JPH04180518A - Production of 50kgf/mm2 class refractory steel for building having excellent toughness of large heat input welded joint - Google Patents

Abstract

PURPOSE:To improve high-temp. yield strength and to impart excellent toughness of a large heat input welded joint by heating a slab which consists of a Cr-Mo system as a basic compsn. and is limited in the value of PCM to a specific temp., and rolling this slab at a specified draft and end temp. CONSTITUTION:The compsn. of the refractory steel for buildings is composed of 0.05 to 0.15% C, 0.05 to 0.60% Si, 0.50 to 1.50% Mn, <=0.020% P, <=0.005% S, 0.10 to 0.40% Cr, 0.10 to 0.40% Mo, 0.005 to 0.060% Nb, 0.005 to 0.030% Ti, 0.0020 to 0.0070% N, and the balance Fe and unavoidable impurities. In addition, the value of the PCM specified by formula is specified to <=0.24%. This slab is heated to >=1050 deg.C and is rolled at >=50% draft at <=900 deg.C. The rolling is ended in a 850 to 780 deg.C range. The 50kgf/mm<2> class refractory steel having >=22kgf/mm<2> yield strength at 600 deg.C and the excellent toughness of the large heat input welded joint is obtd. in this way.

Description

Detailed Description of the Invention (Industrial Application Field 9) The present invention relates to a method for manufacturing fire-resistant steel, and in detail,
The present invention relates to a method for producing a 50 kgf/mm class 2 fire-resistant steel for construction, which has high yield strength even at high temperatures of .degree. C. and has excellent toughness for large heat input welded joints.

(Prior Art) When a steel frame is exposed to high temperatures in the event of a fire, the strength of the steel frame decreases, reducing the strength of the building, so the Building Standards Act requires that the steel frame be covered with a fire-resistant coating.

With conventional Sj-Mn-based 50 kgf/mrn' class steel for construction, when the temperature exceeds 350°C, the long-term yield strength (273 of normal temperature yield strength), which is the strength required for structural members in the event of a fire, is 22 kg.
f/mm', so the temperature of the steel frame does not exceed 350℃! However, they are worried about the fireproof coating, which will be a major hurdle in terms of the construction work and construction period.

``The new fire-resistant design method allows the use of R4←T (fire-resistant steel), which has excellent high-temperature resistance, to reduce the amount of fire-resistant coating.'' .

(Problem N 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/mm2 or more at 600℃, and has a high weld crack susceptibility composition (PC), so it has poor weld crack resistance and is difficult to weld without preheating or postheating. There is. Furthermore, when performing adult heat welding such as electroslab welding and subman arc welding, which are used to increase welding efficiency, weld heat affected zone (HAZ)
), the toughness of the steel is significantly reduced, so we are forced to use dwarf heat welding.

Therefore, in order to reduce or omit the need for fire-resistant coating on architectural steel, we developed a new technology that has high high-temperature yield strength, excellent weldability, and high heat-welded joint toughness δ and base metal properties, making it the same as conventional steel. There is a need for steel that can be designed and constructed.

In addition, from the viewpoint of the deformability of buildings during earthquakes, there is an increasing requirement for architectural steel to have a yield ratio of 8006 or less.

(Means for Solving the Problems) In view of the above-mentioned problems with conventional architectural steel, the present inventors have conducted intensive research1, and as a result, the chemical components, particularly the Cr-Mo system, have been improved. As a result, Nb precipitation strengthening significantly improves high-temperature yield strength without impairing weldability.
Furthermore, it was completed based on the knowledge that excellent high heat input welding joint toughness can be ensured by utilizing TiN, and the first invention is c:o, 05-0.15%, S
i: 0.05-0.60%, l1ln: 0°5
0 to 1.50%, P: 0.020% or less, S: 0.00
5% or less, Cr: 0.10-0.40%, Mo: 0.1
0-0.40%, Nb: 0.005-0.060%,
Ti: 0.005-0.030%, N: 0.0020
After heating a steel piece to a temperature of 1050°C or higher and having a PCM value of 0°2406 or less as defined by the following formula 0, containing ~0°0070%, the balance consisting of Fe and unavoidable impurities. , the rolling reduction ratio is 5006 or more at 900°C or less, and the rolling is finished in a temperature range of 850 to 780°C,
High heat input welded joint with a yield strength of 22 kgf/mm or more at 600'C. 50 kgf/mm for construction with excellent toughness.
This is a method for producing second grade fireproof steel.

