cho, Kashima-gun, Ibaraki Prefecture Sumitomo Metal Industries Co., Ltd. Kashima Works (56) References JP 5-9572 (JP, A) JP Hei 4-276016 (JP, A) Patent 2976852 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/00, 8/02 C21C 5/36, 5/46 C21C7 / 064,7 / 076

Claims (4)

(57) [Claims] (1) At the time of converter tapping, based on CaO, Al ₂ O
_3. A low melting point flux containing at least one of CaF ₂ and SiO ₂ and a slag modifier based on Al and CaCO ₃ are charged into the molten metal, and an inert gas is blown during ladle refining. Thus, a process of melting molten steel having a chemical composition of C: 0.03 to 0.20 wt%, Si: 2.0 wt% or less, Mn: 0.3 to 2.0 wt%, S: 0.002 wt% or less, with the balance being Fe and unavoidable impurities. Forming a slab by continuous casting of the obtained molten steel; finishing temperature of the slab: 800 to 950 ° C .; winding temperature: 400
A method of hot rolling at a temperature of up to 550 ° C., comprising the steps of: 2. A high basicity solvent having a basicity of 8 or more is supplied during the tapping of the converter, and then, based on CaO, Al ₂ O ₃ , CaF ₂ ,
And a low melting flux containing at least one of SiO _2, Al and the CaCO ₃ was charged with slag modifier based on the melt, and the inert gas the molten steel 250 tons per during ladle refining 10-60 Nm ³ by writing (0.04~0.24Nm ³ / Ton) spray, chemical composition C: 0.03~0.20wt%, Si: 0.2 wt% or less, Mn: 0.3 ~2.0 wt%, S: 0.002 wt% or less, the balance A step of smelting molten steel composed of Fe and unavoidable impurities; a step of forming the slab by continuous casting of the obtained molten steel; a finishing temperature of the slab: 800 to 950 ° C .;
A method of hot rolling at a temperature of up to 550 ° C., comprising the steps of: 3. The molten steel according to claim 1, wherein Cr: 1.0 wt% or less;
The method for producing a hot-rolled steel sheet according to claim 1 or 2, further comprising one or more elements of Ti: 0.1 wt% or less and Nb: 0.1 wt% or less. 4. The method according to claim 1, wherein the melted steel has a P content of 0.1 wt% or less,
The method for producing a hot-rolled steel sheet according to any one of claims 1 to 3, further comprising one or more elements of Cu: 0.5 wt% or less and Ni: 0.5 wt% or less.