JPH07188835A - Fireproof cast steel - Google Patents

Abstract

PURPOSE:To produce a fireproof cast steel excellent in fire resisting characteristic and weldability by preparing the fireproof cast steel having a specific composition, in which respective contents of Cr, Mo, V, and Ni are specified, and also having specific carbon equivalent. CONSTITUTION:A fireproof cast steel, which has a composition containing, as essential components, 0.6-4.1%, by weight, C, 0.2-0.8% Si, 0.6-1.4% Mn, 0.2-0.3% Cr, >0.3-0.7% Mo, 0.01-0.1% V, and 0.6-1.3% Ni and having the balance Fe with inevitable impurities and in which CE (carbon equivalent) is regulated to <=0.45%, is prepared. This cast steel is subjected to diffusion annealing at 900-1000 deg.C, air-cooled, and tempered at 600-700 deg.C. By this method, the fireproof cast steel, extremely excellent in characteristics at ordinary temp. as well as in characteristics at high temp., can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a joining part or a member including a joining part used for joining refractory steel materials in various steel structures in the fields of construction, civil engineering and marine structures (hereinafter referred to as joining parts / members). The present invention relates to a refractory cast steel having excellent fire resistance and weldability.

[0002]

2. Description of the Related Art In a structure, particularly a building structure, when a steel frame is exposed to a high temperature during a fire, its strength decreases and the yield strength of the building structure decreases. Was obliged. However, in the "new fireproof design method" that was added in 1989, the use of steel materials with excellent high temperature resistance (hereinafter referred to as fireproof steel materials) reduces the amount of fireproof coating materials, or depending on the application and structure The structure without fireproof coating is now accepted. According to the new standard, the refractory steel is required to retain the yield strength of the conventional steel at 350 ° C even at 600 ° C. For example, a refractory steel material is required to have a yield strength at 600 ° C. that is ⅔ or more of a standard lower limit value of yield strength at room temperature.

Therefore, a new refractory steel material has been developed (for example, Japanese Patent Application Laid-Open No. 2-170943), and further, a fire resistant bolt for use in such a structure (for example, Japanese Patent Application Laid-Open No. 2-247355).
Issue) has also been developed. However, in addition to the above refractory steel materials and refractory bolts, the above-mentioned steel structures are made of cast steel from the viewpoint of structural strength and aesthetics at the joints of joints between columns or columns, or the nodes of truss members, etc. Parts / members such as a diaphragm or a metal fitting for a space truss are required. These joint parts / members are usually joined to the refractory steel by welding.

The above refractory steel materials are supplied as rolled steel materials such as steel plates and H-shaped steels, and steel types having these component compositions cannot be diverted to the cast steel for the above-mentioned joint parts / members which is the object of the present invention. This is because the strength of the latter is generally inferior to that of the former when the rolled steel and cast steel have the same composition. As the refractory steel material, the strength at room temperature is JIS G 3
Corresponds to SM400 and SM490 specified in 106, and 6
Steels having a yield strength at 00 ° C of 2/3 or more of the lower limit of the standard of yield strength at room temperature are used relatively often.

[0005]

Conventionally, a joined part / member, for example, a diaphragm (casting diaphragm) is a cast steel product because it has a complicated shape as shown in FIG.
The material is specified in JIS G 5101 (cast carbon steel products).
SC 450 or SCW 480 specified in JIS G 5102 (cast steel products for welded structures) is used.

The yield strength of these cast steel products at 600 ° C. is about 1/3 of the yield point or yield strength at room temperature, and they cannot be used as refractory cast steel. On the other hand, there is chromium-molybdenum cast steel (JIS G 5151) as a cast steel excellent in high temperature proof stress. This cast steel product is 60
Although the proof stress at 0 ℃ is about 2/3 of the proof stress at room temperature (for example, 190 N / mm ² or more in the annealed material of SCPH 21), the weld crack susceptibility is high, so the weld crack resistance is poor and the preheating or There is a problem in welding work such as post-heating. Therefore, in order to reduce or omit the fireproof coating construction of the structure, in addition to having high temperature proof stress, it has excellent weldability and base material properties, and steel structures having various shapes that can be applied to any design Fireproof cast steel for joint parts / members is required.

Therefore, in the present invention, as a cast product, the strength equivalent to that of the above steel material, that is, the strength at room temperature is JIS
It is intended to provide a refractory cast steel for casting, which is equivalent to SM490 specified in G 3106 and has a yield strength at 600 ° C of 2/3 or more of a yield strength or a lower limit of the yield strength at room temperature.

