JP2936235B2 - Rolled section steel with excellent toughness and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

(Industrial application field) The present invention relates to a rolled section steel excellent in toughness used as a structural member of a building and a method for producing the same. (Prior Art) Due to the increase in height of buildings and stricter safety standards, steel materials used for columns, such as H
For a section steel (hereinafter, referred to as an extremely thick H section steel), higher strength, higher toughness, and lower yield ratio are required. In order to satisfy such required characteristics, conventionally, a heat treatment such as a normalizing process has been performed after the completion of rolling. This heat treatment method causes a significant increase in cost such as an increase in heat treatment cost and a decrease in production efficiency, and has a problem from the viewpoint of economy. In order to solve this problem, it has become necessary to develop a new alloy design and manufacturing method so that high-performance material properties can be obtained as-rolled. In general, when a shaped steel having a flange, for example, an H-shaped steel, is manufactured by universal rolling, the rolling finish temperature, rolling reduction, and cooling rate are reduced at each part of the web, flange, and fillet due to limitations in rolling molding and the specificity of the shape. Make a difference. As a result, the strength, ductility, and toughness vary, and a portion that does not meet the standards such as a rolled steel material for a welding structure (JIS G3106) is generated. In particular, when rolling an extremely thick H-section steel using a continuously cast slab as a raw material, there is a limit to the maximum slab thickness that can be manufactured by the continuous casting facility, so that the reduction ratio is low. In addition, the thicker flange part is hot rolled from the top of the roll molding,
After the rolling is completed, the cooling of the steel material is gradually cooled and the micro-composition is coarsened, so that the effect of fine-graining can hardly be expected even by the controlled rolling method used in the field of thick steel plates. On the other hand, in the field of thick plates, high-strength and high-toughness steels are manufactured by utilizing the precipitation effect of VN. For example, Japanese Patent Publication No. Sho 62-50548, Japanese Patent Publication No. Sho 62-548
Although the technology disclosed in Japanese Patent No. 62 has been proposed, in the prior art with this thick plate, since molten steel is deoxidized with Al, a fine grain having an effect on grain refinement as an intragranular ferrite generation nucleus.
Since Si oxides and the like are not generated, it is difficult to produce a high-strength and high-toughness section steel. (Problems to be Solved by the Invention) The present invention provides a method for producing ferrite (hereinafter referred to as IG) from within austenite grains even under the rolling conditions unique to the shape steel as described above.
Say F) to produce a, a high strength yield strength and tensile strength at 35 kgf / mm ² or more 50 kgf / mm ² or more by grain refinement of the microstructure, 2 mm V-notch Charpy value at and 0 ° C. 3.5 kgf
It is an object of the present invention to provide a shaped steel capable of producing as-rolled shaped steel having excellent toughness having mechanical properties of −m or more and a method for producing the same. (Means for Solving the Problems) The present invention has been made to solve the above-described problems, and generates intragranular ferrite in austenite grains during ferrite transformation from austenite during rolling and cooling, thereby changing the structure. The problem has been solved based on the finding that it is possible to produce a shaped steel with high efficiency and low production cost by the method of grain refinement, and the gist thereof is as follows. (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: 0.006 to 0.015%, Al <
A rolled section steel with excellent toughness, containing 0.005%, with the balance being Fe and unavoidable impurities. (2) Preliminary deoxidation of molten iron to reduce dissolved oxygen to 0.003% by weight.
Melted to ~ 0.015%, C: 0.04 by weight% by alloy addition
~ 0.20%, Si: 0.05 ~ 0.50%, Mn: 0.8 ~ 1.8%, V: 0.05 ~
A steel slab containing 0.20%, N: 0.006 to 0.015%, and Al <0.005%, with the balance being Fe and unavoidable impurities.
After reheating to the temperature range of 00 to 1300 ° C, hot rolling is performed at 750 to 1,050
A method for producing a rolled section steel having excellent toughness, which is completed in a temperature range of ° C. (Operation) Hereinafter, the present invention will be described in detail. Higher strength of steel is achieved by finer ferrite crystals, solid solution strengthening by alloying elements, dispersion strengthening by hardened phases, and precipitation strengthening by fine precipitates. Higher toughness is achieved by crystal grain refinement and solid solution of parent phase (ferrite).
High carbon martensite (hereinafter referred to as M ^* ) and coarse oxides in the hardened phase, which are the starting points of N and C reduction and fracture,
Achieved by reducing precipitates and miniaturizing. 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 present invention performs grain refinement, that is, ferrite refinement, under the conditions of low rolling reduction, high-temperature rolling, and slow cooling under the constraints of the production of rolled steel bars. The method uses the oxides and MnS dispersed in the slab to precipitate VN during rolling and during subsequent cooling, and then transforms this into ferrite transformation nuclei to form intragranular ferrite from within austenite grains, resulting in a fine-grained ferrite structure. It is to get. First, 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 to improve the strength of the steel. If it is less than 0.04%, the strength required for structural steel cannot be obtained. Weld cracking, heat affected zone (hereinafter referred to as HAZ)
Since the toughness and the like are significantly reduced, the lower limit is set to 0.04% and the upper limit is set to 0.20%. Next, Si is necessary for securing the strength of the base material, preliminary deoxidation of molten steel, and the like. However, if it exceeds 0.50%, M ^* of a hardened structure is generated in the HAZ structure, and the toughness of the welded joint is significantly reduced. 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 must 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% in the allowable range of the toughness and crack resistance of the welded portion. Al is a strong deoxidizing element, and if added over 0.005%, manganese / silicon oxide that promotes intragranular ferrite transformation is not formed, resulting in a decrease in toughness. In order to reduce the amount of precipitation of VN, which is a feature of the steel of the present invention, the content is limited to less than 0.005%. N is an extremely important element for VN precipitation, and 0.006%
If it is less than 0.006%, the amount of precipitation of VN is insufficient and a sufficient amount of intragranular ferrite structure cannot be obtained. If the content exceeds 0.015%, the toughness of the base material is reduced, and the surface slab of the steel slab during continuous casting is limited to 0.015% or less. V is extremely important as VN for grain refinement due to formation of intragranular ferrite structure and high strength due to precipitation strengthening. If less than 0.05%, the amount of VN precipitated is insufficient, and 0.20%
If the amount exceeds, the precipitation amount becomes excessive and the base material toughness decreases.
Limited to 0.05-0.20%. The amounts of P and S contained as unavoidable impurities are not particularly limited, but should be reduced as much as possible because welding cracks and lowering of toughness are caused by solidification segregation, and the contents of P and S are desirably 0.02% or less. Preliminary deoxidation of the molten iron to bring the dissolved oxygen to 0.003 to 0.0% by weight
After smelting to 15%, the component adjustment by alloy addition is
Before deoxidation

