JP3403300B2 - 590 MPa class rolled section steel and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high tensile rolled steel having excellent toughness, which is used as a structural member of a building, and a method for producing the steel.

[0002]

2. Description of the Related Art Due to the construction of super-high-rise buildings and stricter safety standards, steel materials used for columns, such as H-section steel with a particularly large plate thickness (hereinafter referred to as extra-thick H-section steel) Are required to have higher strength, higher toughness, and lower yield ratio. In order to satisfy such required characteristics, conventionally, heat treatment such as normalizing treatment is performed after the rolling is completed. The addition of heat treatment causes a significant cost increase such as reduction of heat treatment cost and production efficiency, and there is a problem in economic efficiency. In order to solve this problem, it was necessary to develop a slab and a manufacturing method by a new alloy design that can obtain high-performance material properties as rolled.

Generally, shaped steel with a flange, such as H
When shaped steel is manufactured by universal rolling, the rolling conditions (temperature, reduction rate) from the rolling shaping and the peculiarity of the shape affect the rolling finish temperature, reduction rate, and cooling rate at each part of the web, flange, and fillet. Make a difference. As a result, variations in strength, ductility, and toughness occur between the parts,
For example, a part that does not meet the criteria such as rolled steel for welded structure (JIS G3106) occurs. In particular, when rolling and manufacturing ultra-thick H-section steel using continuously cast slabs as raw materials, there is a limit to the maximum thickness of slabs that can be produced by continuous casting equipment, and a sufficient slab cross-sectional area required for modeling can be obtained. Therefore, the rolling is a low pressure lower ratio rolling. Further, since the high temperature rolling is aimed at in order to obtain the dimensional accuracy of the product by the rolling shaping, the thick flange portion becomes the high temperature rolling, and the steel material after the rolling is gradually cooled. As a result, the microstructure becomes coarser and the strength and toughness decrease.

As a structure refining method in the rolling process, T
Although there is an MCP (Thermo-Mechanical-Controll Process), it is difficult to apply low temperature / large reduction rolling such as TMCP to a steel sheet because the rolling conditions are limited in the shape rolling. Further, in the field of thick steel plates, high strength and high toughness steel is manufactured by utilizing the precipitation effect of VN, for example, Japanese Patent Publication No. 62-5054.
The techniques disclosed in Japanese Patent Publication No. 8 and Japanese Patent Publication No. 62-54862 are proposed. However, when this method is applied to the production of 590 MPa class, since high concentration of solute N is contained, high carbon island martensite (hereinafter referred to as M *) is formed in the bainite structure to be formed. However, there is a problem that it is difficult to clear the standard value because the toughness is significantly reduced.

[0005]

In order to solve the above-mentioned problems, it is necessary to form a low carbon bainite with a small amount of M * formation in the as-rolled shape steel to refine the structure. In order to reduce the γ grain size during rolling and heating, in the steelmaking process, MgO is preliminarily finely crystallized in the slab, and TiN is finely precipitated in the core of the slab to further reduce carbon. In addition, it is necessary to manufacture a slab containing a small amount of microalloy that can obtain a high strength with a small amount. Further, the fillet portion of the joint between the flange of the H-section steel and the web coincides with the central segregation zone of the CC slab, and MnS in this segregation zone is significantly stretched by rolling. High concentration element segregation zone and stretched MnS here
May significantly reduce the drawing value and toughness in the plate thickness direction, and may cause lamellar tearing during welding. It is also a major issue to prevent the formation of MnS having this harmful effect. As described above, it is difficult for the conventional technique to manufacture the desired highly reliable rolled steel having high strength and high toughness online and to provide it inexpensively.

The present invention enables the production of high-strength rolled shaped steel at low cost (as-rolled) without performing heat treatment such as conventional normalizing treatment, and has high strength and toughness used for structural members of buildings. It is an object of the present invention to provide an excellent 590 MPa grade rolled steel and a manufacturing method thereof.

