JPH1068018A - Production of fire resistant steel sheet for structural purpose excellent in welding heat affected zone toughness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fire resistant steel sheet for structural purposes excellent in a welding heat affected zone toughness and advantageously usable as structural members for land structures in civil engineering and construction or the like and marine structures or the like. SOLUTION: A slab having a chemical compsn. contg., by weight, 0.03 to 0.2% C, <=0.6% Si, 0.6 to 2.0% Mn, <=0.030% S, 0.2 to 0.7% Mo, <=0.02% Al, <=0.02% Ti, 0.001 to 0.01% O, <=0.01% N, and the balance Fe with inevitable impurities, also composed of oxides, in which dispersed particles having 1 to 10μm dimension are dispersed into a steel matrix by the average density of >=4 pieces per mm<2> , as for the oxides, the oxide composing elements having a chemical compsn. contg., by atom, 5 to 50% Mn and 50 to 95% to regulate the total to 100% are contained is heated at 1000 to 1280 deg.C, after that, rolling is started and is finished at the Ar3 point or above, thereafter, the obtd. steel sheet is subjected to air cooling to a room temp. or is subjected to accelerated cooling to <=700 deg.C so as to regulate the cooling rate in the surface of the steel to >=3 deg.C/sec and is subsequently subjected to air cooling to form into a steel sheet.

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 structural refractory steel sheet having excellent toughness in a heat-affected zone of a weld. More specifically, the present invention relates to a method for producing a structural refractory steel sheet which has excellent toughness in a heat affected zone of a weld and can be advantageously used as a structural member of a land structure such as civil engineering or a building, or a marine structure.

[0002]

2. Description of the Related Art When a building structure is fired, the steel members constituting the structure are exposed to high temperatures, so that its proof strength is sometimes lower than the lower limit required for the building structure. For example, in the case of a Si—Mn steel sheet conventionally used as a structural member for a building, when exposed to a high temperature exceeding 350 ° C., the proof stress required as a structural member of a building structure (“long-term proof stress”) 217N which is about 2/3 of the normal temperature proof stress).
/ Mm ² . Therefore, in order to maintain the required strength of a steel structure as a building structure even in the event of a fire, it has been conventionally required to carry out fireproof construction on the steel frame by the Building Standards Law. Specifically, slag wool,
Spraying materials and felts made of glass wool asbestos or the like as a base material have been spread, and the surface of a steel frame has been covered with a fire mortar or the like. However, performing fireproof construction on steel frames inevitably results in an increase in construction costs and an extension of the construction period.

[0003] The "New Fireproof Design Law" recently implemented
According to this, the design standards are changed, and if fire-resistant steel material, which is a steel material with excellent fire resistance, that is, high temperature resistance, can be used as a steel frame, fire-resistant construction can be reduced compared to the conventional case, and in some cases, the implementation of fire-resistant construction will be exempted It became. At present, there is a demand for a reduction in construction costs and a shortened construction period, and there is a movement to use refractory steel materials that satisfy the above design criteria.

[0004] As a steel plate for refractory steel excellent in high-temperature proof strength, C-plate widely used for boilers and pressure vessels in recent years has been used.
An r-Mo steel sheet is known (see JIS G4109). This Cr-Mo steel sheet has the following chemical composition by weight%: Si: 0.50% or less Mn: 0.30 to 0.60% P: 0.030% or less S: 0.030% or less Cr: 0.50 to 6.00% Mo: 0.40 to 1.15% Remainder: Fe and unavoidable impurities The proof stress at 600 ° C. is 217 N / mm ² or more. However, on the other hand, the steel plate has a weld cracking susceptibility index Pc (a low-temperature welding cracking susceptibility in which the coefficient of the hydrogen content of the weld metal and the thickness of the test material are included in addition to the chemical composition) as defined by the following equation. Pc = C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 2
0 + Mo / 15 + V / 10 + 5B + H / 60 + t / 600 (where each component is weight%, H is the amount of diffusible hydrogen of the deposited metal (cc / g), and t is the plate thickness (mm)), so that welding is performed. Low heat affected zone toughness. Therefore, it is difficult to perform welding with a large heat input without causing welding cracks.

[0005] Steel materials for building structures are required to reduce the yield ratio (the ratio of the yield strength to the tensile strength) to 80% or less from the viewpoint of preventing the destruction of the building during an earthquake. .