[0] If the concentration is less than 0.003%, intragranular ferrite generation nuclei such as manganese / silicon oxide which promotes intragranular ferrite transformation are reduced, so that fine grains cannot be formed and toughness cannot be improved. If it exceeds 0.015%, even if other conditions are satisfied, the oxides become coarse and become the starting point of brittle fracture, which lowers the toughness.
At 0.003 to 0.015%. The reason for restricting the reheating temperature to the temperature range of 1100 to 1300 ° C is
Production of shaped steel by hot working requires heating at 1100 ° C or higher to facilitate plastic deformation, and it is necessary to sufficiently dissolve elements such as V and Nb. Was set to 1100 ° C. The upper limit is based on heating furnace performance and economy.
It was 1300 ° C. The reason for setting the hot rolling end temperature to 750 to 1,050 ° C is that the lower the temperature, the more the toughness is improved.However, processing below 750 ° C is difficult due to the shaping of the shaped steel. This is because a structure is formed and toughness is reduced. Hereinafter, the effects of the present invention will be further shown by examples. (Example) The prototype steel was melted from a converter and cast into 250-300 mm thick slabs by continuous casting, and then H-shaped steels of various dimensions shown in Table 1 were manufactured by rolling molding for each flange thickness. . Mechanical properties in the center of the plate thickness t ₂ of the flange 2 shown in Figure 1 from (1 / 2t ₂₎ at 1 / 4,1 / 2 width of the flange width total length (B) (1 / 4B, 1 / 2B) Collected and asked. The characteristics of these locations were determined as follows: The flange 1 / 4F shows almost the average mechanical properties of H-section steel,
The reason for this is that the mechanical test characteristics of the H-section steel can be represented by these two locations because the characteristics of the flange 1 / 2F portion are most deteriorated. Table 2 shows the chemical composition of the prototype steel, and Table 3 shows the rolling conditions and mechanical test characteristics. In addition, the rolling heating temperature
It is well-known that the lowering of the heating temperature generally improves the mechanical properties, and the high-temperature heating condition is estimated to indicate the lowest value of the mechanical properties. This is because it was determined that the characteristics of the above can be represented. As shown in Table 3, the shape steel of the steel 1 according to the present invention has a target strength (the above-mentioned JIS G310) with respect to changes in the rolling finish temperature, rolling reduction, flange plate thickness (cooling rate), and flange portion.
6) and the Charpy value at 0 ° C of 3.5 kgf-m or more is sufficiently satisfied. On the other hand, although the sectional steels 10 to 12 of the comparative steel satisfy the strength characteristics, the toughness of the flange 1/2 part is remarkably reduced and the target value cannot be achieved. The reason is that the number of fine oxides + MnS + VN acting as IGF nucleation sites is insufficient due to the Al deoxidation treatment and the non-addition and insufficient amounts of V and N, the IGF is not generated, and the toughness cannot be improved by grain refinement. is there. That is, when all the requirements of the present invention are satisfied, as in the section steel 1 shown in Table 3, the flange plate thickness 1/2 and width 1/2 section which are most difficult to satisfy the mechanical test characteristics of the rolled section steel. Thus, a rolled section steel having sufficient strength and excellent toughness and its production can be provided. 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. (Effect of the Invention) 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 guaranteed, 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 diagram showing a cross-sectional shape of an H-section steel, showing names of respective parts and a sampling position of a mechanical test piece. 1; H-section, 2; flange, 3; web.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Naoki Oda, inventor, Sakai Works, Sakai City, Osaka 1st place, Nippon Steel Corporation Sakai Works (56) References JP-A-3-191020 (JP, A) Kohei 7-37657 (JP, B2) (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00-38/60 C21D 8/00-8/10

Claims (2)

(57) [Claims] C: 0.04 to 0.20% by weight, Si: 0.05 to 0.50% by weight
%, Mn: 0.8 to 1.8%, V: 0.05 to 0.20%, N: 0.006 to 0.015
%, Al <0.005%, with the balance being Fe and unavoidable impurities. 2. The dissolved oxygen is preliminarily deoxidized to dissolve oxygen in weight%.
Melted to 0.003 to 0.015%, and by alloy addition, C:
0.04-0.20%, Si: 0.05-0.50%, Mn: 0.8-1.8%, V: 0.
05-0.20%, N: 0.006-0.015%, including Al <0.005%,
The remainder is a slab consisting of Fe and unavoidable impurities, and after reheating the slab to a temperature range of 1100 to 1300 ° C., hot rolling is performed to 750 to 1
A method for producing a rolled section steel having excellent toughness, which is completed in a temperature range of 050 ° C.