[0007]

The features of the present invention are different from the conventional idea, and a low carbon bainite structure is obtained by adding Mg, finely dispersing the fine oxide and TiN produced by the addition, and adding a microalloy. The point is that a rolled shaped steel with high strength and high toughness was realized by refining the structure by forming.

[0008] In addition, the TMCP adopted is characterized in that the rolling can be efficiently performed even in the hot rolling under the light reduction in the shape rolling instead of the large reduction rolling performed in the thick steel plate. It is a method of water cooling between passes, and repeating rolling and water cooling. INDUSTRIAL APPLICABILITY The present invention casts a slab capable of obtaining a low-carbon bainite fine structure having a low M * content in the as-rolled state, and using this slab, efficient TMCP is performed in a shaped steel rolling to obtain high strength and high toughness It is characterized by manufacturing a shaped steel having

In the steelmaking process, the slab has a fine M content due to the addition of Mg in the slab for the purpose of γ-fine graining during rolling and heating.
TiN is finely dispersed by crystallization of gO and addition of Ti, and in addition, an alloying element is replaced by a small amount of Nb, V, and Mo with the aim of reducing M * in the structure after rolling, and further reduction of B is achieved. Manufacture. Next, this cast slab is rolled and shaped to manufacture a shaped steel.In this rolling shaped steel rolling process, the steel material is water-cooled between hot rolling passes to give a temperature difference between the surface layer portion and the inside of the steel material, and light Even under the rolling condition, the penetration of rolling into the steel at higher temperature is enhanced, and work dislocations that become bainite nuclei in the γ grains are introduced to increase the nuclei. In addition, by controlling the cooling of the γ / α transformation temperature region after rolling, the method of suppressing the growth of the nucleated bainite can make the microstructure fine, which is highly efficient and inexpensive to manufacture. The above problems have been solved based on the finding that it is possible to manufacture controlled rolled steel, and the gist thereof is as follows.

% By weight, C: 0.02-0.06%, Si: 0.05-0.
25%, Mn: 0.8-1.6%, Cu: 0.7-1.5%, Ti: 0.012-0.030
%, Mg: 0.0005 to 0.0050%, Nb: 0.005 to 0.03%, V: 0.01
~ 0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, and
O: 0.003 to 0.006% is included, and the content ratio of Ti and N is Ti.
/ N is 3.0 to 3.5, the balance is Fe and inevitable impurities, and the B content of the inevitable impurities is 0.0003%.
A 590 MPa grade rolled shaped steel characterized by the following and Al content limited to 0.005% or less.

% By weight, C: 0.02-0.06%, Si: 0.05-0.
25%, Mn: 0.8-1.6%, Cu: 0.7-1.5%, Ti: 0.012-0.030
%, Mg: 0.0005 to 0.0050%, Nb: 0.005 to 0.03%, V: 0.01
~ 0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, O: 0.003
.About.0.006% and at least one of Cr: 0.1 to 1.0% and Ni: 0.1 to 2.0%, the Ti / N content ratio Ti / N is 3.0 to 3.5, and the balance is Fe. And unavoidable impurities, of which B content is 0.00
A 590 MPa grade rolled steel having a content of 03% or less and an Al content of 0.005% or less.

% By weight, C: 0.02-0.06%, Si: 0.05-0.
25%, Mn: 0.8-1.6%, Cu: 0.7-1.5%, Ti: 0.012-0.030
%, Mg: 0.0005 to 0.0050%, Nb: 0.005 to 0.03%, V: 0.01
~ 0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, and
O: 0.003 to 0.006% is included, and the content ratio of Ti and N is Ti.
/ N is 3.0 to 3.5, the balance is Fe and inevitable impurities, and the B content of the inevitable impurities is 0.0003%.
And slabs with Al content limited to 0.005% or less 1200
Rolling is started after heating in the temperature range of ~ 1300 ° C, and the flange surface of the shaped steel is water-cooled to 700 ° C or less in the rolling process and rolled in the reheat process. 590MP characterized by cooling to a temperature range of 700 to 400 ° C at a cooling rate of 0.5 to 10 ° C / s and then allowing to cool.
Manufacturing method of grade a rolled steel.

