JP2579841B2 - Method for producing as-rolled intragranular ferritic steel with excellent fire resistance and 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 section steel having excellent fire resistance and toughness used as a structural member of a building.

[0002]

2. Description of the Related Art Fireproof design has been reviewed by the Ministry of Construction's comprehensive project due to the increase in the height of buildings and the sophistication of building design techniques. In March 1987, the "New Fireproof Design Law" was enacted. According to this regulation, the restriction on fire-resistant coating to keep the temperature of steel at 350 ° C or less in the event of a fire according to the old law is lifted, and a fire-resistant coating method that conforms to the balance between the high-temperature strength of steel and the actual load of a building is lifted. Can now be determined. That is, when the design high-temperature strength at 600 ° C. can be secured, the refractory coating can be reduced accordingly.

In response to such a trend, Japanese Patent Laid-Open No.
No. 77523 proposes a low yield ratio steel and a steel material for building having excellent fire resistance and a method for producing the same. The gist of the invention of the prior application is that Mo and Nb are added so that the yield point at 600 ° C. becomes 70% or more of that at normal temperature to improve the high-temperature strength. The design high-temperature strength of the steel material was set to 600 ° C. based on the finding that it is the most economical in view of the balance between the increase in the cost of the steel material due to the alloying element and the cost of the refractory coating.

[0004]

Generally, when a section steel having a flange, for example, an H-section steel, is manufactured by universal rolling, a rolling finish is performed at each part of a web, a flange, and a fillet due to restrictions on rolling molding and its unique shape. Differences occur in temperature, draft and cooling rate. As a result, strength,
Ductility and toughness vary, for example, rolled steel for welded structures (J
Some sites do not meet standards such as IS G3106).

The inventors of the present invention have tried to apply the steel material manufactured by the above-mentioned prior application to various shaped steels, particularly to H-shaped steels having complicated shapes and severe restrictions on rolling molding. , The structure, especially the bainite ratio varies significantly depending on the part, and the room temperature / high temperature strength, ductility, and toughness vary,
Some parts did not meet the criteria. In order to solve the above-mentioned problems, the present invention performs a suitable preliminary deoxidation treatment in a steelmaking process, performs high purification of molten steel, controls dissolved oxygen concentration, and molds a deoxidizing element immediately before solidification. Disperses fine oxides and generates intragranular ferrite (IGF) from austenite grains under the rolling conditions peculiar to shaped steel as described above, refines the microstructure, and provides high-temperature strength characteristics. Less dependent on rolling properties such as rolling finish temperature, rolling reduction ratio and steel sheet thickness (cooling rate)
An object of the present invention is to provide an inexpensive and economical rolled steel excellent in material properties and excellent in fire resistance and toughness.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and its gist is as follows. (1) In addition to vacuum degassing of molten iron, deoxidizing elements Al and S
Preliminary deoxidation treatment is performed by adding i, Ca, Mg alone or in combination with an alloy thereof, and the dissolved oxygen is reduced to 0.003 to 0.003% by weight.
After smelting to 0.015%, by adding an alloy, C:
0.04 to 0.20%, Si: 0.05 to 0.50%,
Mn: 0.4 to 2.0%, Mo: 0.3 to 0.7%,
V: 0.05 to 0.20%, N: 0.006 to 0.01
5%, include Al ≦ 0.005% and adjusted to the molten steel a balance of Fe 及 beauty inevitable impurities, further Ti-Cu in solution steel in a continuous casting mold, Ti-Ni, either Ti-Fe alloy Is continuously added for final deoxidation, and the Ti content is −0.006 ≦ [Ti based on the dissolved oxygen [O%] of the molten steel.
%] - 2 [O%] and cast into weight percent of slab satisfy the relationship of ≦ 0.008, this time the template piece becomes intragranular ferrite nuclei
Contains Ti-based oxide and Si-Mn-based oxide particles dispersedly
Te becomes, after reheating the said template pieces to a temperature range of 1100 to 1300 ° C., and ends the rolling in a temperature range of from 750 to 1,050 ° C. subjected to hot rolling, thus said acid in said rolling conditions
Precipitation of MnS, TiN, and VN with nucleation of halide particles
Of Intragranular Ferrite from Austenite Grains
A pressure characterized by more microstructural refinement
A method for producing an intragranular ferritic section steel having excellent fire resistance and toughness as it is .