[0006] Steel materials (steel plates) for architectural structures have been proposed to meet the various requirements described above. One of them is a steel plate for architectural materials which we previously proposed, which has excellent weldability and fire resistance. (JP-A-6-192730). This steel sheet is
And C: 0.05 to 0.10% Si: 0.05 to 0.35% Mn: 0.60 to 1.50% Nb: 0.010% or less V: 0.020 to 0.070% Mo: 0.10 to 0.40%, with the balance being a chemical composition of Fe and unavoidable impurities;
q ^(W) : 0.30 to 0.40%, Mo * defined by the following formula: 0.52% or more Ceq ^(W) (%) = C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + V / 14 + Cr ‥‥‥ Mo * = 9 / 17C + 4 / 17Mn + Mo + 24 / 17V
+ 92 / 17Nb; After heating the slab to a temperature range of 1000 to 1300 ° C. and performing hot rolling, accelerated cooling is performed from a temperature range of Ar ₃ points or more, and the accelerated cooling is performed at a temperature range of 400 ° C. or less. Is stopped, and tempering is performed in a temperature range of (Ac ₁ + 10 ° C.) or more and (Ar ₃ −20 ° C.) or less.
This steel sheet is intended to lower the Ceq ^(W) and to specify Mo *, thereby improving the toughness of the weld heat affected zone of the welded joint.

However, in recent years, from the viewpoint of improving construction efficiency, large heat input welding such as submerged arc welding (SAW) having a heat input of 30 KJ / mm or more is used for welding structural members such as box columns. It is considered to be applied, and the development of a structural steel sheet having more excellent toughness in the heat-affected zone of the weld is required.

[0008] These are advantageous properties for structural members of land structures including buildings, and offshore structures.

[0009]

SUMMARY OF THE INVENTION Therefore, the present invention is a structural steel sheet which is excellent in fire resistance and further excellent in the toughness of the weld heat affected zone, and is designed and constructed in the same manner as in the prior art. It is an object of the present invention to provide a structural steel sheet that can be used.

Another object of the present invention is to provide a simple method for producing the above structural steel sheet.

[0011]

Means for Solving the Problems The inventor of the present invention has proposed that the steel plate proposed in JP-A-6-192730, which only reduces Ceq ^(W) and specifies Mo *, When large heat input welding exceeding 30 KJ / mm is performed, the toughness of the heat affected zone decreases, and further, the notch of the heat affected zone is evaluated by a Charpy impact test after the large heat input welding. Absorbed energy of position ( _V E _O ) ≧ 47
Recognizing that it is necessary to impart to the steel sheet toughness that satisfies J, and tried to manufacture a steel sheet having such properties, it was mainly composed of a composite oxide consisting of Al, Mn, and Ti. When the particles are finely dispersed and precipitated in the steel matrix, the precipitation of intragranular ferrite is promoted during the cooling of the welding heat cycle, and as a result, the low-temperature toughness in the weld heat affected zone after application of large heat input welding is significant. As a result of more sufficient experiments, a new method for producing a structural refractory steel sheet having excellent toughness in the heat affected zone has been proposed.

That is, in the present invention, C: 0.03 to 0.2% Si: 0.6% or less Mn: 0.6 to 2.0% S: 0.030% or less Mo: 0% by weight%. 2 to 0.7% Al: 0.02% or less Ti: 0.02% or less O: 0.001 to 0.01% N: 0.01% or less, with the balance being Fe and unavoidable impurities Having composition; and made of oxide, having a size of 1 to 10 μm
m are dispersed in the steel matrix at an average density of 4 or more per mm ² , and the oxide contains, in atomic%, Mn: 5 to 50% (Al + Ti): 50 to 95% After heating the steel slab to a temperature range of 1000 to 1280 ° C., starting rolling, and ending the rolling at _three or more Ar points,
A structure having excellent toughness in a weld heat affected zone, characterized in that the obtained steel sheet is allowed to cool to room temperature or accelerated cooling to a temperature of 700 ° C or less at a steel sheet surface cooling rate of 3 ° C / sec or more, and then cooled to room temperature. The present invention relates to a method for producing a refractory steel sheet for use.

According to another aspect of the present invention, Cu: 0.05 to 0.5% Ni: 0.05 to 0.5% Cr: 0.05 to 0.5% Nb: 0.005 to 0.5% by weight 0.03% V: 0.005 to 0.07% Ca: 0.0005 to 0.005% REM: 0.0001 to 0.005% Apply manufacturing method.

In the following description, "%" means "% by weight" unless otherwise specified.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Chemical composition and structure (basic component) of steel slab C: 0.03 to 0.2% C is an element capable of increasing the strength, but if it is less than 0.03%, the effect is sufficient. On the other hand, if it exceeds 0.2%, the weldability and toughness of the steel sheet as a product deteriorate. Therefore, it is limited to 0.03 to 0.2%. Desirably 0.0
6 to 0.12%.

