JPH10147835A - 590mpa class rolled shape steel and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain 590MPa class high tensile strength rolled shape steel having a high strength and excellent in toughness as-rolled at a low cost by specifying the compsn. composed of C, Si, Mn, Ti, Nb, V, Mo, N, O and Fe, limiting the contents of B and Al among inevitable impurities and moreover forming its microstructure of a specified one. SOLUTION: The compsn. of this steel is composed of the one contg., by weight. 0.02 to 0.06% C, 0.05 to 0.25% Si, 0.8 to 1.6% Mn, 0.005 to 0.025% Ti, 0.04 to 0.10% Nb, 0.01 to 0.10% V, 0.05 to 0.40% Mo, 0.002 to 0.006% N and 0.003 to 0.006% O, furthermore contg., at need, Cr, Ni and Cu by 0.1 to 1.0%, and the balance Fe with inevitable impurities, and in which, among the impurities, the content of B is limited to <=0.0003% and Al to <=0.005%. Moreover, its microstructure is formed of bainite by 50 to 90 area %, and the balance ferrite-pearlite and high carbon insular martensite by <=5 area %. Furthermore, the hardness of the surface layer to 3mm from the surface is preferably regulated to <=250 by Hv.

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 section steel having excellent toughness used as a structural member of a building and a method for producing the same.

[0002]

2. Description of the Related Art Due to the increase in height of buildings and stricter safety standards, steel materials used for pillars, for example, H-beams having a particularly large thickness (hereinafter referred to as extra-thick H-beams) have been developed. There is a demand for higher strength, higher toughness, and lower yield ratio. In order to satisfy such required characteristics, conventionally, a heat treatment such as a normalizing process has been performed after the completion of rolling. The addition of heat treatment causes a significant increase in cost, such as a decrease in heat treatment cost and production efficiency, and has a problem in economy. In order to solve this problem, it is necessary to develop a slab and a manufacturing method by a new alloy design that can obtain high-performance material properties as rolled.

Generally, a section steel having a flange, for example, H
When a section steel is manufactured by universal rolling, the rolling conditions (temperature, rolling reduction) from the roll molding and the uniqueness of the shape limit the rolling finish temperature, rolling reduction, 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, there are portions that do not meet the standards such as rolled steel materials for welded structures (JIS G3106). In particular, when rolling an extremely thick H-beam using a continuous cast slab as a raw material, there is a limit to the maximum thickness of a slab that can be produced by a continuous casting facility, and a sufficient slab cross-sectional area required for molding can be obtained. Therefore, the rolling is a low reduction ratio rolling. Furthermore, since high-temperature rolling is performed in order to obtain the dimensional accuracy of the product by rolling molding, the flange portion having a large thickness is subjected to high-temperature rolling, and the steel material after rolling is also gradually cooled. As a result, the microstructure becomes coarse and the strength and toughness are reduced.

As a method of refining the structure in the rolling process, T
Although there is MCP (Thermo-Mechanical-Controll Process), since rolling conditions are limited in section steel rolling, it is difficult to apply low-temperature, large-reduction rolling such as TMCP to a steel sheet. In the field of thick steel plates, high strength and high toughness steels are produced by utilizing the effect of precipitation of VN, for example, Japanese Patent Publication No. Sho 62-5054.
No. 8 and Japanese Patent Publication No. Sho 62-54862 have been proposed. However, when this method is applied to the production of a 590 MPa class, since high concentration of solute N is contained, high carbon island-like martensite (hereinafter referred to as M *) is generated in the bainite structure. In addition, 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 * generated as it is in the form of rolled steel to refine the structure. For this purpose, in the steelmaking process, Ti-O is finely crystallized in advance in the steel slab in order to reduce the γ grain size at the time of rolling and heating, and TiN is finely precipitated in the nucleus, thereby adding low carbon. Therefore, it is necessary to manufacture a slab to which a small amount of microalloy, which can obtain high strength in a small amount, is added. The fillet at 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 elongated by rolling. Here, high concentration elemental segregation zone and stretched MnS
The steel sheet significantly reduces the drawing value and toughness in the sheet thickness direction, and may cause lamella tearing at the time of welding. Therefore, it is also a major problem to prevent the production of MnS having this harmful effect. As described above, it is difficult for the conventional technology to manufacture a rolled section steel having high reliability and high strength with high reliability on-line and to provide it at low cost.