(2) In addition to the vacuum degassing treatment, the molten iron is subjected to a preliminary deoxidation treatment by adding the deoxidizing elements Al, Si, Ca, and Mg alone or in combination with their alloys, and the dissolved oxygen is reduced to 0.1% by weight.
After smelting to 003 to 0.015%, by adding an alloy, C: 0.04 to 0.20% and Si: 0.05 to 0.
50%, Mn: 0.4-2.0%, Mo: 0.3-0.
7%, V: 0.05 to 0.20%, N: 0.006 to
0.015%, Al ≤ 0.005%, plus Cr
≦ 0.7%, Ni ≦ 1.0%, Nb ≦ 0.05%, Cu
≦ 1.0%, the balance being adjusted to molten steel consisting of Fe and unavoidable impurities, and further adding Ti—Cu, Ti—Ni, Ti-
Any one of the Fe alloys is continuously added for final deoxidation, and the Ti content is -0.006 ≦ to the dissolved oxygen [O%] of the molten steel.
[Ti%] − 2 [O%] ≦ 0.008 is cast into a slab of weight% which satisfies the relationship, wherein the slab is an intragranular ferrite nucleus.
Dispersed Ti-based oxide and Si-Mn-based oxide particles
After the slab is reheated to a temperature range of 1100 to 1300 ° C., hot rolling is performed and rolling is completed in a temperature range of 750 to 1050 ° C. , thus under the rolling conditions.
Composite of MnS, TiN, VN with the oxide particles as nuclei
Of intragranular ferrite from austenite grains by precipitation
A method for producing an intragranular ferritic section steel excellent in fire resistance and toughness as rolled , characterized in that the microstructure is refined by generation .

[0008]

Hereinafter, the present invention will be described in detail. The high temperature strength of steel material is almost the same as the strengthening mechanism at room temperature below 700 ° C, which is almost half the melting point of iron, and is similar to the strengthening mechanism at normal temperature. Fine grain size of ferrite, solid solution strengthening by alloying elements, dispersion strengthening by hardened phase. , Governed by precipitation strengthening by fine precipitates.

In general, an increase in the high-temperature strength is achieved by increasing the softening resistance at a high temperature by adding precipitation strengthening by adding Mo and Cr and suppressing the disappearance of dislocations. But Mo, C
The addition of r significantly increases the hardenability, and the ferrite of the base metal +
A pearlite structure is easily formed into a bainite structure. When a component steel that easily produces a bainite structure is applied to a rolled section steel, a difference occurs in the rolling finish temperature, rolling reduction, and cooling rate at each part of the web, flange, and fillet due to its unique shape. The proportion of the bainite structure changes greatly depending on the location. As a result, there are variations in room-temperature / high-temperature strength, ductility, and toughness, and portions that do not meet standards. in addition,
The addition of these elements significantly hardens the weld and reduces toughness.

A feature of the present invention is that oxide particles such as Ti-based oxides and Si / Mn-based oxides dispersed in steel are controlled by controlling the amount of dissolved oxygen in molten steel and selecting a procedure for adding a deoxidizing element. Utilizing the promotion effect of intragranular ferrite transformation from within austenite grains due to the complex precipitation of MnS, TiN and VN with nucleus as the core, the change in the microstructure ratio of bainite and ferrite in each part of the section steel is reduced, and that to achieve uniformity of mechanical properties, high-temperature strength to precipitation strengthening by V carbonitride
It has been improved.

The heat affected zone (hereinafter referred to as HAZ) is heated to a high temperature just below the melting point of iron, causing remarkable coarsening of austenite grains, resulting in coarsening of the structure and remarkably lowering toughness. . Ti dispersed in steel according to the present invention
Oxide particles, such as oxides and Si-Mn oxides, have an excellent needle-like intragranular ferrite generation function, and use this as a nucleus to generate an intragranular ferrite structure, which has the feature of remarkably refining the structure and improving toughness. ing.

Next, the reasons for limiting the range of the basic components of the target steel of the present invention will be described. First, C is added as an effective component for improving the strength of steel, and if it is less than 0.04%, the strength required for structural steel cannot be obtained.
% In excess, the base material toughness, weld cracking resistance, H
The upper limit is 0.20% because AZ toughness is significantly reduced.
And

Next, Si is necessary for securing the strength of the base material and for generating Si-based oxides. If it exceeds 0.50%, high-carbon martensite having a hardened structure is formed in the heat-treated structure, and toughness is increased. Is significantly reduced. If the content is less than 0.05%, a necessary Si-based oxide cannot be formed, so the Si content is limited to 0.05 to 0.50%. Mn must be added in an amount of 0.4% or more in order to ensure the strength and toughness of the base material, but the upper limit is set to 2.0% in an allowable range such as the toughness and crack resistance of the welded portion.

