JPH05345915A - Production of thin web wide flange shape excellent in workability and reduced in yield ratio - Google Patents

Abstract

PURPOSE:To easily obtain a thin web wide flange shape excellent in workability, improved in earthquake resistance, and reduced in yield ratio by performing temp. control at cooling and recuperation, with high precision, in the intermediate rolling step of a steel containing respectively specified amounts of C, Si, Mn, Ti, Al, and Nb. CONSTITUTION:A steel having a composition which consists of, by weight, 0.04-0.2% C, 0.01-0.5% Si, 0.3-1.8% Mn, <=0.02% Ti, <=0.06% Al, <=0.02% Nb, and the balance Fe and where the carbon equivalent represented by the prescribed expression is regulated to <=0.4% is hot-rolled. At this time, in the intermediate rolling step before finish rolling, rolling is done while repeating a stage where cooling is performed until the surface layer temp. at the side face of a flange becomes <=700 deg.C and a stage where the cooling is stopped and recuperation is performed until the surface layer temp. becomes >=700 deg.C. The lower limit of the finishing temp. of the above finish rolling is regulated to >=750 deg.C, and the flange just after finish rolling is forcedly cooled. Further, by performing, e.g. the acceleration of ferrite transformation and the acceleration of the refining of untransformed austenite, the structure in the surface layer part can be refined and hardenability can be reduced.

Description

Detailed Description of the Invention

[0001]

[Industrial application] When a thin web H-section steel having a web thickness smaller than the flange thickness is manufactured by hot rolling, it is possible to prevent web waves and suppress the increase in hardness of the flange cooling surface caused by forced cooling. At the same time, the present invention relates to a method for producing a low-yield-ratio thin-walled web H-section steel which is excellent in workability such as drilling and has improved seismic performance by suppressing an abnormal increase in yield strength.

[0002]

2. Description of the Related Art As a thin web H-section steel having a large section modulus relative to the weight per unit length and being excellent in economical efficiency, a build-up H-section steel by welding has been the mainstream in the past. Various manufacturing means by rolling have been proposed.
That is, the most important issue in the method for producing a thin web H-section steel by rolling was how to suppress the generation of web waves, but recently, various practical measures have been proposed.

As is well known as a web wave of a thin web H-section steel, an internal compressive stress exceeding the buckling limit of the web is generated in the web due to residual stress caused by a temperature difference in the cooling process of the flange and the web. It appears as a wavy shape defect on the web.

As a technique for economically producing a thin web H-section steel without web waves, the applicants of the present invention have previously disclosed Japanese Unexamined Patent Publication No.
The technology of Japanese Patent No. 205028 is provided. The gist of this proposal is a method for producing a thin web H-section steel in which the flange of H-section steel immediately after hot finish rolling is forcibly cooled to prevent web waves, and the upper limit of the water cooling time during which web waves do not occur during forced cooling. Alternatively, the lower limit of the temperature difference between the flange and the web immediately after water cooling and the lower limit of the water cooling time at which the thermal stress of the web after forced cooling to normal temperature is less than the buckling stress of the web or the temperature difference between the flange and the web immediately after water cooling. The upper limit is set in advance for each size of H-section steel and the cooling water amount density, and the flange is forcibly cooled within this upper and lower limit range so that the temperature difference of the flange web at the end of water cooling falls within a certain range. It is a means.

However, it has been found that the forced cooling may cause the flange to be hardened and the surface hardness of the flange to be excessively increased. If the surface hardness becomes too high,
Processing such as drilling becomes difficult, which is not preferable.

In recent years, the construction industry has introduced a new seismic design method and demanded that the properties of steel materials have a low yield ratio {normal: TR = (yield strength / tensile strength) × 100 (%)}. There is. Therefore, in order to manufacture a thin web H-section steel that can meet the requirements of the building industry, not only the prevention of web waves but also a new manufacturing technique satisfying the suppression of the hardness increase of the flange surface and the reduction of the yield ratio is required. Will be needed.

