JP2601961B2 - Manufacturing method of rolled section steel with excellent toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[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. To solve this problem, it was necessary to develop a new alloy design and manufacturing method so that high-performance material properties could be obtained as-rolled.

Generally, a section steel having a flange, for example, H
When a section steel is manufactured by universal rolling, the web, flange,
Differences occur in the rolling finish temperature, rolling reduction, and cooling rate at each part of the fillet. As a result, the strength, ductility, and toughness vary, for example, a rolled steel material for a welded structure (JIS G3106)
Some parts do not meet the criteria. In particular, when rolling an extremely thick H-section steel as a continuous cast slab as a raw material, the maximum thickness of the slab that can be manufactured by the continuous casting facility is limited, so that the reduction ratio is low. Further, the flange portion having a large thickness is subjected to high-temperature rolling due to the restriction on the rolling molding, and the steel material after rolling is gradually cooled, whereby the microstructure becomes coarse. As a grain refining method, there is a controlled rolling method used in the field of thick steel plates, but it is difficult to apply the method due to restrictions on modeling.

On the other hand, in the field of thick steel sheets, high strength and high toughness steels are manufactured by utilizing the precipitation effect of VN.
Japanese Patent Publication No. 50548 and Japanese Patent Publication No. Sho 62-54862 propose a technique. However, since the molten steel is subjected to Al deoxidation treatment by a conventional method, it is used as an intragranular ferrite generation nucleus to reduce the grain size. A fine composite oxide showing an effect was not generated, and the grain refinement of the structure was not sufficient. That is, in the conventional Al deoxidation, Al was added at the initial stage of the smelting process, and the purpose was to deoxidize the molten steel and float off the generated Al ₂ O ₃ to achieve high purification. That is, the theme was how to reduce the oxygen concentration of molten steel and the number of primary deoxidized oxides in steel. The present invention is characterized in that, unlike the conventional idea, a fine composite oxide capable of being used as an intragranular ferrite transformation nucleus is precipitated by controlling the deoxidation process and used.

[0005]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention performs an appropriate deoxidation treatment in a steelmaking process, highly purifies molten steel, controls dissolved oxygen concentration, and finally adds Al. By controlling the order of addition of Al and the amount of Al added, and dispersing a large number of fine composite oxides in the steel, even under the rolling conditions peculiar to the section steel as described above, even within the austenitic grains, the intragranular ferrite is reduced. (Hereinafter I
GF), and by micronizing the microstructure,
To provide a method for manufacturing a rolled section steel excellent in material properties, inexpensive, economical, and excellent in toughness, in which the dependence of rolling finish temperature, rolling reduction ratio, steel sheet thickness (cooling rate), etc. on the material properties is insensitive. It is in.

[0006]

SUMMARY OF THE INVENTION The present invention provides a method for producing IGF in austenite grains at the time of ferrite transformation from an austenite phase during rolling and cooling to refine the microstructure, thereby achieving high efficiency and low production cost. The present invention has solved the above-mentioned problem based on the finding that it is possible to produce various shaped steels. The gist of the present invention is that (1) C:
0.04 to 0.20%, Si: 0.05 to 0.50%,
Mn: 0.8 to 1.8%, V: 0.05 to 0.20%,
N: 0.004 to 0.015%, Ti: 0.005 to
The molten steel containing 0.025%, the balance being Fe and unavoidable impurities, is adjusted to 0.003 to 0.015% by weight of dissolved oxygen by pre-deoxidation treatment, and then metal aluminum or ferroaluminum is added. And the Al
The content is 0.005 to 0.015% by weight%, and -0.004 ≦ [Al%] based on the dissolved oxygen [O%] of the molten steel.
Casting into a slab satisfying the relationship of −1.1 [O%] ≦ 0.006, reheating the slab to a temperature range of 1100 to 1300 ° C., then hot rolling, and a temperature range of 750 to 1050 ° C. And (2) C: 0.04 to 0.2% by weight.
0%, Si: 0.05 to 0.50%, Mn: 0.8 to
1.8%, V: 0.05 to 0.20%, N: 0.004
0.015%, Ti: 0.005 to 0.025%, Cr ≦ 0.7%, Mo ≦ 0.3%, Nb ≦
Molten steel containing one or more of 0.05%, Ni ≦ 1.0%, Cu ≦ 1.0%, and the balance being Fe and unavoidable impurities, is subjected to a preliminary deoxidation treatment to dissolve dissolved oxygen by weight. %, Adjusted to 0.003 to 0.015%, and further deoxidized by adding metallic aluminum or ferroaluminum.
l is 0.005 to 0.015% by weight, and -0.004 ≦ [Al
%]-1.1 [O%] ≦ 0.006, and the resulting slab is reheated to a temperature range of 1100 to 1300 ° C., and then hot-rolled to a temperature of 750 to 1050 ° C. This is a method for producing a rolled section steel having excellent toughness, characterized by terminating rolling in a temperature range.