Al is a strong deoxidizing element and has a content of 0.005%.
%, It promotes intragranular ferrite transformation.
No i-based oxide, Si-Mn-based oxide, etc. are formed, resulting in a decrease in toughness, and excess solid solution Al combines with N to form AlN, and VN which is a feature of the steel of the present invention.
Was limited to 0.005% or less in order to reduce the amount of precipitation.

N is an extremely important element for the precipitation of VN. If it is less than 0.006%, the amount of VN deposited is insufficient, and a sufficient amount of intragranular ferrite structure cannot be obtained.
Since the high temperature strength at ℃ cannot be secured, the content is set to 0.006% or more. When the content exceeds 0.015%, the toughness of the base material is reduced, and the steel slab is subjected to surface cracking during continuous casting, so that the content is limited to 0.015% or less.

Mo is an element effective for securing the base material strength and the high-temperature strength. If it is less than 0.3%, sufficient high-temperature strength cannot be ensured even by the combined action of VN and precipitation strengthening, and the effect of VN is not sufficient.
If it exceeds 7%, the hardenability is excessively increased, and the base material toughness and the HAZ toughness are deteriorated. V is extremely important as VN for formation of intragranular ferrite structure, grain refinement thereof, and securing high-temperature strength. When V is less than 0.05%, the amount of VN precipitated is insufficient. Since the amount of precipitation becomes excessive and the toughness of the base material and the toughness of the welded portion decrease, the content is limited to 0.05 to 0.20%.

Although the amounts of P and S contained as unavoidable impurities are not particularly limited, they should be reduced as much as possible because welding cracks and toughness are reduced due to solidification segregation. Less than 02%. The above are the basic components of the target steel of the present invention. For the purpose of increasing the base metal strength and improving the toughness of the base metal, Cr, Ni,
One or two or more of Nb and Cu can be contained.

First, Ni is an extremely 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 was made 1.0%. Cr is effective in strengthening the base material and strengthening at high temperatures by improving hardenability and precipitation hardening. However, excessive addition exceeding the upper limit is detrimental from the viewpoint of toughness and curability, so the upper limit was set to 0.7%.

Nb is effective for toughening the base material, but excessive addition exceeding the upper limit is detrimental from the viewpoint of toughness and hardenability, so Nb is set to 0.05% or less. Cu strengthens the base material,
Although it is an element effective for weather resistance, the upper limit is set to 1.0% in consideration of temper brittleness due to stress relief annealing, welding crack resistance, hot working crack, and the like. The vacuum degassing treatment of molten iron and the preliminary deoxidation treatment by adding Al, Si, Ca, and Mg metals or their alloys in combination make the molten iron highly purified, and at the same time, dissolve dissolved oxygen by 0.003 to 0.015% by weight. This is an extremely important process for controlling the speed.

If the high purity of the molten iron is insufficient and a coarse oxide remains in the molten steel, it is used as a nucleation site to form Ti.
Fine secondary deoxidized oxide, which has an effect on the formation of intragranular ferrite generated by adding the alloy to the mold, adheres and agglomerates to form a coarse oxide, which reduces the number of the coarse oxide and reduces the toughness due to the coarse oxide. If the [O] concentration after the pre-deoxidation is less than 0.003%, T
Intragranular ferrite generation nuclei such as i-based oxides are reduced, grain refinement cannot be performed, and toughness cannot be improved. On the other hand, 0.015%
If it exceeds, even if other conditions are met, the oxide becomes coarse in molten steel and during solidification and becomes the starting point of brittle fracture,
Decreases toughness. Therefore, the [O] concentration after the preliminary deoxidation is limited to 0.003 to 0.015% by weight.

In the preliminary deoxidation treatment, vacuum degassing and A
l, Si, Ca, Mg, etc. are selected because the vacuum degassing directly removes oxygen in the molten steel as gas and CO gas, and the strong deoxidation of Al, Si, Ca, Mg, etc. Oxide-based inclusions formed were used because they are very effective in cleaning molten steel because they are easy to float and remove.

The addition of Ti as a final deoxidizer in a mold is a process for uniformly dispersing and depositing a fine Ti-based oxide in a slab. This is because the primary deoxidized oxide precipitated in the molten steel stage is liable to be agglomerated and coarsened. Therefore, it is necessary to tap and solidify the steel as short as possible after adding Ti. This is because the method of adding Ti in a continuous casting is most effective.

Ti-Cu, Ti-N
i, Ti-Fe alloy was selected because it is necessary to uniformly diffuse Ti added in molten steel in a short time as possible in a continuous casting mold, and that a Ti alloy having a low melting point is effective, Cu, Ni, and Fe are metals that hardly affect the material properties. The main composition of each alloy is 30-60% by weight of Ti, and the balance is Cu, T
i: 30 to 80%, balance is Ni, Ti: 40 to 75%,
The balance is made of Fe, and all are alloys having a lower melting point than pure Ti. The addition may be performed by processing these alloys into wires or granules and adding them continuously to the mold.