By the way, as a method for suppressing the increase in hardness, there is a method for suppressing uneven hardness in a steel sheet disclosed in Japanese Patent Laid-Open No. 182916/1984. However, in the case of H-section steel, for example, the cross-sectional shape is generally complicated and the plate thickness of each part is often different, so the steel material temperature during rolling differs depending on each part. Also, in the case of shaped steel, the length of rolled material is long, and reverse rolling by the same rolling mill is common, so even if forced cooling is performed only once during rolling, there is sufficient time to reheat during rolling. is there. Therefore, the above-mentioned JP-A-59-1829 is used.
It is impossible to apply the technology of Japanese Patent No. 16 to the hot rolling process of shaped steel. Further, Japanese Patent Laid-Open No. 19422/1990 discloses a method for suppressing an increase in hardness of a cooling surface in a process of forcibly cooling a steel material after rolling in order to manufacture a high-strength shaped steel. However, this method is rather a technique aimed at increasing the strength, and proposes how to suppress the increase in hardness of the cooling surface in the process, and control of the yield strength or the yield ratio is mentioned. Not not.

[0008]

When a thin web H-section steel is manufactured by the hot rolling method for forcibly cooling the flange, it prevents web waves and suppresses an increase in hardness of the flange surface and an abnormal increase in yield strength. This provides a method for producing a low-yield ratio thin web H-section steel having excellent workability such as drilling.

[0009]

The web wave generated during the cooling of the thin web H-section steel can be basically prevented by the above-mentioned cooling means, but the temperature of the flange at the start of water cooling is high and the water cooling is not possible. It was found that when the time was long, the outer surface of the flange was burned and the hardness was significantly increased, so that the predetermined material could not be satisfied. Table 2 Steel No. 3-1 is an example, but it can be seen that even if the web wave can be prevented, the hardness of the flange surface is significantly increased and the strength is increased, resulting in insufficient elongation. This is as shown in FIG.
This is due to the bainite structure on the outer surface of the flange outer surface.

There is no specific value for the hardness of the outer surface of the flange, but the Vickers hardness Hv (10)
≦ 200. This is a thick steel plate TMCP (Thermo Mec
hanical Control Process) With reference to steel,
It was decided based on information from the steel processing industry.

The inventors of the present invention have conducted various studies and, prior to the forced cooling immediately after the finish rolling for preventing web waves, by refining the surface layer portion hot structure on the outer surface of the flange, It was found that quenching becomes difficult by cooling after finish rolling. According to this method, the quench hardening phenomenon of the flange water-cooled surface can be prevented without lowering the productivity. However, emphasizing the reduction of hardenability, when performing low-temperature rolling that excessively lowers the rolling temperature of the flange by excessive water cooling during rolling, the hot structure will be excessively refined, and the increase in hardness will not be suppressed. Although it could be achieved, it was found that the yield strength increasing effect due to excessive grain refinement became remarkable and the yield ratio increased extremely. Although the target of the upper limit value of the yield ratio is not clearly defined at present, it has been set to 80% or less based on the recent demand for steel materials for high-rise buildings.

For example, as shown in Table 2 Steel No. 3-4, although the increase in hardness of the web wave and the surface of the flange is suppressed, the yield ratio may increase and the target value may exceed 80%. As shown in FIG. 3 (b), this is because the surface layer on the outer surface of the flange has an extremely fine ferrite and pearlite structure, and the strain introduced during rolling seems to remain warm-worked. Due to the existence of an organization. When this structure was observed with a scanning electron microscope, subgrain boundaries with a diameter of 2 to 3 μm were observed as shown in FIG. 3C, which confirms that low-temperature rolling was performed.

As a result of further studies, the present inventors have found that
We have found that a low yield ratio can be achieved by appropriately refining the hot structure. That is, it is necessary to regulate the lower limit of the finish rolling temperature to control the predetermined flange surface layer thickness to a fine hot structure, and to add Ti and Al that form TiN and AlN and have a structure refinement effect. It was found that the regulation of the amount and the regulation of the Nb addition amount that raises the non-recrystallization temperature of the austenite phase are effective.