Si: 0.6% or less Si is an essential element for deoxidizing molten steel.
%, The weldability of the steel sheet deteriorates. Therefore, 0.6
% Or less.

Mn: 0.6 to 2.0% Mn is an essential element in the present invention as a constituent element of a composite oxide that becomes a ferrite precipitation nucleus, and also deoxidizes molten steel and the strength and toughness of a steel sheet. However, if it is less than 0.6%, the effect is not sufficiently exerted. On the other hand, if it exceeds 2.0%, the hardenability is increased, and as a result, the weldability and weldability of the steel sheet are increased. The heat-affected zone toughness deteriorates. Therefore, it is limited to 0.6 to 2.0%.

S: 0.030% or less S, like P, is an impurity element inevitably contained in steel. When a large amount of S is present, precipitates, such as MnS, serving as weld crack initiation points are formed. For this reason, it is desirable that the content of S is low.
030% was set as an allowable upper limit. In order to further improve the base metal toughness and the toughness of the weld heat affected zone, and to reduce the segregation at the center of the slab, the content is preferably 0.01% or less.

Mo: 0.2-0.7% Mo is an element essential for ensuring high-temperature strength.
Significant increase in proof stress at 600 ° C, but 0.2%
If it is less than 0.70%, the effect will not be sufficiently exhibited. On the other hand, if it exceeds 0.70%, the weldability will be deteriorated, and the toughness will be deteriorated during large heat input welding. Therefore, it is limited to 0.2 to 0.7%. 0.4
~ 0.7% is desirable.

Al: 0.02% or less Al is an essential element in the present invention as a constituent element of the composite oxide serving as a ferrite precipitation nucleus.
%, An increase in island martensite in the heat affected zone is caused. Therefore, it is limited to 0.02% or less. For the formation of the composite oxide, it is desirable to add at least about 0.0001% of Al by calculation.

Ti: 0.02% or less Ti, like Al, is an essential element in the present invention as a constituent element of a composite oxide that becomes a ferrite precipitation nucleus. If it exceeds 0.02%, coarse TiC Alone precipitates, and the toughness of the base metal and the heat affected zone of the weld deteriorates. Therefore,
It is limited to 0.02% or less. Note that it is desirable to add at least about 0.0001% of Ti in the calculation for forming the composite oxide.

O: 0.001 to 0.01% O is required to be at least 0.001% in order to form a composite oxide which becomes a ferrite precipitation nucleus. However, the presence of excessive O in the steel adversely affects the toughness of the base metal. Therefore, it is limited to 0.001 to 0.01%.

N: 0.01% or less N is an impurity element inevitably contained in steel, and if present excessively, adversely affects the toughness of the base material. In the present invention, N is 0.01% or less. If the influence is small. Therefore, it is limited to 0.01% or less.

The balance consists of Fe and inevitable impurities.

(Additional Components) The steel slab used in the present invention can further improve strength and toughness by adding the following elements as a group of strength improving elements in addition to the above basic components.

Ni: 0.05 to 0.5% Ni improves the strength and toughness of the base material without adversely affecting the weldability and the toughness of the heat affected zone.
When the amount is less than 0.5%, the effect is not sufficiently exhibited. On the other hand, when the amount exceeds 0.5%, the price is increased to such an extent that it is unacceptable especially as a structural member for a building structure. Therefore, it is limited to 0.05 to 0.5%.

Cu: 0.05 to 0.5% Cu exerts substantially the same effect as Ni, and also exhibits an effect on an increase in high-temperature strength and an improvement in corrosion resistance and weather resistance due to Cu precipitates. If it is less than 0.05%, the effect is not sufficiently exhibited, while if it exceeds 0.5%, Cu cracks occur during hot rolling, making molding difficult. Therefore, 0.05
Limited to ~ 0.5%.

Cr: 0.05-0.5% Cr is an element that can increase the strength of the base material and the welded portion and further improve the weather resistance. On the other hand, if it exceeds 0.5%, the weldability and the toughness of the heat affected zone are poor. Therefore, 0.05-
Limited to 0.5%.

Nb: 0.005 to 0.03% Nb is an element capable of improving high-temperature strength (fire resistance) by precipitation hardening and transformation strengthening and improving toughness by grain refinement. %, The effect is not sufficiently exhibited. On the other hand, when it exceeds 0.03%, the toughness of the welded heat-affected zone is deteriorated during large heat input welding. Therefore, 0.005
Limited to 0.03%.