[0007]

Hereinafter, the present invention will be described in detail. Higher strength of steel is achieved by refinement of ferrite crystals, solid solution strengthening by alloy elements, dispersion strengthening by hardened phases, precipitation strengthening by fine precipitates, and the like. Higher toughness is achieved by reducing the crystal grain size and dissolving N,
This is achieved by reduction of C, reduction and refinement of high-carbon martensite and coarse oxides and precipitates in a hardened phase which is a starting point of fracture.

In general, toughness is reduced by increasing the strength of steel, and contradictory measures must be taken to increase strength and toughness.
The only metallurgical factor that satisfies both at the same time is grain refinement. The feature of the present invention is that the steel is made up of Al, Ti, Mn, and Si elements by controlling the amount of dissolved oxygen in the molten steel in the steelmaking process and adding a deoxidizing element that adds a small amount of Al immediately before tapping. MnS, Ti with composite oxide particles as nuclei
By dispersing and precipitating complex precipitates of N and VN, utilizing the effect of promoting intragranular ferrite transformation from within austenite grains during hot rolling, during rolling under the constraints of rolling section steel, The precipitates are used as nuclei to generate intragranular ferrite, to refine the crystal, and to achieve high strength and high toughness of the base material as it is rolled.

Next, the reasons for limiting the range of the basic components of the shaped steel according to 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 for structural steel cannot be obtained, and if it exceeds 0.20%, excessive addition of The lower limit is set to 0.04 because it significantly lowers the base metal toughness, weld cracking resistance, and toughness of the weld heat affected zone.
% And the upper limit was set to 0.20%.

[0010] Next, Si is necessary for securing the strength of the base material and for pre-deoxidizing the molten steel.
Generates high-carbon martensite having a hardened structure in the structure, and significantly reduces the toughness of the welded joint. If the content is less than 0.05%, the necessary preliminary deoxidation of molten steel cannot be performed, so the Si content is limited to the range of 0.05 to 0.50%. Mn needs to be added in an amount of 0.8% or more to ensure the strength and toughness of the base material, but the upper limit is set to 1.8% within an allowable range of the toughness and cracking properties of the welded portion.

V is an extremely important element as VN for grain refinement due to formation of an intragranular ferrite structure and high strength due to precipitation strengthening. If less than 0.05%, the amount of VN precipitated is insufficient. If it exceeds 0.20%, the amount of precipitation will be excessive and the base material toughness will decrease, so it was limited to 0.05 to 0.20%.

N is an extremely important element for the precipitation of VN. If the content is less than 0.004%, the amount of VN precipitated becomes insufficient, and a sufficient amount of intragranular ferrite structure cannot be obtained.
04% or more. When the content exceeds 0.015%, the toughness of the base material is reduced, and surface cracking of the steel slab during continuous casting is performed, so that the content is limited to 0.015% or less.

[0013] Ti is used as a deoxidizing agent to form a Ti-based oxide, which has the effect of promoting the formation of intragranular ferrite during rolling and precipitating fine TiN to promote the reduction of austenite grain size and the formation of intragranular ferrite. Improve the toughness of materials and welds. Therefore, at 0.005% or less, T
Since the i content is insufficient and the effect as an intragranular ferrite generation nucleus is reduced, the lower limit of the Ti content is set to 0.005% or more. However, if it exceeds 0.025%, excess Ti becomes TiC
, Precipitation hardening occurs and the toughness of the heat affected zone is significantly reduced.