The problem is solved based on the above knowledge, and the gist thereof is as follows.

When the flange of the H-section steel immediately after hot finish rolling is forcibly cooled, the lower limit of the temperature difference between the flange and the web immediately after water cooling at which the web wave does not occur during the forced cooling and after reaching the room temperature after forced cooling The upper limit of the temperature difference between the flange and the web immediately after water cooling, at which the thermal stress of the web becomes less than the buckling stress of the web, is obtained in advance for each size of the H-section steel and the cooling water amount density, and the upper and lower limits of the temperature difference. In the method for producing a thin web H-section steel in which the flange is forcibly cooled inside, C: 0.04 to 0.20% Si: 0.01 to 0.50% Mn: 0.3 to 1.80% by weight%. Ti: 0.02% or less Al: 0.060% or less Nb: 0.02% or less, if necessary Mo: 0.3% or less V: 0.2% or less Cr: 0.7% or less Cu: 1.0% or less Ni: 1.0% or less B: 0.003 % Or less Ca: 0.001 to 0.005% of 1 type or 2 or more types, and carbon equivalent Ce
q. (= C + Si / 24 + Mn / 6 + Ni / 40 + Cr
/ 5 + Mo / 4 + V / 14) (%) is 0.40% or less, and the balance is Fe and steel inevitable impurities are subjected to hot rolling, and the flange outer surface is forced at the intermediate rolling stage before the finish rolling. While repeating the water cooling process of cooling and cooling the surface layer temperature of the flange outer surface once to 700 ° C or less, and the recuperation process of stopping the cooling and reheating the surface layer temperature of the flange outer surface to more than 700 ° C. A method for producing a thin web H-section steel, which comprises performing rolling, setting the lower limit of the finish rolling finish temperature to 750 ° C. or higher, and performing forced flange cooling immediately after finish rolling.

[0016]

The present invention will be described in detail below.

FIG. 1 shows the rolling condition in the intermediate rolling stage of the method of the present invention in relation to the passage of time and the temperature change of the H-section steel. FIG. 2 is an example of the equipment arrangement, in which water cooling devices 2a are arranged on the front and rear surfaces of the intermediate rolling mill 1, and a water cooling device 2b for preventing cooling web waves is arranged on the rear surface of the finish rolling mill 3 in the next process. is doing. In FIG. 1, the average temperature of the flange portion gradually decreases, while the surface temperature of the flange portion has a sawtooth shape because cooling and recuperation are repeated alternately. By rolling while alternately performing cooling and recuperation, it is possible to reduce the hardenability by refining the structure of the surface layer portion of the flange outer surface by promoting ferrite transformation and refining untransformed austenite. It is a thing.

In order to lower the yield ratio, it is sufficient to lower the yield strength (hereinafter, σ _y ) and increase the strain hardening index. However, it has been concluded that it is extremely difficult to control the strain hardening index in general structural steel, and it is effective to find a method of lowering σ _y .

Σ _y is expressed by equation (1).

Σ _y = σ _o + σ _d + k · d ^−1/2 formula (1) σ _o : solid solution strengthening, precipitation strengthening σ _d : work strengthening by introducing dislocations d: crystal grain size, that is, ferrite and pearlite components If the ratio is constant, it is dominated by the amount of solute carbon in ferrite and its grain size,
Furthermore, when undergoing warm working by low temperature rolling,
Subgrain boundaries are formed by the introduction of dislocations, which is further affected.

Therefore, in order to reduce the yield ratio, it is important to select a component system that does not excessively reduce the ferrite grain size and control the rolling conditions and cooling conditions.

That is, the thin web H-section steel according to the present invention has a low yield ratio and is excellent in earthquake resistance, which realizes proper control of component system selection, processing in the intermediate rolling stage and water cooling conditions. By doing so, the microstructure is composed of a relatively large ferrite.