The present invention makes it possible to produce high-strength rolled steel at low cost (as it is rolled) without performing conventional heat treatment such as normalizing treatment, and to provide high strength and toughness for structural members of buildings. It is an object of the present invention to provide a 590 MPa class rolled section steel excellent in the above and a method for producing the same.

[0007]

A feature of the present invention is that, unlike the conventional idea, a low-carbon bainite structure is obtained by adding Ti, adding fine Ti oxide and TiN, thereby forming a fine dispersion and adding a microalloy. Thus, a rolled section steel having high strength and high toughness has been realized by refining the structure by the formation of steel.

In addition, the feature of the TMCP adopted is that the structure can be efficiently refined even in the hot rolling under light pressure in the shape steel rolling in place of the large rolling under pressure performed in a thick steel plate. There is a method of cooling with water between passes and repeating rolling and water cooling. The present invention casts a slab that can obtain a microstructure of low-carbon bainite with a low M * content as it is rolled, and uses this slab to perform efficient TMCP in rolling a section steel to achieve high strength and high toughness. It is characterized by producing a shaped steel having

[0009] In the steel making process, the slab is made to have a fine T
Crystallization of i-O and fine dispersion of TiN, and in addition, aiming at reduction of M * in the structure after rolling, a small amount of alloying elements such as Nb,
Substitute by adding V and Mo, and further reduce B to manufacture. Next, this slab is rolled and shaped to produce a shaped steel,
In this rolling section steel rolling process, the steel material is water-cooled between hot rolling passes to give a temperature difference between the surface layer and the inside of the steel material, so that even under light rolling conditions, the infiltration into the higher temperature steel material can be reduced. In addition, work dislocations which become bainite forming nuclei in γ grains are introduced to increase the number of formed nuclei. in addition,
By controlling the cooling of the γ / α transformation temperature range after rolling,
According to the method of suppressing the growth of the nucleated bainite, the microstructure can be refined, and the above-mentioned problem is solved based on the finding that it is possible to produce a highly-efficient and low-cost controlled rolled steel section. The summary is as follows.

[0010] By weight%, C: 0.02 to 0.06%, Si: 0.05 to 0.
25%, Mn: 0.8-1.6%, Ti: 0.005-0.025%, Nb: 0.04--0.
10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.002
~ 0.006%, and O: 0.003-0.006%, with the balance being Fe and unavoidable impurities, and having a chemical composition in which the B content of the unavoidable impurities is limited to 0.0003% or less and the Al content to 0.005% or less. And the area ratio of bainite is 50-
590 MPa class rolled section steel having a microstructure of 90%, with the balance being ferrite pearlite and high carbon island martensite, wherein the high carbon island martensite has an area ratio of 5% or less.

In weight%, C: 0.02-0.06%, Si: 0.05-0.
25%, Mn: 0.8-1.6%, Ti: 0.005-0.025%, Nb: 0.04--0.
10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.002
~ 0.006%, O: 0.003-0.006%, and Cr: 0.1-1.0%, N
i: at least one of 0.1 to 1.0% and Cu: 0.1 to 1.0%
Including the seeds, the balance consisting of Fe and inevitable impurities, of the inevitable impurities having a chemical composition in which the B content is limited to 0.0003% or less and the Al content to 0.005% or less, and the area ratio of bainite is 50 to 50%. 590 MPa class rolled section steel characterized by having a microstructure of 90% with the balance being ferrite pearlite and high carbon island martensite, wherein the high carbon island martensite has an area ratio of 5% or less.

C: 0.02-0.06% by weight, Si: 0.05-0.
25%, Mn: 0.8-1.6%, Ti: 0.005-0.025%, Nb: 0.04--0.
10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.002
~ 0.006%, and O: 0.003 ~ 0.006%, the balance is made of Fe and inevitable impurities, of the inevitable impurities B content is 0.0003% or less and Al content is limited to 0.005% or less slab 1200 Rolling is started after heating to a temperature range of ~ 1300 ° C, and a water-cooling / rolling cycle in which the flange surface of the section steel is water-cooled to 700 ° C or less in the rolling process and rolled in the recuperation process is performed once or more, and after the rolling is completed. 70 at a cooling rate of 0.5-10 ° C / s
A method for producing a 590 MPa class rolled section steel, which is cooled after cooling to a temperature range of 0 to 400 ° C.