Further, since it is intended to be a refractory cast steel for large-scale cast steel joint parts / members, it has a mechanical property of SM490-equivalent refractory cast steel and has a yield strength or yield strength of about 315 at room temperature.
In N / mm ² or more, proof stress of 600 ° C. is an object of the invention refractory cast steel having about 217 N / mm ² or more. In addition, in order to aim at fire-resistant cast steel for large cast steel joint parts and members,
An object of the heat treatment method is to secure the above mechanical performance by air cooling treatment.

[0009]

In view of the above problems, the present inventors have conducted diligent research, and as a result, the chemical component is Mn-C.
It is possible to significantly improve the high-temperature yield strength without impairing the weldability even if the r-Mo system is used as a basic component to further enhance the hot strength by V and Ni is added to omit the quenching treatment. It is an invention that was completed by obtaining the knowledge.

(1) The invention of claim 1 is a refractory cast steel having the following characteristics (component composition is wt%). (A) As a main component, C: 0.09 to 0.15%, Si: 0.2 to 0.8
%, Mn: 0.6 to 1.4%, Cr: 0.2 to 03% Mo: 0.3 to 0.7%, V: 0.01 to 0.1% Ni: 0.6 to 1.3%, with the balance being Fe and inevitable impurities. (B) CE (carbon equivalent) is 0.45% or less.

(2) According to the second aspect of the present invention, the cast steel having the above-described composition is diffusion-annealed at 900 to 1000 ° C., air-cooled, and then 600.
The refractory cast steel according to claim 1, which is tempered at about 700 ° C.

(3) The invention of claim 3 is a joint part / member for a steel structure manufactured from the refractory cast steel according to claim 2.

[0013]

The reason for limiting the chemical components in the present invention will be described below. C is an element that contributes to the strength increase, but if it is less than 0.09%, it is difficult to secure the strength,
Also, if added in excess of 0.15%, the weldability and toughness deteriorate. Therefore, its content is 0.09 ~
The range is 0.15%.

Si is an essential element for ensuring the deoxidizing effect and castability. If it is less than 0.2%, the deoxidizing effect is small, and it is desirable that Si is large, but 0.8 is preferable.
If it is added in excess of%, the weldability is deteriorated. Therefore, the amount added is in the range of 0.2 to 0.8%.

Mn is an element necessary for securing its deoxidizing effect, castability, strength and toughness, but if it is less than 0.6%, such an effect is small, and 1.
If added in excess of 4%, it deteriorates weldability and toughness. Therefore, it is set to 0.6% to 1.4%.

Cr is an element effective in improving the high temperature strength, but if it is less than 0.2%, such an effect cannot be obtained, and if it exceeds 0.3%, the weldability is impaired. Therefore, the added amount is 0.2 to 0.3%.

Mo is an essential element for securing high temperature strength. In particular, Mo is a necessary element because its resistance to temper softening at 600 ° C. is remarkably higher than that of other elements. If the content is 0.3% or less, such an effect cannot be obtained, and if it is added over 0.7%, the weldability is impaired. Therefore, the amount of addition is 0.3 to 0.7%.

V is an element effective in increasing the strength by precipitation hardening, but if it is less than 0.01%, such hardening can hardly be expected, and if it is added in excess of 0.1%, the weldability deteriorates. To do. Therefore, the amount added is 0.01-0.1
The range is%.

Ni generally improves the hardenability as the content increases, but as a result of research, the following points were found. Figure 1 shows the results of an investigation of the yield strength or yield strength at room temperature and the yield strength at 600 ° C, with the other components being fixed and the Ni content being varied from 0 to 1.5%. From this figure, Ni is 0.6-1.
The yield strength at 600 ° C within the range of 3% was the subject of the present invention.
217 N / mm ² or more. The proof stress at room temperature is Ni
315 N / mm which is the subject of the present invention in the range of 0.1 to 1.5%
More than ² was obtained. Therefore, the Ni content is 0.6-1.3%
And In the above range, the strength and toughness of the base material can be improved without adversely affecting the weldability and toughness of the heat-affected zone described later. Further, as long as the properties of the cast steel according to the present invention are not changed, a trace amount of impurity elements and other elements may be contained.

Also, in order to prevent welding defects such as cold cracking during welding, it is necessary to suppress the maximum hardness Hv of the heat affected zone to 350 or less. This maximum hardness can be achieved mainly by keeping the carbon equivalent (CE) below a certain limit. Here, the carbon equivalent (CE) is calculated by the following formula. CE (wt%) = C + Mn / 6 + Si / 24 + Ni / 4
0 + Cr / 5 + Mo / 4 + V / 14 Since welding defects such as cold cracking during welding depend on various welding conditions, the maximum hardness test of the weld heat affected zone under the conditions for welding the joint parts / members which is the object of the invention. I went.