Al is a strong deoxidizing element, and 0.015
%, A complex oxide that promotes intragranular ferrite transformation is not formed, resulting in a decrease in toughness, and excess solid solution Al combines with N to form AlN, which is a feature of the invention steel, VN. Was limited to 0.015% or less in order to reduce the amount of precipitation. Further, if the content is less than 0.005%, the target composite oxide containing Al cannot be produced, so the content was made 0.005% or more. In addition, the amount of Al is expressed by weight% with respect to the dissolved oxygen [O%] of the molten steel, -0.004% ≦ [Al%]-1.1 [O
%] ≦ 0.006%, the reason is that if the weight percentage of Al in this relational expression is excessive relative to the [O] concentration, the number of composite oxides produced decreases, and A large number of Al ₂ O _3, which are ineffective as ferrite-forming nuclei, are generated, and the structure cannot be refined, toughness is reduced. This was limited because of the significant reduction in oxides. The reason for adding Al at the end of the steelmaking process is as follows.
The reason 1 is that Al ₂ O ₃ which has a strong deoxidizing power and is stable is generated, and the desired low melting point composite oxide is hardly generated.

The amounts of P and S contained as inevitable impurities are not particularly limited, but welding cracks due to solidification segregation,
Since toughness is reduced, the content of P and S should be less than 0.02%.

The basic components of the steel of the present invention have been described above. For the purpose of increasing the strength of the base material and improving the toughness of the base material, Cr, M
One, two or more of o, Nb, Ni, and Cu can be contained.

First, Cr is effective in strengthening the base material by improving the hardenability. However, an excessive addition exceeding 0.7% is harmful from the viewpoint of toughness and curability,
The upper limit was set to 0.7%. Mo is an element effective for securing the base material strength, but is limited to 0.3% or less because it is expensive. Nb is effective for toughening the base material, but an excessive addition exceeding 0.05% is harmful from the viewpoint of toughness and curability, so Nb was set to 0.05% or less. Ni is a very effective element for increasing 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 is 1.
0%. Cu is an element effective for strengthening the base material and weathering resistance. However, the upper limit is set to 1.0% in order to promote temper embrittlement by stress relief annealing, weld cracking, and hot work cracking.

The reason why the molten steel is preliminarily deoxidized to control the dissolved oxygen to 0.003 to 0.015% by weight is as follows.
It is extremely important to clean the molten steel and to generate fine composite oxides in the slab at the same time. If the [O] concentration after the preliminary deoxidation is less than 0.003%, the amount of the composite oxide of the intragranular ferrite nucleus which promotes the intragranular ferrite transformation decreases, and the grain cannot be refined to improve the toughness. On the other hand, 0.0
If it exceeds 15%, even if other conditions are satisfied, the oxide becomes coarse and becomes the starting point of brittle fracture, and the [O] concentration after preliminary deoxidation for reducing toughness is 0.003% by weight.
To 0.015%.

The above-mentioned preliminary deoxidation treatment is performed by vacuum degassing, Al,
This was performed by deoxidation of Si, Ca, and Mg. The reason is that vacuum degassing directly removes oxygen from molten steel as gas and CO gas, and oxide inclusions generated by strong deoxidation such as Al, Si, Ca, and Mg easily float and are easily removed. This is because it is extremely effective in purifying the water.

The reason why the reheating temperature is limited to the temperature range of 1100 to 1300 ° C. is that heating of 1100 ° C. or more is necessary in order to facilitate plastic deformation in the production of a shaped steel by hot working. Since it is necessary to sufficiently dissolve elements such as Nb, the lower limit of the reheating temperature is set to 1100 ° C. The upper limit was set to 1300 ° C. in view of the performance and economy of the heating furnace.

The reason why the hot working end temperature is set to 750 to 1050 ° C. is that although the toughness is improved as the rolling is performed at a lower temperature, working at a temperature lower than 750 ° C. is difficult due to the shaping of the shaped steel.
Excessive processing forms a coarse-grained structure and lowers toughness.

[0022]

EXAMPLES The effects of the present invention will be further illustrated by the following examples. The prototype steel was melted in the converter, and after adding the alloy, it was pre-deoxidized, the oxygen concentration of the molten steel was measured, and the amount of Al corresponding to the amount was added, and continuous casting was performed to produce a 250-300 mm thick slab. After casting, H-shaped steels having various dimensions shown in Table 1 were produced by rolling molding for each flange thickness. The mechanical characteristics are shown in FIG. 1 at the center (1 / 2t ₂ ) of the thickness t ₂ of the flange 2, which is １ ／ of the entire flange width (B), １ ／ width (フ ラ ン ジ B, 1).
/ 2B) to obtain a test piece. The characteristics of these locations were determined by the following: Flange 1 / 4F shows almost average mechanical properties of H-section steel, and Flange 1 / 2F has the lowest properties. This is because the mechanical test characteristics of the section steel can be represented.