It should be noted that the Ti content is determined by changing the dissolved oxygen [O
%] To -0.006 ≦ [Ti%] − 2 [O%] ≦
The limitation that the weight% satisfying the relation of 0.008 is given is that in this relational expression, when the weight% is excessive with respect to the [O] concentration, the excess Ti becomes excessive Ti.
N forms N and reduces the amount of VN precipitation which is a feature of the present invention. When Ti is too small relative to the [O] concentration by weight%, Ti-based oxide and Si-Mn-based become ferrite nuclei in grains. The total number of oxides is limited because it does not exceed the required number of 40 / mm ² .

The reason why the reheating temperature is restricted 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 shaped steel by hot working, and V, In order to increase the high-temperature yield point of Mo, these elements must be sufficiently dissolved to form a solid solution.
The lower limit of the reheating temperature was 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 the lower the temperature, the more the toughness is improved, but it is difficult to work at a temperature lower than 750 ° C.
This is because excessive processing produces a coarse-grained structure and lowers toughness. Hereinafter, the effects of the present invention will be further shown by examples.

[0027]

[Example] A prototype steel was melted from a converter and vacuum degassed.
Preliminary deoxidation treatment is performed by adding an alloy of Al, Si, Ca, and Mg. Further, a Ti alloy is continuously added by a continuous casting mold, cast into a 250-300 mm thick slab, and then flanged by rolling molding. H of various dimensions shown in Table 1 for each thickness
Shaped steel was manufactured.

The mechanical properties are shown by the thickness t of the flange 2 shown in FIG.
_{2 at} the center (1/2 · t ₂ ), 1 / of the overall flange width B
Specimens were sampled from 4, 1/2 widths (1 / 4B, 1 / 2B). The welded joint Charpy test piece 2 was taken from 1/4 width of the full-length (1 / 4B) at the center of plate thickness of the flange shown in FIG. 3 (1/2 · t _2). It should be noted that the characteristics of these portions were obtained because the flange 1 / 4F portion shows almost the average mechanical properties of the H-section steel, and the flange 1 / 2F portion has the lowest characteristics. This is because it was determined that the mechanical test characteristics of the section steel could be represented.

[0029] The toughness of the weld was determined by using a groove (Fig. 2) and K
Multi-layer latent welding was performed using a mold groove (FIG. 3) and evaluated by a 2 mm V notch Charpy test. Welding current is 700
A, voltage 32V, welding speed 30cm / min, heat input 45kJ
/ Cm one-electrode arc welding. Tables 2 and 3 (continued in Table 2) show the chemical composition of the prototype steel, and Tables 4 and 5 (continued in Table 4)
Indicates rolling conditions and mechanical test characteristics. The rolling heating temperature was set to 1280 ° C. The reason is that it is generally known that mechanical properties are improved by lowering the heating temperature, and it is estimated that high-temperature heating conditions show the lowest value of mechanical properties, and this value lowers the properties at lower heating temperatures. This is because it was judged that he could represent.

As shown in Tables 4 and 5, the steels 1 to 8 according to the present invention have room temperature strength of the target base material mechanical properties with respect to changes in the rolling finish temperature and flange plate thickness (cooling rate).
High temperature strength at 00 ° C and Charpy value of 3.5kg at 0 ° C
satisfies fm and above. Furthermore, welded joints and H
The Charpy value of the AZ portion at 0 ° C. sufficiently satisfies 3.5 kgf-m or more. On the other hand, the steels 9 to 12 of the comparative steels satisfy the normal temperature strength and the high temperature strength, but none of the toughness of the flange thickness 1/2 part and the width 1/2 part satisfy the target values. The cause is that the oxygen concentration of molten steel before the addition of Ti deviates from the lower limit of the limit range for steels 9, 11, and 12 by Al deoxidation treatment,
Although the oxygen concentration of the steel 10 is within the limited range due to the preliminary deoxidation of the Ca—Si alloy, like the steels 9, 11, and 12, T
Since the mold addition treatment of i-alloy is not added, IGF
This is because the number of fine oxides + MnS + TiN acting as nucleation sites was insufficient, IGF was not generated, and toughness could not be improved by grain refinement.