It is for this reason that the lower limit of the finishing temperature is regulated in the present invention. When the finishing temperature becomes lower than 750 ° C. due to the enhanced cooling of the outer surface of the flange in the intermediate rolling stage, as shown in FIG. It was found that the ultrafine grains and the warm-worked structure shown in b) remarkably appeared and the yield ratio increased, so the lower limit value was set to 750 ° C.

On the other hand, when the finishing temperature can be secured,
As shown in FIG. 4, an H-shaped steel having a low yield ratio and a thin web having a substantially normal fine-grained structure having almost no processed structure and suppressing an increase in hardness can be obtained.

Next, the reasons for limiting the basic composition range of the shaped steel of the present invention will be described.

First, C is added as an effective component for improving the strength of steel. If it is less than 0.04%, the strength required as a structural steel cannot be obtained, and if it exceeds 0.20%, it is excessive. The addition markedly lowers the base material toughness, weld crackability, weld heat affected zone (hereinafter referred to as HAZ) toughness, etc., so the upper limit was made 0.20%.

Next, Si is necessary for securing the strength of the base metal and pre-deoxidizing molten steel, but if it exceeds 0.50%, HAZ
It forms island martensite with a hardened structure in the structure, and significantly reduces the toughness of the welded joint. Further, if less than 0.01%, the toughness of the base material deteriorates, so the Si content is limited to this range.

Mn is 0.3 in order to secure the strength and toughness of the base material.
% Or more is required, but the upper limit was set to 1.8% within the allowable range of the toughness and crackability of the welded portion.

Ti is an element effective for refining the austenite grains during heating of the billet and finely converting the austenite grains by fixing N and finely converting it to TiN. Therefore, the upper limit is set to 0.02% because it accelerates and further causes toughness reduction due to carbide formation.

Although Al is a powerful deoxidizing element and is effective for deoxidizing during steel refining, it is an alloying element which fixes N to become AlN and has a grain refining effect, and when it is more than 0.060%. Since the HAZ toughness is significantly deteriorated, the upper limit is set to 0.
It was set to 060%.

Nb forms fine carbonitrides, raises the non-recrystallization temperature of the austenite phase, and improves the toughness of the base material which makes it possible to refine the rolling structure by rolling in the non-recrystallization temperature range. Since it is an effective element, and excessive addition is harmful from the viewpoint of toughness and hardenability, the content was made 0.02% or less.

The amounts of P and S contained as inevitable impurities are not particularly limited, but weld cracks due to solidification segregation,
Since the toughness is reduced, it should be reduced as much as possible, and the P and S contents are preferably 0.02% and 0.02% or less, respectively.

The above are the basic components of the steel of the present invention. However, for the purpose of increasing the strength of the base metal or improving the toughness of the base metal, Mo, V, Cr, Cu, Ni, B and Ca may be added as necessary. It may contain one kind or two or more kinds.

First, Mo is an element effective for securing the strength of the base material, but since it is expensive, it is limited to 0.3% or less.

V is important as VN for fine graining and high strength by precipitation strengthening, but if it exceeds 0.2%, the amount of precipitation becomes excessive and the toughness of the base material decreases, so V is 0.2% or less. Limited to.

Cr is effective for strengthening the base material due to improvement in hardenability, but excessive addition is harmful from the viewpoint of toughness and hardenability, so the upper limit was made 0.7%.

Cu is an element effective for strengthening the base material and weathering resistance, but the upper limit is set to 1.0% in consideration of weld crackability, hot work cracking and the like.

Ni is an extremely effective element for enhancing the toughness of the base material, but the addition of more than 1.0% increases the alloy cost and is not economical, so the upper limit was made 1.0%.

Further, B is an effective element capable of ensuring sufficient hardenability with a trace amount of 0.003% or less when it is necessary to compensate for the insufficient hardenability, but when added excessively, Since it causes grain boundary embrittlement, the upper limit was made 0.003%.