Table 2 shows the chemical composition of the prototype steel, and Table 3 shows the rolling conditions and mechanical test characteristics. Note that the rolling heating temperature was 12
The reason why the temperature is adjusted to 80 ° C. is that it is generally known that the reduction of the heating temperature improves the mechanical properties, and it is estimated that the high-temperature heating condition indicates the lowest value of the mechanical properties. This is because it has been determined that the characteristics in the above can be represented.

As shown in Table 3, steels 1 to 8 according to the present invention
Satisfies a target base metal strength (JIS G3106) and a Charpy value of 34 (J) or more at 0 ° C. or more with respect to changes in rolling finish temperature and flange plate thickness (cooling rate). On the other hand, the comparative steels 9 to 12 satisfy the standard in the base metal strength, but none of the toughness of the flange thickness 板 and the width 部 portion satisfy the target values. The cause is steel 9.1
Reference numerals 1 and 12 denote ordinary Al deoxidation treatments, in which Al was added in the initial stage of the steelmaking process, and the limitation range of the amount of Al with respect to the oxygen concentration of the molten steel-0.004% ≤ [Al%] -1.
1 [O%] ≦ 0.006%, and the oxygen concentration of the molten iron in the pre-deoxidation was within the limit range, but the steel 10 was not subjected to the Ti addition treatment. This is because the number of fine composite oxides containing Ti serving as nucleation sites + MnS + TiN + VN was insufficient, so that IGF was not generated and toughness could not be improved by grain refinement.

That is, when all the requirements of the present invention are satisfied, as shown in Tables 1 to 8 in Table 3, the flange plate thickness 1/2 and width, which hardly satisfy the mechanical test characteristics of the rolled section steel Even in a half part, a rolled section steel having sufficient strength and excellent toughness can be manufactured. 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.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

According to the present invention, the rolled section steel has sufficient strength even at a flange plate thickness of 1/2 and a width of 1/2 section where mechanical test characteristics are most difficult to be assured, and has excellent toughness. Manufacturing can be performed as it is rolled, and industrial effects such as improvement in reliability of large buildings, safety assurance, and economic efficiency are extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a view showing a cross-sectional shape of an H-section steel, showing names of respective parts and a sampling position of a mechanical test piece.

[Explanation of symbols]

 1 H-section steel 2 Flange 3 Web

Continuation of the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location C22C 38/00 301 C22C 38/00 301A 38/14 38/14

Claims (2)

(57) [Claims] 1. C: 0.04 to 0.20% by weight, S
i: 0.05 to 0.50%, Mn: 0.8 to 1.8%,
V: 0.05 to 0.20%, N: 0.004 to 0.01
5%, Ti: 0.005 to 0.025%, the balance being molten steel consisting of Fe and unavoidable impurities, by pre-deoxidation treatment, dissolved oxygen by weight 0.003 to 0.015.
% And further deoxidized by adding metallic aluminum or ferroaluminum, and the Al content is 0.005% by weight.
-0.004 ≦ [Al%] − 1.1 [O%] ≦ 0.00 with respect to the dissolved oxygen [O%] of the molten steel.
6 and cast the slabs 1100-1
After reheating to a temperature range of 300 ° C., hot rolling is performed, and
A method for producing a rolled section steel having excellent toughness, wherein rolling is completed in a temperature range of 1050 ° C. 2. C: 0.04 to 0.20% by weight, S
i: 0.05 to 0.50%, Mn: 0.8 to 1.8%,
V: 0.05 to 0.20%, N: 0.004 to 0.01
5%, Ti: 0.005 to 0.025%, Cr ≦ 0.7%, Mo ≦ 0.3%, Nb ≦ 0.05%,
The molten steel containing one or more of Ni ≦ 1.0% and Cu ≦ 1.0%, the balance being Fe and unavoidable impurities, was subjected to a preliminary deoxidation treatment to reduce dissolved oxygen to 0.04% by weight.
After being adjusted to 03 to 0.015%, it is further deoxidized by adding metallic aluminum or ferroaluminum. The Al content is 0.005 to 0.015% by weight and the dissolved oxygen [O%] of the molten steel. -0.004 ≦ [Al%]-1.1
[O%] Cast into a slab that satisfies the relationship of 0.006, re-heat the slab to a temperature range of 1100 to 1300 ° C, perform hot rolling, and finish rolling at a temperature range of 750 to 1050 ° C. A method for producing a rolled section steel having excellent toughness, characterized in that: