JPH03173715A - Production of 50kgf/mm2 class refractory steel for construction use excellent in weldability - Google Patents

Abstract

PURPOSE:To provide high yield strength at high temp. and superior weldability by subjecting a slab of a refractory steel in which Pcm value is specified to rolling at respectively specified rolling reduction and temp. CONSTITUTION:A steel having a composition which consists of 0.07-0.15% C, 0.05-0.60% Si, 0.50-1.50% Mn, 0.005-0.060% Nb, and the balance Fe with inevitable impurities and in which the value of Pcm specified by an equation is regulated to <=0.24% is cast. The resulting cast slab is heated up to >=1050 deg.C, and rolling reduction at <=900 deg.C is regulated to >=50%, and further, rolling is finished at 850-780 deg.C. By the above procedure, a 50kgf/mm<2> class refractory steel for construction use having >=22kgf/mm<2> yield strength at 600 deg.C and excellent in weldability can be obtained. If necessary, one or more kinds among 0.005-0.060% V, 0.05-0.50% Cu, 0.05-0.50% Ni, 0.005-0.030% Ti, and 0.0005-0.0050% Ca are incorporated to the above steel composition. By this method, the necessity of fireproofing coating can be reduced or obviated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing fire-resistant steel, and more specifically, to a method for producing fire-resistant steel.
The present invention relates to a method for manufacturing 50 kgf/+m'' class fire-resistant steel for architectural use, which has high yield strength even at high temperatures and has excellent weldability.

(Prior Art) In building structures, when steel frames are exposed to high temperatures in the event of a fire, their strength decreases and the strength of the building decreases, so the Building Standards Act requires the construction of fireproof coatings on steel frames.

Conventional 51-Mn-based architectural steel loses its long-term yield strength (the strength required for structural members in the event of a fire) when the temperature exceeds 350°C.
Since the strength is less than 22 kgf/mm2 (2/3) of the room temperature proof strength, a fireproof covering was applied to prevent the temperature of the steel frame from exceeding 350°C, which would be a barrier in terms of construction cost and construction period.

However, in the recently added New Fireproof Design Method 1, the amount of fireproof coating can be reduced by using steel materials (fireproof steel materials) with excellent high-temperature resistance.

(Problem to be solved by the invention) Currently, as a steel with excellent high-temperature yield strength, there is a Cr-Mo steel sheet that is widely used for boilers and pressure vessels.
This steel plate has a yield strength of 22 kgf/m+e" or more at 600°C, but due to its high weld crack susceptibility composition (PCM), it has poor weld crack resistance and has difficulties in welding work such as requiring preheating and postheating. be.

Therefore, in order to reduce or eliminate the need for fire-resistant coating on architectural steel, we need a steel that has high high-temperature yield strength, excellent weldability and base material properties, and can be designed and constructed in the same way as conventional steel. has been done.

Furthermore, from the viewpoint of the deformability of buildings during earthquakes, there is an increasing requirement for architectural steel to have a yield ratio of 80% or less.

(Means for Solving the Problems) In view of the above-mentioned problems with conventional architectural steel, the present inventors have conducted extensive research, and as a result, the present invention has been developed using a chemical composition, particularly a Cr-Mo system, as a basic component. This was completed based on the knowledge that Nb precipitation strengthening can significantly improve high-temperature yield strength without impairing weldability.
The first invention has C: 0°05 to 0.15%, Si:
0.05-0.60%, Mn: 0.50-1.50%,
Cr: 0.10-0.60%, Mo: 0.10-0.6
0%, Nb: 0.005 to 0.060%, the balance consists of Fe and unavoidable impurities, and the PCH value specified by the following formula 0 is 0.24% or less.
After heating to a temperature of 50°C or higher, the rolling reduction at 900°C or lower is set to 50% or higher, and the rolling is finished in a temperature range of 850 to 780°C, resulting in a yield strength of 22 kg at 600"C.
50kg for architectural use with excellent weldability of f/+am” or more
This is a method for manufacturing f/m+a class 2 fireproof steel.

The second invention is V: 0.005 to 0.060%, Cu: 0
．． 05-0.50%, Ni: 0.05-0.50%,
Ti: 0.005-0.030%, Ca: 0.000
The manufacturing method according to claim (1), which contains one or more selected from 5 to 0.0050% (Function) Below, the reasons for limiting the chemical components in the present invention will be explained.