V: 0.005 to 0.07% V is an element exhibiting substantially the same effect as Nb, although the effect on the high-temperature proof stress is inferior. However, 0.005
%, The effect is not sufficiently exhibited.
If it exceeds 7%, the toughness of the heat affected zone is adversely affected. Therefore, it is limited to 0.005 to 0.07%.

Ca: 0.0005% to 0.005% Ca controls the form of sulfide (MnS), increases (Charpy) absorbed energy to improve low-temperature toughness, and improves hydrogen-induced cracking resistance. Is also effective. However, if the content is less than 0.0005%, the effect is not sufficiently exhibited. On the other hand, if the content is more than 0.005%, CaO and CaS are generated in large amounts to form large inclusions, which impair not only the toughness of the steel but also the cleanliness. In addition, the weldability also deteriorates. Therefore, 0.
0005 to 0.005%.

REM: 0.0001% to 0.005% REM (rare earth element) is an element that exerts substantially the same effect as Ca and also causes the same problem as Ca when the added amount increases. Excessive addition is not desirable from an economic viewpoint. Therefore, it is limited to 0.0001 to 0.005%.

[0033] In the structure steel matrix, particles composed of the composite oxide are dispersed. The particles are 1-10 μm in size and are dispersed in the steel matrix with an average density of 4 or more per mm ² . When the size is less than 1 μm, it is too small as a precipitation nucleus of intragranular ferrite, while when it exceeds 10 μm, it is too large, and none of them functions sufficiently as a ferrite precipitation nucleus. This result was found based on experimental results. Further, the dispersion density of the particles was limited to 4 or more per 1 mm ² . If it is less than this, the structure improving effect does not sufficiently appear and the toughness of the heat affected zone is not improved to a satisfactory degree.

The particles consist of a composite oxide, which is
Atomic%: Mn: 5 to 50% (Al + Ti): 50 to 95%, limited to those containing oxide constituent elements having a chemical composition of 100% in total. Therefore, the content of Mn is 50
Atomic%, the upper limit of the content of (Al + Ti) is 50
Atomic%. In some cases, the total content of Mn and (Al + Ti) does not reach 100 atomic%. Mn or (Al
When (+ Ti) is contained in the above range, it exerts its function effective as a precipitation nucleus of intragranular ferrite during cooling after welding. It is considered that the particles have high crystal matching with ferrite and thus easily function as precipitation nuclei of ferrite.

Manufacturing Method A steel slab satisfying the above conditions is heated to a temperature range of 1000 to 1280 ° C., and rolling is started. After rolling is completed at _three or more Ar points, the obtained steel sheet is air-cooled or cooled to room temperature. After cooling to a temperature of 700 ° C. or less at a steel sheet surface cooling rate of 3 ° C./sec or more, air cooling is performed to room temperature.

(Process) The slab is heated before rolling. Heat to 1000 ° C. or higher to dissolve the added element.
On the other hand, if the temperature exceeds 1280 ° C., the austenite grains become too coarse, and it is difficult to reduce the size by hot rolling. Therefore, the heating temperature range is limited to 1000 to 1280 ° C.

(Step) After the heating, hot rolling is performed, and the rolling end temperature is set to a high temperature of _three or more Ar points. This is because when rolling is completed at less than Ar ₃ , Mo precipitates and the precipitates become too large, and the high-temperature proof stress is significantly reduced.

(Step) After the rolling, the steel sheet is allowed to cool to room temperature (typically air cooling) or at a steel sheet surface cooling rate of 3 ° C./sec or more.
After cooling to a temperature of 00 ° C. or lower, it is allowed to cool to room temperature. When the plate thickness is large (40 mm or more), when accelerated cooling is performed, an appropriate amount of pro-eutectoid ferrite is generated, and a low yield ratio can be realized without deteriorating the strength. In the case of a thick wall (about 50 to 1000 mm), the steel sheet surface temperature is cooled from a temperature in the range of Ar ₃ −60 ° C. to Ar ₃ −120 ° C. to 400 ° C. or less at a steel sheet surface cooling rate of 3 ° C./sec or more Later, it is desirable to air-cool. In addition, medium meat (40
(About 50 mm), it is preferable to cool the steel sheet at a cooling rate of 3 ° C./sec or more to a temperature in the range of 500 ° C. to 700 ° C. after rolling, and then air-cool.