[0040] Ca is a strong deoxidizing element, and further binds to the impurity S to prevent the damage of S. However, if its amount is too large, it deteriorates the toughness and weldability.
The range was made 0.001 to 0.005%.

However, these elements have the carbon equivalent Ceq. Need to be contained so that the amount is not more than a predetermined amount. That is, carbon equivalent Ceq. (%) Is Ceq. = C + Si / 24 + M
n / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14, and Ceq. Is more than 0.40%, it is not preferable because the toughness is lowered and the yield strength and the tensile strength are significantly increased even if the steel is manufactured under appropriate rolling conditions and water cooling conditions. Therefore, Ceq. Is 0.40% or less.

In the present invention, 700 ° C., which is the boundary temperature for recuperation of cooling, strictly varies depending on the steel material composition, but in the rolled H-section steel having a tensile strength of 40 to 50 kg / mm ² class, this temperature is actually operated. It has been determined as a result of various tests that the object of the present invention can be sufficiently achieved if it is controlled as the reference value of.

[0043]

EXAMPLE A thin web H-section steel was produced under the rolling conditions and cooling conditions shown in Table 2 for steels having the chemical compositions shown in Table 1. Table 2 also shows the obtained mechanical properties, the hardness of the flange water-cooled surface, and the occurrence of web waves.

In Table 1, Steel 1 to Steel 8 are steels having chemical compositions within the scope of the present invention. On the other hand, Steels 9 to 11 are steels having chemical compositions outside the scope of the present invention, and Steel 9 is Nb.
Steel 10 has a large amount of Nb and Ti, and Steel 11 has a carbon equivalent Ceq. Is a high example.

In Table 2, steel numbers 2-1 and 2-
2 shows that different manufacturing conditions were applied to steel 2 in Table 1, and the same applies to steel numbers 3-1 and 3-2.

As is clear from Table 2, the steels of the present invention 1,
2-1, 3-2, 3-3, 3-5, 4, 5, 6, 7-
Nos. 1 and 8-1 were produced with appropriate chemical components and under appropriate rolling and cooling conditions, and the yield ratio was stably satisfying the target value of 80% or less, and the flange surface hardness was also A sound thin web H-section steel satisfying the target value Hv (10) of 200 or less and having no web wave was obtained.

On the other hand, in Comparative Steel 2-2, Comparative Steel 3-4 and Comparative Steel 7-2, the flange surface was 700 ° C. in the intermediate rolling stage.
Since the number of times of water cooling becomes relatively large below and the finishing temperature became low, the structure was refined or sub-grain boundaries were generated, and as a result, the yield ratio was high and exceeded the target value of 80%.

Since the comparative steel 3-1 was not subjected to flange water cooling in the intermediate rolling stage, the outer surface of the flange was quenched due to water cooling after rolling, and the hardness was remarkably increased to the target value Hv.
(10) It exceeds 200. This steel has extremely deteriorated workability such as drilling due to this increase in hardness.

In Comparative Steel 8-2, the number of times of water cooling at which the surface of the flange was 700 ° C. or less in the intermediate rolling stage was 0, so the outer surface of the flange was quenched by water cooling after rolling, and the hardness was remarkably increased. The value exceeds Hv (10) 200.

Further, since the comparative steel 9 contains a large amount of Nb, the grain refinement becomes excessive and the yield ratio is high, exceeding the target value of 80%. Comparative steel 10 has a high yield ratio because both Nb and Ti are excessive. Steel 11 has a carbon equivalent of Ceq. Is high, the strength is high and the elongation is remarkably reduced.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

In the conventional thin web H-section steel manufacturing method, the increase in hardness and the abnormal increase in strength of the flange surface cannot be avoided due to the cooling after the finish rolling. Not only the prevention but also the cooling in the intermediate rolling stage and the temperature control of the recuperation are accurately performed, so that the hardness increase of the flange surface can be suppressed and the yield ratio can be reduced. Further, since the addition of expensive alloying elements such as Ti and Nb is unnecessary or can be reduced, it is possible to extremely economically manufacture a low yield ratio thin web H-section steel which has excellent workability such as drilling and drilling and has improved seismic performance.