In weight%, C: 0.02-0.06%, Si: 0.05-0.
25%, Mn: 0.8-1.6%, Ti: 0.005-0.025%, Nb: 0.04--0.
10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.002
~ 0.006%, O: 0.003-0.006%, and Cr: 0.1-1.0%, N
i: at least one of 0.1 to 1.0% and Cu: 0.1 to 1.0%
After heating the slabs containing seeds, the balance consisting of Fe and unavoidable impurities, with the B content of 0.0003% or less and the Al content of 0.005% or less among the unavoidable impurities, heated to a temperature range of 1200 to 1300 ° C. Rolling is started, water-cooling / rolling cycle of rolling the flange surface of the section steel to 700 ° C or less in the rolling process and rolling in the reheating process is performed at least once,
A method for producing a 590 MPa class rolled section steel, comprising cooling at a cooling rate of 10 ° C./s to a temperature range of 700 to 400 ° C. and then allowing it to cool.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. Strengthening of steel is achieved by refinement of ferrite crystals, solid solution strengthening by alloying elements, dispersion strengthening by hardened phases, precipitation strengthening by fine precipitates, and the like. Also,
Higher toughness is achieved by miniaturization of crystal, reduction of solid solution N and C of mother phase (ferrite), reduction and miniaturization of high carbon martensite and coarse oxides and precipitates of hardened phase which is a starting point of fracture. And so on.

In general, the toughness is reduced by increasing the strength of steel, and it is necessary to contradict high strength and toughness.
The only metallurgical factor that satisfies both at the same time is crystal refinement. The feature of the present invention is to achieve high strength and high toughness by dispersing fine Ti oxide and TiN by adding Ti in the steel making process and by refining the structure by forming low carbon bainite based on microalloying alloy design. is there.

In addition, according to the present invention, in the hot rolling step, the flange surface is water-cooled between hot rolling passes, and the rolling step is repeated at the time of reheating to impart a rolling reduction effect to the center portion of the flange in the thickness direction. And T
The effect of refining the structure by the MCP is enhanced, and the refining of the structure improves the mechanical properties of the base material in each part of the H-section steel, and reduces the variation to achieve homogenization.

The reasons for limiting the range of components and the control conditions of the section steel of the present invention are 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, if the addition exceeds 0.06%, the base material toughness, weld cracking resistance, weld heat affected zone (hereinafter abbreviated as HAZ) toughness, etc. are significantly reduced.
The upper limit is set to 0.06%.

Next, Si is necessary for securing the strength of the base material, pre-deoxidizing the molten steel, etc. If it exceeds 0.25%, high-carbon island-like martensite is formed in the hardened structure of the base material and HAZ, It significantly reduces the toughness of the base metal and the welded joint. If the content is less than 0.05%, the preliminary deoxidation of the molten steel cannot be sufficiently performed, so the Si content is limited to the range of 0.05 to 0.25%. Mn must be added at 0.8% or more to secure the strength of the base material, but the upper limit is set to 1.6% from the allowable concentration for the toughness, cracking, and the like of the base material and the welded portion.

Ti suppresses generation of M * by precipitating TiN and reducing solid solution N. The finely precipitated TiN also contributes to the refinement of the γ phase. By the action of these Tis, the structure is refined and the strength and toughness are improved. Therefore, if the content is less than 0.005%, the amount of TiN deposited is insufficient, and these effects cannot be exhibited. Therefore, the lower limit of the Ti content is set to 0.005%. However, if the content exceeds 0.025%, excessive Ti precipitates TiC, and the precipitation hardening deteriorates the toughness of the base material and the heat affected zone by welding, so that the content is limited to 0.025% or less.

Nb is added for the purpose of increasing hardenability and increasing strength. To achieve this effect, the Nb content needs to be 0.04% or more. However, if it exceeds 0.10%, Nb
Since the amount of precipitated carbonitride increases and the effect as solid solution Nb saturates, the content is limited to 0.10% or less. The addition of a small amount of V can make the rolling structure finer and strengthening it by precipitation of vanadium carbonitride, so that a lower alloy can be obtained and welding characteristics can be improved. To achieve this effect, the V content must be 0.01% or more. However, excessive addition of V hardens the weld and
Since the base material has a high yield point, the upper limit of the content is set to V: 0.
10%.