% By weight, C: 0.02-0.06%, Si: 0.05-0.
25%, Mn: 0.8-1.6%, Cu: 0.7-1.5%, Ti: 0.012-0.030
%, Mg: 0.0005 to 0.0050%, Nb: 0.005 to 0.03%, V: 0.01
~ 0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, O: 0.003
.About.0.006% and at least one of Cr: 0.1 to 1.0% and Ni: 0.1 to 2.0%, the Ti / N content ratio Ti / N is 3.0 to 3.5, and the balance is Fe. And unavoidable impurities, of which B content is 0.00
A slab with a content of 03% or less and an Al content of 0.005% or less
Rolling is started after heating in the temperature range of 1200 to 1300 ° C, the flange surface of the shaped steel is water-cooled to 700 ° C or less in the rolling process, and rolled in the recuperation process. Later 700-400 with 0.5-10 ° C / s cooling rate
It is characterized that it is left to cool after being cooled to a temperature range of ℃ 5
90MPa grade rolled steel manufacturing method.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. Higher strength of steel is achieved by refining ferrite crystals, solid solution strengthening by alloying elements, dispersion strengthening by hardening phase, precipitation strengthening by fine precipitates, and the like. Also,
High toughness is achieved by making crystals finer, reducing the solid solution N and C of the parent phase (ferrite), and reducing and miniaturizing high carbon martensite and coarse oxides and precipitates in the hardening phase that is the starting point of fracture. Etc.

Generally, as the strength of steel becomes higher, the toughness decreases, and it is necessary to deal with the conflict between the higher strength and the higher toughness.
The only metallurgical factor that satisfies both requirements is the refinement of crystals. The feature of the present invention is to achieve high strength and high toughness by dispersing fine Mg oxide and TiN by adding Mg in the steel making process and microstructuring by low carbon bainite microstructure based on microalloying alloy design. is there.

In addition, in the present invention, in the hot rolling step, the flange surface is water-cooled between hot rolling passes and the step of rolling at the time of recuperation is repeated to impart a reduction infiltration effect to the central portion of the flange thickness. However, even in this part, T
The effect of microstructure refinement by MCP is enhanced, and the microstructure refinement improves the mechanical properties of the base material in each portion of the H-section steel and reduces variations to achieve homogenization.

The reasons for limiting the composition range and control conditions of the shaped steel of the present invention will be described below. First, C is added to strengthen the steel. If it is less than 0.02%, the strength required for structural steel cannot be obtained. Further, when the content exceeds 0.06%, the base metal toughness, weld crack resistance, weld heat affected zone (hereinafter abbreviated as HAZ) toughness, etc. are significantly reduced, so the lower limit is 0.02%,
The upper limit was 0.06%.

Next, Si is necessary for securing the strength of the base material and pre-deoxidizing molten steel. When it exceeds 0.25%, high carbon island martensite is formed in the hardened structure of the base material and HAZ. Remarkably reduces the toughness of the base material and weld joint. Further, if it is less than 0.05%, the predeoxidation of molten steel cannot be sufficiently performed, so the Si content is limited to the range of 0.05 to 0.25%. Although 0.8% or more of Mn needs to be added to secure the strength of the base metal, the upper limit was set to 1.6% from the allowable concentration with respect to the toughness and crackability of the base metal and the weld.

Cu retains in the α temperature range and is slowly cooled to precipitate a Cu phase on dislocations in the α phase, and the precipitation hardening increases the room temperature strength of the base material. However, C in this α
If the u phase precipitation is less than 0.7%, it is within the solid solubility limit of Cu in α, and since precipitation does not occur, strengthening by Cu precipitation cannot be obtained. Further, at 1.5% or more, the precipitation strengthening is saturated, so Cu:
Limited to 0.7-1.5%.

Ti precipitates TiN and suppresses the formation of M * by reducing the solid solution N. The finely precipitated TiN also contributes to the grain refinement of the γ phase. By the action of these Ti, the structure is refined and the strength and toughness are improved. Therefore, if it is less than 0.012%, the precipitation amount of TiN is insufficient and these effects cannot be exhibited, so the lower limit value of the Ti amount was made 0.012%. However, if it exceeds 0.03%, excess Ti precipitates TiC, and the precipitation hardening deteriorates the toughness of the base material and the weld heat affected zone, so the content was limited to 0.03% or less.

The Mg alloy used for adding Mg is Si-Mg-Al and
It is Ni-Mg. The reason for using Mg alloy is M
This is because the g-containing concentration is reduced, the deoxidation reaction during addition to molten steel is suppressed, the safety during addition is ensured, and the Mg yield is improved. Limiting Mg to 0.0005 to 0.005% is
Is also a strong deoxidizing element, and the crystallized Mg oxide is easily floated and separated in molten steel, so even if it is added in excess of 0.005%, the yield does not rise any further, so the upper limit was made 0.005%.
If it is less than 0.0005%, the dispersion density of the target Mg-based oxide is insufficient, so the lower limit was made 0.0005%. Note that the Mg-based oxide here is mainly described as MgO, but according to an electron microscope analysis and the like, this oxide is Ti and a trace amount of Al.
And a complex oxide such as Ca contained as an impurity is formed.

Nb is added for the purpose of increasing hardenability and strength. To achieve this effect, the Nb content needs to be 0.005% or more. However, if it exceeds 0.03%, Nb
Since the precipitation amount of carbonitrides increases and the effect as solid solution Nb is saturated, it is limited to 0.03% or less. The addition of a small amount of V makes it possible to make the rolling structure finer and strengthens it by the precipitation of vanadine carbonitride, so that it can be made into a low alloy and the welding characteristics can be improved. In order to exhibit this effect, the V content needs to be 0.01% or more. However, excessive addition of V causes hardening of the weld,
Since it causes a high yield point of the base material, the upper limit of the content is V: 0.
It was set to 1%.

Mo is an element effective for ensuring the strength of the base material. In order to exhibit this effect, the Mo content needs to be 0.05% or more. However, if it exceeds 0.4%, Mo carbide (Mo2 C)
Was precipitated and the effect of improving the hardenability as solid solution Mo was saturated, so it was limited to 0.4% or less. N forms a solid solution in α and increases the strength, but in the upper bainite structure, M * is produced and the toughness is deteriorated, so the solid solution N needs to be reduced as much as possible. However, N in the present invention is combined with Ti to finely precipitate TiN in the steel to reduce solid solution N, and
It is added for the purpose of suppressing the crystal grain growth due to N and exerting the effect of refining the structure. Therefore, in order to achieve this effect, if the N amount is less than 0.004%, the TiN precipitation amount is insufficient, and
If it exceeds 010%, the amount of precipitation will be sufficient, but coarse TiN will precipitate and impair the toughness, so N was limited to 0.004 to 0.010%.

In addition, "Ti / N content ratio Ti / N is 3.
0 to 3.5 "is defined as the content of Ti and N so that almost all the added Ti and N are stoichiometrically precipitated as TiN and the solid solution Ti and N, which cause the toughness decrease, are reduced as much as possible. Limited ratio. When B is added in a small amount, the hardenability is increased and the strength is increased. However, since it was found that when the content of B exceeds 0.0003%, M * is formed in the upper bainite structure and the toughness is remarkably lowered, B is limited to 0.0003% or less as an impurity.

Al is set to 0.005% or less because Al is a strong deoxidizing element. If the content of Al exceeds 0.005%, the production of MgO is hindered and fine dispersion cannot be achieved. Limited to less than%. O (oxygen) is indispensable for the production of Mg-O, and it is necessary to contain 0.003% or more of it.
The O content is limited to 0.003 to 0.006% in order to coarsen the O particles and reduce the toughness.