[0039]

EXAMPLES Preparation of Samples Various steel slabs were prepared by ingot casting after melting in a laboratory-scale vacuum refining furnace. At that time, the timing and order of addition of the deoxidizing element were variously changed in order to change the composition of the oxide constituting the dispersed particles, and the mold dimensions were variously changed in order to change the dispersion density. The chemical composition of the obtained steel slab and the oxide particles present in the steel are as shown in Table 1 for the examples of the present invention and Table 3 for the comparative examples. As a result of making the refining conditions the same, substantially the same composition of the dispersed particles was obtained. The dispersion density of the dispersed particles is 5
The surface of the micro sample was observed and measured with a 00 × optical microscope. The composition of the oxide constituting the dispersed particles is such that the particles
The analysis was performed using an M-EDX apparatus.

The steel slabs shown in Tables 1 and 3 were replaced by Tables 2 and 3, respectively.
Rolling was performed under the conditions shown in FIG. 4 to obtain steel sheets having the thickness shown in the right column.

The cut out each test piece from the test fabricated steel, strength at room temperature, i.e., tensile ^{strength: YS (N / mm 2)} , shear ^{strength: TS (N / mm 2)} , yield ratio: YR (%) and Charpy absorbed energy: _{V EO} (J), proof stress at 600 ° C. (stress at a point where residual strain becomes 0.2% when an offset, that is, when a load is removed) (N / mm ² ), and weldability are evaluated. For this purpose, the Charpy absorbed energy: _V E _O (J) of the heat-affected zone of the weld, especially at the notch position, after exposure to the reproducible welding heat cycle was measured.

As a reproduction welding heat cycle exposure treatment, the steel sheet was cut off from both sides by a thickness of about one quarter and removed, and then the width was 11 mm and the thickness was 11 m.
m, a test piece having a length of 60 mm was collected and heated at a maximum heating temperature of 1400 ° C., and then 800 to 500
Cool to 300C over 300 seconds. This is welding heat input: 3
It corresponds to 0 kJ / mm. And after the heat cycle exposure,
The test piece was processed into a JIS No. 4 test piece and subjected to a Charpy impact test. The measurement results are shown in Table 5 for the examples of the present invention.
Table 6 shows comparative examples.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

[0047]

[Table 5]

[0048]

[Table 6]

Evaluation When a steel slab that satisfies the chemical composition (whole steel slab, dispersed particles), size and dispersion density of the steel stipulated by the present invention is rolled according to the process of the present invention, a steel sheet is produced. Shows excellent base metal strength at normal temperature and high temperature, and 30KJ / m
The heat affected zone showed excellent toughness even after exposure to a reproducible welding heat cycle simulating a heat input of m. On the other hand, when a steel sheet was manufactured from a slab having a small number of dispersed particles, the steel sheet exhibited the same performance as that of the inventive example in terms of base metal strength, but the toughness of the heat-affected zone was significantly deteriorated. Example of the present invention in which the effect of MnS was studied by reducing the amount of S in steel (No.
In 8), precipitation of MnS was hardly observed, and excellent heat-affected zone toughness was exhibited.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masahiko Hamada 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Inside Sumitomo Metal Industries, Ltd. (72) Inventor Takeshi Ichinose 4-5-Kitahama, Chuo-ku, Osaka, Osaka No. 33 Inside Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] C: 0.03 to 0.2% Si: 0.6% or less Mn: 0.6 to 2.0% S: 0.030% or less Mo: 0.2 to 0% by weight% 7% Al: 0.02% or less Ti: 0.02% or less O: 0.001 to 0.01% N: 0.01% or less, and the balance has a chemical composition composed of Fe and unavoidable impurities. And particles composed of an oxide and having a size of 1 to 10 μm are dispersed in a steel matrix at an average density of 4 or more per 1 mm ² , and the oxide is represented by atomic% and Mn: 5 to 50 % (Al + Ti): contains 50 to 95%, and contains oxide constituent elements having a chemical composition of 100% in total; rolling the steel slab after heating to a temperature range of 1000 to 1280 ° C.,
After finishing rolling at Ar ₃ or more, the obtained steel sheet is allowed to cool to room temperature or 700 ° C. at a steel sheet surface cooling rate of 3 ° C./sec or more.
A method for producing a refractory steel sheet for a structure having excellent toughness in a heat affected zone of a weld, characterized in that the steel sheet is cooled to a temperature of not more than ° C and then cooled to a room temperature. 2. Ni: 0.05-0.5% Cu: 0.05-0.5% Cr: 0.05-0.5% Nb: 0.005-0.03% by weight% V: The structural refractory steel sheet according to claim 1, wherein a steel slab further containing one or more of 0.005 to 0.007% Ca: 0.0005 to 0.005% REM: 0.0001 to 0.005% is used. Manufacturing method.