Mo is an element effective for securing the strength of the base material. In order to achieve this effect, the Mo content needs to be 0.05% or more. However, if it exceeds 0.4%, Mo carbide (Mo2C)
Was precipitated, and the effect of improving the hardenability as solid solution Mo was saturated, so the content 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 generated and the toughness is deteriorated. Therefore, it is necessary to reduce the solute N as much as possible. However, N in the present invention is combined with Ti to cause fine precipitation of TiN in the steel and to reduce solid-solution N,
It is added for the purpose of suppressing the crystal grain growth due to N and exhibiting the structure refinement effect. Therefore, in order to achieve this effect, if the N content is less than 0.002%, the amount of TiN deposited is insufficient, and the amount of TiN is not sufficient.
If it exceeds 005%, the amount of precipitation is sufficient, but the toughness is impaired due to an increase in the amount of solute N in α.

B increases the hardenability by adding a small amount, and contributes to an increase in strength. However, it has been found that when B is contained in excess of 0.0003%, M * is formed in the upper bainite structure and the toughness is significantly reduced.
%. The reason why the content of Al is set to 0.005% or less is that Al
Is a strong deoxidizing element, and if the content exceeds 0.005%, Ti-
Since the generation of O is inhibited and fine dispersion cannot be performed, Al
Was also limited to 0.005% or less as an impurity.

O (oxygen) is indispensable for the production of Ti—O, which requires a content of more than 0.003%.
When the content exceeds 006%, the generated Ti-O particles are coarsened and the toughness is reduced, so the O content is 0.003 to 0.00.
Limited to 6%. The amounts of P and S contained as unavoidable impurities are not particularly limited, but may cause welding cracks and decrease in toughness due to solidification segregation. Therefore, P and S contents should be reduced as much as possible, and the contents of P and S are each limited to less than 0.02%. It is desirable to do.

In addition to the above elements, at least one of Cr, Ni and Cu can 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. In order to achieve this effect, the Cr content must be 0.1% or more. However, an 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 for improving the toughness of the base material. To achieve this effect, the Ni content must be 0.1% or more. However, the addition of more than 1.0% increases the alloy cost and is not economical, so the upper limit was made 1.0%. Cu is an element that enhances the toughness of the base material, and the Cu content is required to be 0.1% or more to exhibit this effect. However, the addition exceeding 1.0% lowers the high-temperature ductility such as causing surface cracks in the slab, so Cu was limited to 0.1 to 1.0%.

The rolled section steel of the present invention has a capacity of 590 MPa (60 kg).
f / mm ² ) class tensile strength and toughness at the same time,
It is necessary to have a microstructure in which the area ratio of bainite is 50 to 90% and the balance is composed of ferrite pearlite and high carbon island martensite, and the area ratio of the high carbon island martensite is 5% or less. is there. The 590 MPa class rolled steel of the present invention desirably has a surface hardness of 3 mm or less from the surface of Hv250 or less in order to form bolt holes when used as a beam.

The above microstructure and surface hardness can be realized by the method of the present invention. That is, the slab having the above chemical composition is reheated to a temperature range of 1200 to 1300 ° C. The reason for limiting the reheating temperature to this temperature range is that in the production of shaped steel by hot working, plastic deformation is easily performed.
The lower limit of the reheating temperature was set to 1200 ° C., since heating at 200 ° C. or higher was necessary and elements such as V and Nb had to be sufficiently dissolved. The upper limit was set to 1300 ° C. in view of the performance and economy of the heating furnace.

Water-cooling between hot rolling passes, the surface temperature of the flange is reduced to 700 ° C. or less during rolling, and a water-cooling / rolling cycle of rolling in the reheating process between the next rolling passes is performed at least once. Water cooling between the rolling passes gives a temperature difference between the surface layer and the inside of the flange to penetrate the processing strain into the interior even under light rolling conditions. This is for performing the TMCP efficiently.

Rolling in the recuperation process after cooling the flange surface temperature to 700 ° C. or lower is performed in order to suppress quenching and hardening of the surface due to accelerated cooling after finish rolling and to soften the surface. The reason is that the flange surface temperature is 700 ° C
Once cooled, the γ / α transformation temperature is temporarily cut off, and the surface layer is reheated and heated by the next rolling. Rolling is performed in the γ / α two-phase coexisting temperature range. A mixed structure with the fine α thus formed is formed. Thereby, the hardenability of the surface layer can be significantly reduced, and the hardening of the surface layer caused by accelerated cooling can be prevented.