C is an element that contributes to increasing strength, but it is difficult to ensure strength at less than 0.05%, and C is an element that contributes to increasing strength.
When added in a large amount exceeding 5%, weldability and toughness deteriorate. Therefore, the amount added is 0.05 to 0
．． The range is 15%.

Si is an essential element for deoxidation, but at 0.05%
If it is less than 0.60%, the deoxidizing effect will be small (and if it is added in excess of 0.60%, weldability will deteriorate. Therefore, the amount added is in the range of 0.05 to 0.60%.

Mn is an element necessary to ensure the strength and toughness of steel, but if it is less than 0.50%, this effect will be small, and if it is added in large amounts exceeding 1.50%, weldability will deteriorate. Therefore, the amount added should be in the range of 0.50 to 1.50%.

Cr is an element effective in improving high-temperature strength, but 0.1
It is difficult to expect such an effect below θ%, and 0.60
If added in a large amount exceeding %, weldability deteriorates. Therefore, the amount added is in the range of 0.10 to 0.60%.

Mo is an essential element for ensuring high-temperature strength and significantly increases yield strength at 600°C.However, if it is less than 0.1θ%, such an effect cannot be obtained.
Also, if added in excess of 0.60%, weldability will be impaired.
Therefore, the amount added is in the range of 0.10 to 0.60%.

Nb is an element that increases high-temperature strength through precipitation hardening and transformation strengthening, and improves toughness through grain refinement. However, if it is less than 0.005%, such an effect cannot be obtained, and if it is added in excess of 0.060%, the toughness of the welded joint deteriorates. Therefore, the amount added is o,
The range is from oos to 0.060%.

In addition, in the second invention of the present invention, in addition to the above-mentioned elements, V 5Cus Nts Ti and Ca
One or more of the following can be added:
It is an effective element for increasing strength through precipitation hardening, but o, o
If the content is less than os%, hardly any such effect can be expected, and if it is added in excess of 0.060%, weldability deteriorates. Therefore, the amount added is 0.005 to 0.
The range is 0.060%.

Cu is an element that is effective in increasing strength through solid solution strengthening, but if it is less than 0.05%, this effect is small (and
Adding more than 0.50% will impair hot workability and weldability, so the amount added should be between 0.05 and 0.50%.
% range.

Ni is an element effective in improving toughness, but at 0.05%
If the amount is less than that, such an effect cannot be obtained. Also, 0.50
Even if it is added in an amount exceeding 20%, such an effect will be saturated and it will be economically wasteful. Therefore, the amount added is 0.05~
The range is 0.50%.

Ti suppresses the coarsening of austenite grains and generates fine ferrite, so it is an effective element for reducing embrittlement of the toughness of the welded joint.

However, if less than 0.005%, such effects cannot be exhibited, and if added in excess of 0.030%, the toughness of the welded joint will deteriorate. Therefore, the amount added is in the range of 0.005 to 0.030%.

Ca is an element that improves properties in the thickness direction in trace amounts, but if it is less than o, oos%, there is no such effect;
When added in excess of o, ooso%, such effects are saturated and large inclusions are generated, making it easy to cause ultrasonic defects. Therefore, the amount added is 0.000
The range is 5 to o, ooso%.

In addition, in both the first invention and the second invention, in order to prevent cold cracking during welding, the weld cracking susceptibility composition (Po) is set to 0.
．． Limited to 24% or less.

Next, the reasons for limiting the heating and rolling conditions in the present invention will be explained.

The reason why the heating temperature is limited to 1050° C. or higher is to dissolve Nb, which is necessary for ensuring room temperature strength and high temperature strength, into the steel. Further, the rolling reduction ratio of 900°C or less is required to be 50% or more in order to obtain fine-grained austenite that is effective in ensuring toughness. Furthermore, regarding the rolling end temperature, if the rolling end temperature is less than 780°C, the yield ratio will increase due to grain refinement of ferrite and work hardening of ferrite due to two-phase region rolling, and the yield ratio will be 80% or less. can't get it.

Further, when the rolling end temperature exceeds 850°C, the austenite becomes red grains and the toughness deteriorates. therefore,
The rolling end temperature is limited to a temperature range of 850 to 780°C.

(Example) The present invention will be described below with reference to Examples.