The second invention is V: 0.005 to 0.060%, Cu: 0
．． 05-0.5096, Ni: 0.05-0
．． 50%, Ca: 0.0005 to 0.0050%
, REV: 1 selected from 0.001 to 0.020%
A method for manufacturing a 50 kgf/mm class 2 fire-resistant steel for construction with excellent high heat input weld joint toughness according to claim 11, which contains one or more of the following:

(Function) Below, the reason for limiting the chemical components in the present invention will be explained.

C is an element that contributes to an increase in strength, or if it is less than 0.05%, it is difficult to ensure strength, and if it is less than 0.1%, it is difficult to ensure the strength.
If a large amount of Qo is added with 5", it will deteriorate weldability and toughness, so the amount of FM added will be 0.
05 to 0.15"t+.

Si is an essential element for deoxidation, or 0.05%
If it is less than 0.60, the deoxidizing effect is low. If added in excess of more than %, weldability deteriorates. I wanted to,
For songs where the amount of addition is in the range of 0.05 to 0.60%,
It is an element necessary to ensure the strength and toughness of steel, and if it is less than 0.50%, this effect will be small, and if it is added in a large amount exceeding 1.50%, it will deteriorate weldability. Moreover, the toughness is also deteriorated. Therefore, the amount added should be in the range of 0.50 to 1.50%.

If P exceeds 0.020%, micro-segregation will cause deterioration of high heat input weld joint toughness, base metal toughness, and weld cracking resistance, so it is set to 0.020% or less.

If S exceeds 0.005%, it tends to form coarse A-based inclusions and deteriorates the toughness of the base material.
% or less.

C is an element effective in improving high-temperature strength, or is 0.H
If the IC' is less than 2, an effect like 2 is expected:, 0.
If a large amount of splint exceeds 400o8, weldability will deteriorate. Therefore, the amount added is in the range of 0.10 to 0.40%.

MO is an essential element for ensuring high-temperature strength, and significantly increases yield strength at 600°C.

However, if it is added in an amount less than 0.10%, such an effect cannot be obtained, and if it is added in an amount exceeding 0.40%, weldability is impaired. Therefore, the amount added is 0.10 to 0.40
% range.

Nb is an element that increases strength through precipitation strengthening and transformation strengthening, and improves toughness through grain refinement. However, if it is less than 0.005%, such effects cannot be obtained, and if it is added in excess of 0.060%, the toughness of high heat input welded joints will deteriorate. Therefore, the amount added is 0
．． The range is 0.005% to 0.060%.

T1 is an element that is effective in suppressing the coarsening of austenite grains in the HAZ due to TiN, as well as generating internal ferrite, thereby reducing deterioration of the toughness of high heat input welded joints. However, if the amount is less than 0.0050, such an effect may not be exhibited, and if it is added in excess of 0.0300t+, the toughness of the welded joint will deteriorate. Therefore, the amount added is in the range of 0.005 to 0.030%.

N improves the toughness of high heat input welded joints by combining with T1. However, if it is less than 0.0020%, such an effect cannot be exhibited;
If added in a large amount exceeding 0.070%, the toughness of welded joints will deteriorate. Therefore, the amount added is 0.0020~0
．． The range is 0.0070%.

In addition, in the second invention of the present invention, in addition to the above-mentioned elements, V, Cu, Ni, Ca, and RE may be added as necessary.
One or more selected from M can be added.

Is ■ an element effective in increasing strength through precipitation strengthening?
If it is less than 0.005%, such an effect cannot be expected, and if it is added in excess of 0.060%, weldability deteriorates. However, the corresponding 0 kan is 0.00
The range is 5 to 0.060%.