The amounts of P and S contained as unavoidable impurities are not particularly limited, but they cause weld cracking due to solidification segregation and decrease in toughness, so they should be reduced as much as possible, and the amounts of P and S are both 0.02%. It is desirable to limit it to less than. In addition to the above elements, one or two kinds of Cr and Ni may be contained for the purpose of increasing the strength of the base material and improving the toughness of the base material.

Cr is effective in strengthening the base material by improving the hardenability. To achieve this effect, the Cr content needs to be 0.1% or more. However, excessive addition exceeding 1.0% is harmful from the viewpoint of toughness and curability, so the upper limit was made 1.0%. Ni is an extremely effective element that enhances the toughness of the base material. To achieve this effect, the Ni content needs to be 0.1% or more. However, the addition of more than 2.0% increases the alloy cost and is not economical, so the upper limit was made 2.0%.

The slab that has undergone the above-mentioned treatment is then subjected to 1200-1.
Reheat to a temperature range of 300 ° C. The reason why the reheating temperature is limited to this temperature range is that the manufacturing of shaped steel by hot working requires heating of 1200 ° C. or higher to facilitate plastic deformation and that elements such as V and Nb are sufficiently added. Since it is necessary to form a solid solution, the lower limit of the reheating temperature is 1200 ° C. The upper limit was set to 1300 ° C. in view of the performance and economical efficiency of the heating furnace.

The water cooling / rolling step of water cooling between hot rolling passes, cooling the flange surface temperature to 700 ° C. or less during rolling, and rolling in the recuperation process between the next rolling passes is repeated once or more. The reason for this is that water cooling between rolling passes creates a temperature difference between the surface layer and the inside of the flange, and in order to permeate the processing strain into the inside even under light reduction conditions, it is possible to perform low temperature rolling in a short time by water cooling. This is to realize the TMCP efficiently.

After the flange surface temperature is cooled to 700 ° C. or lower, rolling in the recuperation process is carried out in order to suppress quench-hardening and soften the surface due to accelerated cooling after finish rolling. The reason is that the flange surface temperature is 700 ° C.
If cooled below, the γ / α transformation temperature is cut once, the surface layer temperature rises to the recuperative temperature until the next rolling, and rolling is performed in the γ / α two-phase coexisting temperature range, and γ grain refinement and processing To form a mixed structure with the fine α. This is because the hardenability of the surface layer portion can be significantly reduced, and the hardening of the surface layer caused by accelerated cooling can be prevented.

After completion of rolling, 0.5 to 10 ° C / s continues.
The reason for cooling to 700 to 400 ° C. at the cooling rate and allowing to cool is to suppress the grain growth of ferrite and refine the bainite structure by accelerated cooling to obtain high strength and high toughness. Then, the accelerated cooling is stopped at 700 to 400 ° C. When the temperature is stopped at a temperature higher than 700 ° C., a part of the surface layer portion becomes Ar 1 point or more and the γ phase remains, and this γ phase is
The terminating temperature of accelerated cooling was set to 700 ° C. or less because ferrite coexisting with the coexisting ferrite transforms into ferrite and the ferrite grows and becomes coarser. Also, with cooling below 400 ° C,
The high carbon martensite formed between the laths of the bainite phase during the subsequent cooling cannot be decomposed due to the precipitation of cementite during the cooling and exists as a hardening phase. This high carbon martensite acts as a starting point of brittle fracture and causes toughness deterioration. For these reasons,
The stop temperature for accelerated cooling was limited to 700 to 400 ° C.

[0032]

[Examples] Prototype shaped steel was melted in a converter, added with an alloy, and then pre-deoxidized to adjust the oxygen concentration in the molten steel.
Alloys were sequentially added, and continuous casting was performed to cast 250 to 300 mm thick slabs. The cooling of the slab was controlled by selecting the amount of water in the secondary cooling zone below the mold and the drawing speed of the slab. Although not shown in the drawing, the slab was heated and rough rolling was omitted, and the slab was rolled into H-section steel by a universal rolling apparatus train shown in FIG. Water cooling between rolling passes is performed by providing water cooling devices 5a before and after the intermediate universal rolling mill 4 and repeating spray cooling and reverse rolling on the outer surface of the flange. The outer side surface of the flange was spray-cooled by the cooling device 5b installed in.