The welding conditions are as follows. Test method: JIS Z3101 Welding condition: Welding material; Shinko LB-490FR 4mmφ Preheating temperature; Room temperature (no preheating) Welding condition: 170A-25V-15cm / min. Welding heat input; 16.3 kJ / cm Material to be welded: Cast steel of the present invention (A to E in Table 1), 120 x 200 x
A 35 mm sample material was heated to 950 ° C, air-cooled, tempered at 600 ° C, and mechanically cut to prepare a test material.

Test results: (see Table 3 below) A (CE; 0.44%); Hv304 B (CE; 0.45%); Hv298 C (CE; 0.44%); Hv286 D (CE; Hv295 E (CE; 0.43%); Hv298 The maximum hardness of the heat-affected zone is Hv3.
It was 50 or less, and the weldability was good. So CE
Was 0.45%.

Next, the limitation of heat treatment conditions in the present invention will be described. In the present invention, considering that the structure before heat treatment is a cast structure, diffusion annealing is performed at 900 to 1000 ° C. and air cooling is performed. In this case, ferrite-
A bainite structure is obtained. The reason why the heat treatment is air cooling is that
This is because the cast steel of the present invention is intended for jointed parts / members which are relatively large cast steel products, and is air-cooled in a simpler process than oil quenching or the like. It should be noted that although oil cooling instead of air cooling complicates the manufacturing process slightly as described above, there is no problem from the viewpoint of obtaining the strength desired by the present invention.
As the metal structure in this case, a ferrite-bainite structure is obtained as in the case of air cooling.

The reason why the tempering temperature is limited to 600 to 700 ° C. is that the effect of temper hardening by Mo, Ni, Cr, Si, etc. is maximized. Therefore, at temperatures below 600 ° C, products with poor toughness are likely to be produced.
This is because if the temperature exceeds 0 ° C, the strength becomes too low.

[0025]

EXAMPLES The present invention will be described below with reference to examples. Using a high-frequency electric furnace, the test materials with the chemical composition shown in Table 1 were melted, and 120 × 200 × 35 using a carbon dioxide template.
mm and 120 × 200 × 60 mm test materials were cast, and after air cooling
A tempering treatment was performed at 600 to 700 ° C. Test pieces were taken from these test materials and subjected to a room temperature tensile test and 500 ℃, 60
A high temperature tensile test was performed at 0 ° C and 700 ° C.

Table 1 shows the chemical components of the inventive materials A to E and the comparative materials FL. Table 2 shows the strength of the comparative material and the fire-resistant cast steel according to the present invention. A yield strength of 315 N / mm ² or more is obtained at room temperature, and a target proof stress of 217 N / mm ² or more is obtained at 600 ° C. Among the comparative materials, F, G and H are Ni
The yield strength at 600 ° C does not reach the target of 217 N / mm ² because the content is too low or too high. I, J, K,
L does not achieve the target because any one or more of Ni, Cr, Mo and V is outside the scope of the invention. In addition, Table 3 shows the results of the maximum hardness test of the weld heat affected zone of the refractory cast steel according to the present invention.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

EFFECTS OF THE INVENTION The refractory cast steel of the present invention is excellent in high temperature characteristics as described above and also extremely good in room temperature characteristics, and therefore is used for various structures in the fields of construction, civil engineering and marine structures. It can be applied to joint parts and members used for joining existing refractory steel materials. Therefore, even if it is a steel structure that uses steel plates or H-shaped designs,
Can be manufactured.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Ni content and a room temperature strength or a proof stress at 600 ° C. in a refractory cast steel according to the present invention.

FIG. 2 is a joint part to which the refractory cast steel according to the present invention is applied.
It is a figure which shows the shape of a diaphragm as an example of a member.

Claims (3)

[Claims] 1. The following characteristics (component composition is wt%):
Fire-resistant cast steel with. (A) As a main component, C: 0.09 to 0.15%, Si: 0.2 to 0.8
%, Mn: 0.6 to 1.4%, Cr: 0.2 to 0.3% Mo: 0.3 to 0.7% or less, V: 0.01 to 0.1% Ni: 0.6 to 1.3%, with the balance being Fe and inevitable impurities.
(B) CE (carbon equivalent) is 0.45% or less. 2. The refractory cast steel according to claim 1, wherein the cast steel having the composition of components is diffusion annealed at 900 to 1000 ° C., air-cooled, and then tempered at 600 to 700 ° C. 3. A joint part / member for a steel structure manufactured from the refractory cast steel according to claim 2.