After the end of the rolling, the temperature is continuously maintained at 0.5 to 10 ° C./s.
The reason why the cooling rate is set to 700 to 400 ° C. and then allowed to cool is to suppress nucleation and grain growth of ferrite by accelerated cooling and refine the bainite structure to obtain high strength and high toughness. Next, the reason why the accelerated cooling is stopped at 700 to 400 ° C. is that, when the temperature is stopped at a temperature exceeding 700 ° C., a part of the surface layer becomes an Ar1 point or more and a γ phase remains, and this γ phase coexists Since the ferrite is transformed into a ferrite core and the ferrite grows and becomes coarse, the stop temperature of the accelerated cooling is set to 700 ° C. or less. In addition, when the cooling is performed at a temperature lower than 400 ° C., high carbon martensite generated between laths of the bainite phase during the subsequent cooling is not decomposed due to precipitation of cementite during cooling, and exists as a hardened phase. . This high carbon martensite acts as a starting point of brittle fracture and causes a decrease in toughness. For these reasons, the stop temperature of the accelerated cooling was limited to 700 to 400 ° C.

[0031]

EXAMPLE A prototype steel was melted in a converter, an alloy was added, preliminary deoxidation was performed, and the oxygen concentration of the molten steel was adjusted.
Alloys were sequentially added and cast into 250-300 mm thick slabs by continuous casting. The cooling of the slab was controlled by selecting the amount of water in the secondary cooling zone below the mold and the speed of drawing the slab. The slab was heated and rolled into an H-beam by a universal rolling mill row shown in FIG. 1, although illustration of the rough rolling step was omitted. Water cooling between rolling passes is provided with a water cooling device 5a before and after the intermediate universal rolling mill 4, and spray cooling and reverse rolling are repeated on the outer surface of the flange. Accelerated cooling after rolling is performed after finishing rolling by the finishing universal rolling mill 6. The outer surface of the flange was spray-cooled by the cooling device 5b installed in the above.

The mechanical properties are shown in FIG. 2 at the center (1 / 2t2) of the thickness t2 of the flange 2 at 1 / 4,1 / of the overall flange width (B).
2 From the width (1 / 4B, 1 / 2B), it was determined using test specimens collected.
The properties of these parts were determined as follows: The flange 1 / 4F shows the average mechanical properties of the H-section steel, and the flange 1 / 2F has the lowest properties. This is because it was judged that the mechanical test characteristics of the H-section steel could be represented by the above.

Tables 1 and 3 show the chemical composition values of the steels of the present invention and the comparative steels, and Tables 2 and 4 show the oxygen concentrations and the Ti-based oxides of the molten steel of these steels before the addition of Ti. Oxide and T
Tables 5 and 6 show the rolling / accelerated cooling conditions. Next, Tables 7 and 8 show the mechanical test characteristic values of these H-section steels, the surface hardness of the flange side surfaces, and the bainite and M * areas. In addition, the rolling heating temperature is 1300 ℃
It is well-known that γ grains are generally refined by lowering the heating temperature to improve mechanical test characteristics.
Under high temperature heating conditions, it is estimated to show the lowest value of mechanical properties,
This is because it was determined that this value could represent the mechanical test characteristics at a lower heating temperature. The underlined numerical values in each table are outside the scope of the present invention.

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

On the other hand, in the H-section steel 6, the content of Si and Mo is
The H-section steel 7 has a carbon content, the H-section steel 8 has a Ti and N content, the H-section steel 9 has a boron and Al content, and the H-section steel 10
In this case, since the Nb content exceeds each upper limit and the M * area ratio that deteriorates toughness exceeds 5%, the Charpy absorbed energy value at −10 ° C. cannot exceed the target of 47 J or more. In addition, in the H-section steel 6 and the H-section steel 9, the hardenability increases, and the flange surface hardness exceeds Hv250.

In the H-section steel A4, the oxygen concentration of the molten steel before the addition of Ti is low and less than the lower limit of the oxygen content.
Since the number of generated system oxides is reduced, the structure becomes coarse and the toughness is reduced, so that the target Charpy absorbed energy value cannot be cleared. On the other hand, in the H-section steel A5, since the oxygen content exceeds the upper limit, the Ti-based oxide is coarsened, so that the toughness value cannot be cleared. Then H
Since the section steel A6 does not satisfy the requirements of water cooling during rolling and cooling rate after rolling, the target value of strength cannot be cleared.