That is, when all the requirements of the production method of the present invention are satisfied, as shown in Tables 4 and 5,
It has sufficient strength and toughness even in the flange plate thickness 1/2 and width 1/2 parts where the mechanical test characteristics of rolled section steel are the least guaranteed, and it is sufficient in the flange plate thickness 1/2 and width 1/4 part. It is possible to produce a rolled section steel having excellent room temperature, high temperature strength and weld HAZ toughness, and excellent in fire resistance and toughness. Note that the rolled section steels to which the present invention is applied are not limited to the H-section steels of the above-described embodiment, but may also be applied to section steels having flanges such as I-section steel, angle steel, channel steel, and unequal-thickness angle steel. Of course, it can be applied.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

[Table 5]

[0037]

According to the present invention, the rolled section steel has sufficient strength and toughness even at a flange plate thickness of 1/2 and a width of 1/2 part where mechanical test characteristics are hardly guaranteed, and high-temperature characteristics and weldability are obtained. It is possible to manufacture rolled section steel with excellent fire resistance and toughness that can achieve the purpose of fire resistance with the coating thickness of the refractory material being 20 to 50% of the conventional one as it is, reducing the construction cost and shortening the construction period. The cost can be significantly reduced, and the industrial effects such as the improvement of the reliability of large buildings, the securing of safety, and the 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 and a sampling position of a mechanical test piece.

FIG. 2 is a schematic view of a groove shape and a welding shape of a weld joint part.

FIG. 3 is a schematic view of a K-shaped groove shape and a welded shape of a weld joint.

[Explanation of symbols]

 1 H-section steel 2 Flange 3 Web

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication location C22C 38/00 301 C22C 38/00 301A 38/14 38/14

Claims (2)

(57) [Claims] 1. Deoxidizing element A in addition to vacuum degassing of molten iron
Preliminary deoxidation treatment is performed by adding l, Si, Ca, Mg alone or in combination with their alloys, and the dissolved oxygen is reduced to 0.00% by weight.
After smelting to 3 to 0.015%, by alloy addition, C: 0.04 to 0.20% by weight, Si: 0.05 to 0.50% by weight.
%, Mn: 0.4 to 2.0%, Mo: 0.3 to 0.7
%, V: 0.05-0.20%, N: 0.006-0.
015%, Al ≦ 0.005%, the balance being adjusted to molten steel consisting of Fe and unavoidable impurities, and further adding Ti-Cu, Ti-Ni, Ti-F to the molten steel in a continuous casting mold.
e alloy is continuously added for final deoxidation, and the Ti content is -0.006 ≦ [T
i%]-2 [O%] ≦ 0.008% by weight
Slabs , and the slabs serve as intragranular ferrite nuclei.
Contains dispersed Ti-based oxide and Si-Mn-based oxide particles
After the slab is reheated to a temperature range of 1100 to 1300 ° C, hot rolling is performed and rolling is completed in a temperature range of 750 to 1050 ° C.
Complex precipitation of MnS, TiN and VN with oxide particles as nuclei
Of Intragranular Ferrite from Austenitic Grains by Alumina
Characterized in that the microstructure is refined by
A method for producing an intragranular ferritic section steel excellent in fire resistance and toughness as rolled . 2. Deoxidizing element A in addition to vacuum degassing of molten iron
Preliminary deoxidation treatment is performed by adding l, Si, Ca, Mg alone or in combination with their alloys, and the dissolved oxygen is reduced to 0.00% by weight.
After smelting to 3 to 0.015%, by alloy addition, C: 0.04 to 0.20% by weight, Si: 0.05 to 0.50% by weight.
%, Mn: 0.4 to 2.0%, Mo: 0.3 to 0.7
%, V: 0.05-0.20%, N: 0.006-0.
015%, Al ≤ 0.005%, plus Cr ≤
0.7%, Ni ≦ 1.0%, Nb ≦ 0.05%, Cu ≦
1.0% of one or more kinds, the balance being adjusted to molten steel consisting of Fe and unavoidable impurities, and further adding Ti-Cu, Ti-Ni, Ti-F to the molten steel in a continuous casting mold.
one of e alloy continuously added to final deoxidation, Ti content to the molten steel of dissolved oxygen [O%] -0.006 ≦ [T
i%]-2 [O%] ≦ 0.008% by weight
Slabs , and the slabs serve as intragranular ferrite nuclei.
Contains dispersed Ti-based oxide and Si-Mn-based oxide particles
After the slab is reheated to a temperature range of 1100 to 1300 ° C, hot rolling is performed and rolling is completed in a temperature range of 750 to 1050 ° C.
Complex precipitation of MnS, TiN and VN with oxide particles as nuclei
Of Intragranular Ferrite from Austenitic Grains by Alumina
Characterized in that the microstructure is refined by
A method for producing an intragranular ferritic section steel excellent in fire resistance and toughness as rolled .