Example 1 The test steel plate was a steel plate containing the chemical components shown in Table 1.
According to the heating and rolling conditions shown in the table, the plate thickness was 2511M.
It was completed in Test pieces were taken from these steel plates and subjected to a room temperature tensile test, a Charvia AI impact test, a maximum hardness test, and a 600° C. high temperature tensile test. The results are also listed in Table 2. The highest hardness test is JIS Z
It was carried out according to 3101.

Table 1 shows the plate thickness and chemical composition of the invention steels A-F and comparative IC-, and Table 2 shows the PCll, heating and rolling conditions, tensile properties, impact properties, weldability and high temperature properties, respectively.

(Left below) As is clear from Table 2, the steels A-F of the present invention have an angle of 600°
The proof stress in C is 22 kgf/mm" or more, showing excellent high temperature proof stress, and the maximum hardness is less than HV350, showing good weldability. The tensile properties at room temperature are 50 kgf/mm" or more.
Of course, it satisfies the value of kgf/mm2 class.

The yield ratio fully satisfies the requirement of 80% or less for architectural steel materials. Furthermore, the fracture surface transition temperature in the Charvi impact test is -35°C or less.

On the other hand, the PCM of comparative steels G and H is far outside the limited range of the present invention, so the maximum hardness is HV35.
0 or more, the weldability is poor, and the fracture surface transition temperature is also high. Comparative ml and J both have PCH of 0.24% or less, but the former has Mo added and the latter has Nb added. Also, the yield strength at 600°C is low for comparison steel, which is a conventional 50kgf/mm2 steel plate for construction, and the yield strength at 600°C is 18kgf/-■2.
and low.

Example 2 A test steel plate was finished to a thickness of 25 m according to the chemical composition and heating/rolling conditions shown in Table 3. Test pieces were taken from these steel plates and subjected to a room temperature tensile test and a Charby impact test. The results are also listed in Table 3. In addition,
The chemical composition is the same as that of w4A of the present invention shown in Table 1.

Table 3 shows the plate thickness, chemical composition, PC) 1% heating and rolling conditions, tensile properties, and impact properties of the invention steels At~^4 and comparative steels A5~88, respectively.

(Left below) Steels A1 to A4 of the present invention have a heating temperature of 1050-1200.
°C range, the rolling reduction rate is 50% or more below 900 °C, and the rolling end temperature is within the range of 790~850''C.
The tensile properties at room temperature satisfy the value of 50 kgf/s+m'' class. Also, the yield ratio is 80% or less, and the fracture surface transition temperature in the Charvy impact test is also -20°C or less.

On the other hand, comparative steel A5 has a low strength at room temperature because the heating temperature is 1000°C, so Nb is not sufficiently dissolved in solid solution. Comparative steel A6 has a small rolling reduction of 40% below 900°C. Therefore, comparative steel A7, in which fine-grained austenite is not obtained and the fracture surface transition temperature is high, has a low rolling end temperature of 750°C, so it is rolled in a two-phase region and the yield ratio at room temperature exceeds 80%. In addition, the comparative mA8 has a rolling end temperature of 880
°C, and the fracture surface transition temperature is high due to insufficient grain refinement of austenite.

Although the above embodiment relates to a method for manufacturing thick steel plates, it goes without saying that the present invention can also be applied to the manufacture of other steel products, such as long steel and shaped steel.

(Effect of the invention)

Claims (2)

[Claims] (1) C: 0.05-0.15%, Si: 0.05-0
．． 60%, Mn: 0.50-1.50%, Cr: 0.1
0-0.60%, Mo: 0.10-0.60%, Nb:
A steel billet containing 0.005 to 0.060%, the balance consisting of Fe and unavoidable impurities, and the value of P_C_M defined by the following formula [1] is 0.24% or less is 1050
After heating to a temperature of ℃ or higher, the rolling reduction at 900℃ or lower is set to 50% or higher, and the rolling is finished in a temperature range of 850 to 780℃, and the yield strength at 600℃ is 22kgf/mm^
Architectural use 5 with excellent weldability characterized by a rating of 2 or more
0kgf/mm^2 class fireproof steel manufacturing method. ▲ Contains mathematical formulas, chemical formulas, tables, etc. ▼ [1] (2) V: 0.005-0.060%, Cu: 0.05
~0.50%, Ni:0.05~0.50%, Ti:0
．． 005-0.030%, Ca: 0.0005-0.0
The manufacturing method according to claim 1, characterized in that the method contains one or more selected from 0.050%.