That is, when all the requirements of the production method of the present invention are satisfied, H-section steels 1 to 5 and A1 shown in Tables 7 and 8 are obtained.
As in A3, it is possible to produce high-strength rolled steel with sufficient strength and low-temperature toughness even at the flange plate thickness 1/2 and width 1/2 where it is most difficult to guarantee the mechanical test characteristics of the rolled steel. become. The rolled section steel to which the present invention is applied is not limited to the H section steel of the above embodiment, but is also applicable to section steels having flanges such as I section steel, angle steel, channel steel, and unequal thickness angle 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]

According to the present invention, the alloy-designed cast slab and the rolled section steel to which the controlled rolling method is applied have sufficient strength even in a flange plate thickness 1/2 and a width 1/2 part where mechanical test characteristics are hardly guaranteed. The production of shaped steel with excellent toughness becomes possible as it is rolled, and the industrial effects such as improvement of reliability, safety and economical efficiency of large steel structures are extremely remarkable. .

[Brief description of the drawings]

FIG. 1 is a schematic view of an example of an apparatus arrangement for performing the method of the present invention.

FIG. 2 is a diagram illustrating a cross-sectional shape of an H-section steel and a sampling position of a mechanical test piece.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... H-shaped steel 2 ... Flange 3 ... Web 4 ... Intermediate rolling mill 5a ... Water cooling device of the front and rear surface of an intermediate rolling mill 5b ... Finishing rolling machine rear surface cooling device 6 ... Finishing rolling mill

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noriaki Onodera 1 Chikuko Hachiman-cho, Sakai-shi, Osaka Nippon Steel Corporation Sakai Works

Claims (6)

[Claims] 1% by weight C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Nb: 0.04 to 0.10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.002 to 0.006%, and O: 0.003 to 0.006%, with the balance being Fe and inevitable impurities, of which B content is 0.0003% or less and Al content. Is limited to 0.005% or less, and the area ratio of bainite is 50 to 90%, and the balance consists of ferrite pearlite and high carbon island martensite, and the area ratio of the high carbon island martensite 590 MPa class rolled section steel characterized by having a microstructure of not more than 5%. 2. The hardness of the surface layer within a depth of 3 mm from the surface is H.
5. The method according to claim 1, wherein the value of v is not more than 250.
90MPa class rolled section steel. 3% by weight C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Nb: 0.04 to 0.10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.003 to 0.006%, O: 0.003 to 0.006%, and Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, and Cu: at least one of 0.1 to 1.0%. The balance consists of Fe and inevitable impurities, and the B content of the inevitable impurities is 0.0003%
And a chemical composition in which the Al content is limited to 0.005% or less, the area ratio of bainite is 50 to 90%, and the balance is composed of ferrite / pearlite and high carbon island martensite. 5 characterized by having a microstructure in which the area ratio of martensite is 5% or less.
90MPa class rolled section steel. 4. The hardness of the surface layer within a depth of 3 mm from the surface is H.
5. The method according to claim 3, wherein the value is not more than v250.
90MPa class rolled section steel. 5% by weight C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Nb: 0.04 to 0.10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.002 to 0.006%, and O: 0.003 to 0.006%, with the balance being Fe and inevitable impurities, of which B content is 0.0003% or less and Al content. Rolling is started after heating the slab having a temperature of 1200 to 1300 ° C limited to 0.005% or less, and the flange surface of the shaped steel is water-cooled to 700 ° C or less in the rolling process, and is rolled in a reheating process. Perform one or more rolling cycles, and after rolling is completed, 0.5 to 10
A method for producing a 590 MPa class rolled section steel, which is cooled at a cooling rate of 700C / s to a temperature range of 700 to 400C and then left to cool. 6% by weight C: 0.02 to 0.06%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Nb: 0.04 to 0.10%, V: 0.01 to 0.10%, Mo: 0.05 to 0.40%, N: 0.002 to 0.006%, O: 0.003 to 0.006%, and Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, and Cu: at least one of 0.1 to 1.0%. The balance consists of Fe and inevitable impurities, and the B content of the inevitable impurities is 0.0003%
Below and the slab with the Al content limited to 0.005% or less is 1200
Rolling is started after heating to a temperature range of ~ 1300 ° C, and at least one water-cooling / rolling cycle is performed, in which the flange surface of the section steel is water-cooled to 700 ° C or less in the rolling process and rolled in the recuperation process. 590MP, characterized by cooling to a temperature range of 700-400 ° C at a cooling rate of 0.5-10 ° C / s and then allowing to cool
Production method for a-grade rolled steel bars.