The mechanical characteristics are shown in FIG. 2, which is 1 / 4,1 / of the total flange width (B) at the center (1 / 2t2) of the plate thickness t2 of the flange 2.
2 From the width (1 / 4B, 1 / 2B), it was determined using the collected test pieces.
The characteristics of these points were determined because the flange 1 / 4F shows the average mechanical characteristics of H-section steel and the flange 1 / 2F has the lowest deterioration of these characteristics. This is because it was judged that the mechanical test characteristics of H-section steel can be represented by.

Tables 1 and 3 show the chemical composition values of the steels of the present invention and comparative steels, and Tables 2 and 4 show the oxygen concentration before addition of Ti and the composite of Mg-based oxide and TiN. The number of bodies is shown in Tables 5 and 6, and the rolling / accelerated cooling conditions for those H-section steels, and then in Tables 7 and 8 the mechanical test characteristic values.
It is well known that the rolling heating temperature is set to 1300 ° C. Generally, it is well known that the γ grains become finer and the mechanical test characteristics are improved by lowering the heating temperature. This is because it was judged that this value can represent the mechanical test characteristics at heating temperatures below that. The underlined numerical values in each table are outside the scope of the present invention.

As shown in Tables 7 and 8, H according to the present invention is used.
In each of the section steels 1 to 5 and the H section steels A1 to A3, both the yield strength and the tensile strength satisfy the JIS standard values for the 590 MPa class steel. That is, the yield strength exceeds its lower limit of 445 MPa, and the tensile strength is also
It exceeds 590 MPa, and their yield ratio (YS / TS) is
It satisfies the low YR value of 0.8 or less. The Charpy impact value also exceeds 47 (J) at -10 ° C, which sufficiently satisfies the JIS standard value.

On the other hand, the H-section steel 6 has a Mo content of the H-section steel 7
Has carbon and Si contents, H-section steel 8 has Ti / N value, H-section steel 9 has nitrogen and Ti content, H-section steel 10 has boron content, and H-section steel 11 has Al content. , Exceeding each upper limit,
Charpy absorbed energy value at -10 ° C is 4
You cannot clear more than 7J. In addition, in H-section steel 7, Cu
The yield strength cannot exceed 445 MPa because the content is less than the lower limit. In the H-section steel 12, the Mg content is less than the lower limit value, and in the H-section steel A4, the oxygen concentration of the molten steel before Ti addition is low and is less than the lower limit value of the oxygen content. Since the number of particles generated is decreased, the γ grain size during heating is coarsened, the toughness is reduced, and the target Charpy absorbed energy value cannot be cleared. On the contrary,
In the H-section steel A5, since the oxygen content exceeded the upper limit value,
The number of Mg-based oxides disperses more than necessary, so that the toughness value cannot be cleared. Then, in H-section steel A6,
Since the requirements of water cooling during rolling and cooling acceleration after rolling are not satisfied, the target values of strength and toughness cannot be met.

That is, when all the requirements of the manufacturing method of the present invention are satisfied, H-section steels 1 to 5 and A shown in Tables 7 and 8 are obtained.
1 to A3, the production of high-tensile rolled shaped steel with sufficient strength and low temperature toughness even in the flange plate thickness 1/2 and width 1/2 part where the mechanical test characteristics of rolled shaped steel are the most difficult to guarantee. It will be possible. The rolled shaped steel to which the present invention is applied is not limited to the H-shaped steel of the above-described embodiment, but is also applicable to shaped steel having a flange such as I-shaped steel, chevron steel, grooved steel, and unequal-thickness chevron steel. Of course you can.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

[0043]

[Table 6]

[0044]

[Table 7]

[0045]

[Table 8]

[0046]

EFFECTS OF THE INVENTION The alloy-designed slab according to the present invention and the rolled section steel to which the controlled rolling method is applied have sufficient strength even at a flange plate thickness of 1/2 and a width of 1/2 where the mechanical test characteristics are the most difficult to guarantee. It is possible to manufacture shaped steel with excellent toughness as it is rolled, and industrial effects such as improvement of reliability of large steel structures, ensuring safety, and economic efficiency are extremely remarkable. .