[Brief description of drawings]

FIG. 1 is a graph showing a temperature change of a rolled material according to the present invention.

FIG. 2 is an explanatory schematic diagram of an example of device arrangement for carrying out the method of the present invention.

3 (a), (b) and (c) are diagrams showing an optical microscope photograph and an electron microscope photograph of a metal structure of a flange surface layer section cross-section when manufactured by a conventional method (comparative method).

FIG. 4 is a view showing an optical microscope photograph of a flange surface layer section of a metal structure produced by the method of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Intermediate rolling mill 2a ... Water-cooling device for front and rear surfaces of intermediate rolling mill 3 ... Finishing rolling mill 2b ... Cooling device for rear surface of finishing rolling mill

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[Procedure amendment]

[Submission date] June 5, 1991

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Claim 1

[Correction method] Change

[Correction content]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

In recent years, the construction industry has introduced a new seismic design method to require that the properties of steel materials have a low yield ratio {usually: Y R = (yield strength / tensile strength) × 100 (%)}. ing. Therefore, in order to manufacture a thin web H-section steel that can meet the requirements of the building industry, not only the prevention of web waves but also a new manufacturing technique satisfying the suppression of the hardness increase of the flange surface and the reduction of the yield ratio is required. Will be needed.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Teruyuki Wakatsuki No. 1 Tsukiko Yawata-cho, Sakai City New Nippon Steel Co., Ltd. Inside the Sakai Works (72) Inventor Akira Inagaki No. 1 Tsukiko Hachiman-cho, Sakai City New Japan (72) Hiroyuki Hasegawa, Inventor Hiroyuki Hasegawa, No. 1 Tsukiko Hachiman-cho, Sakai City Nippon Steel Stock Corporation, Sakai Steel Works

Claims (2)

[Claims] 1. When the flange of H-section steel immediately after hot finish rolling is forcibly cooled, the lower limit of the temperature difference between the flange and the web immediately after water cooling in which no web wave is generated during the forced cooling and the room temperature after forced cooling. The upper limit of the temperature difference between the flange and the web immediately after water cooling, in which the thermal stress of the web until reaching the buckling stress of the web is equal to or less than the buckling stress of the web, is obtained in advance for each size of H-section steel and the density of cooling water, and the temperature difference above -In the manufacturing method of the thin web H-section steel in which the flange is forcibly cooled within the lower limit, C: 0.04 to 0.20% Si: 0.01 to 0.50% Mn: 0.3 to 1. 80% Ti: 0.02% or less Al: 0.060% or less Mb: 0.02% or less, and carbon equivalent Ceq.
(= C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5
+ Mo / 4 + V / 14) (%) is 0.40% or less and the balance is Fe and unavoidable impurities, and the steel is subjected to hot rolling, and the outer surface of the flange is forcibly cooled in the intermediate rolling stage before the finish rolling. Then, rolling is repeated while repeating a water cooling step of cooling the surface layer temperature of the outer surface of the flange once to 700 ° C or less and a recuperation step of stopping cooling and reheating the surface layer temperature of the outer surface of the flange to more than 700 ° C. The lower limit of the finish rolling finish temperature is 750 ° C. or higher, and the flange is forcibly cooled immediately after the finish rolling.
A method for producing a thin web section steel having a low yield ratio and excellent workability. 2. The steel composition according to claim 1, if necessary Mo: 0.3% or less V: 0.2% or less Cr: 0.7% or less Cu 1.0% or less Ni: 1.0 % Or less B: 0.003% or less Ca: 0.001 to 0.005% Addition of one or more kinds of low yield ratio thin wall web H-section steel excellent in workability Method.