[Brief description of drawings]

FIG. 1 is a schematic diagram of an example of device arrangement for carrying out the method of the present invention.

FIG. 2 is a view showing a cross-sectional shape of H-section steel and a sampling position of a mechanical test piece.

[Explanation of symbols]

1 ... H section steel 2 ... Flange 3 ... Web 4 ... Intermediate rolling mill 5a ... Water cooling device on front and rear surfaces of intermediate rolling mill 5b ... Cooling device for rear surface of finishing rolling mill 6 ... Finishing rolling mill

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-8-283901 (JP, A) JP-A-7-216498 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. C: 0.02-0.06% by weight%, Si: 0.05-0.25%, Mn: 0.8-1.6%, Cu: 0.7-1.5%, Ti: 0.012-0.030%, Mg: 0.0005-0.0050%, Nb: 0.005-0.03%, V: 0.01-0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, and O: 0.003-0.006%, and the Ti / N content ratio Ti / N is 3.0 to 3.5
The balance is Fe and unavoidable impurities, and the B content in the unavoidable impurities is limited to 0.0003% or less and the Al content to 0.005% or less.
Grade rolled steel. 2. C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Cu: 0.7 to 1.5%, Ti: 0.012 to 0.030%, Mg: 0.0005 to 0.0050% in weight%. Nb: 0.005-0.03%, V: 0.01-0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, O: 0.003-0.006%, and Cr: 0.1-1.0% and Ni: 0.1-2.0% At least one of
And a Ti / N content ratio Ti / N of 3.0 to
3.5, the balance consisting of Fe and inevitable impurities,
Of the unavoidable impurities, the B content is 0.0003% or less and Al
590 characterized by limiting the content to 0.005% or less
MPa class rolled steel. 3. C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Cu: 0.7 to 1.5%, Ti: 0.012 to 0.030%, Mg: 0.0005 to 0.0050% by weight%. Nb: 0.005-0.03%, V: 0.01-0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, and O: 0.003-0.006%, and the Ti / N content ratio Ti / N is 3.0 to 3.5
The balance consists of Fe and unavoidable impurities, and after heating the slab with the B content of the unavoidable impurities limited to 0.0003% or less and the Al content of 0.005% or less to a temperature range of 1200 to 1300 ° C. After rolling, the flange surface of the shaped steel is water-cooled to 700 ° C or less in the rolling process and rolled in the recuperating process.
A method for producing a 590 MPa grade rolled steel, which comprises cooling to a temperature range of 700 to 400 ° C at a cooling rate of 10 ° C / s and then allowing to cool. 4. C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Cu: 0.7 to 1.5%, Ti: 0.012 to 0.030%, Mg: 0.0005 to 0.0050% in weight%. Nb: 0.005-0.03%, V: 0.01-0.1%, Mo: 0.05-0.4%, N: 0.004-0.010%, O: 0.003-0.006%, and Cr: 0.1-1.0% and Ni: 0.1-2.0% At least one of
And a Ti / N content ratio Ti / N of 3.0 to
3.5, the balance consisting of Fe and inevitable impurities,
Of the unavoidable impurities, the B content is 0.0003% or less and Al
The slab with the content limited to 0.005% or less is heated to the temperature range of 1200 to 1300 ° C, then rolling is started, and the flange surface of the shaped steel is water-cooled to 700 ° C or less in the rolling process and rolled in the recuperation process. Water cooling / rolling cycle is performed once or more, and after the rolling is completed, 0.
A method for producing a 590 MPa grade rolled steel, which comprises cooling to a temperature range of 700 to 400 ° C at a cooling rate of 5 to 10 ° C / s and then allowing to cool.