Is Cu an effective element for increasing strength by solid solution strengthening? If it is less than 0.05%, this effect is small, and
Addition of more than 0.50% of 110 impairs hot workability and weldability. Therefore, the amount added is 0.05 to 0.05.
The range is 50%.

Ni is an effective element for improving toughness, or 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%.

Ca is an element that improves properties in the thickness direction in trace amounts, or if it is less than 0.05%, such effects cannot be obtained, but on the other hand, when added in excess of 0.0050%, As the effect reaches saturation, large inclusions are generated and ultrasonic defects are more likely to occur. Therefore, the amount added is 0
．． The range is 0005% to 0.0050%.

Like Ca, REV is an element that improves the properties in the thickness direction in a trace amount, or if it is less than 0.00%, such an effect cannot be obtained, but on the other hand, if it is added in an amount exceeding 0.020%. At some point, this effect reaches saturation, and large inclusions are likely to occur, causing ultrasonic defects. Therefore, the amount added is in the range of 0.001 to 0.020%.

In addition, in both the first invention and the second invention, the weld crack susceptibility composition (PC,) is limited to 0.24% or less in order to prevent cold cracking during welding.

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 the toughness of the base material. Furthermore, regarding the rolling end temperature, if the rolling end temperature is less than 780°C, the yield ratio increases due to grain refinement of the ferrite and work hardening of the ferrite due to two-phase region rolling, resulting in a yield ratio of 80% or less. I can't do anything.

Further, when the rolling end temperature exceeds 850°C, the toughness of the base material deteriorates because austenite becomes coarse grains. 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.

The test steel plate was a steel slab containing the chemical components shown in Table 1.
Using the rolling conditions shown in the table, the plate was finished to a thickness of 25 mm. Test pieces were taken from these steel plates and subjected to a room temperature tensile test, a Charvi impact test, a maximum hardness test, and a 600
A high temperature tensile test at ℃ and a simulated thermal cycle test were conducted.

The results are shown in Table 2. The highest hardness test is JI
It was carried out according to S Z 3101, and the reproduction thermal cycle conditions were heating at 1350°C for 5 seconds, and the cooling time from 800 to 500°C was 220 seconds, which corresponded to a plate thickness of 30 mm and a heat input of 400 kJ/Cm. Table 1 shows the present invention 11A-
Table 2 shows the plate thickness, chemical composition, and PCM of E and comparative steel F~0, and the rolling conditions, tensile properties, impact properties, weldability, high temperature properties, and high heat input weld joint toughness, respectively.

As is clear from Table 2, the invention steels 8 to E are 60
The proof stress at 0°C is: 22 kgf mm' or more, showing excellent high temperature proof strength, and the maximum hardness is less than HV350, showing good weldability. Furthermore, the high heat input weld joint toughness is also good in a reproduced thermal cycle test. . Note that the tensile properties at room temperature naturally satisfy the value of 50 kgf/mm2 class, and the yield ratio fully satisfies the 80% or less required for architectural steel materials. In addition, the fracture surface transition temperature in the Charvi impact test was below -35°C, while comparative steel F had a temperature of 600°C.
The yield strength is high at 26 kgf/mm2, the toughness of the high heat input welded joint is low because no Ti is added, and the maximum hardness is HV350 or higher because the PCM is outside the limited range of the present invention. Comparative steel G has a PCM content of 0.24% or less, but like F, the high heat input welding joint toughness is low due to the addition of Ti1Mj.

Additionally, the fracture surface transition temperature is high. Comparative Steel H has poor high heat input welded joint toughness because it has T1 added or N is highly outside the limited range of the present invention. On the other hand, comparative steels I and J
The toughness of the high heat input welded joint is good, and the FAT type is MO, which is effective in maintaining high temperature strength.The latter has no added Cr, so the yield strength at 600℃ is 22 kg l,・'
mm2 is not satisfied. In addition, the conventional 50 kgf for construction
, 'mm2 grade steel plate, PCM or 0.24
9ti or less, or Cr, Mo, and Nb are not added, the yield strength at 600°C is low, and the T
1 The toughness of high heat input welded joints is also poor due to no additives.

Next, since the comparative steel has a low heating temperature of 1000℃,
The room-temperature strength and high-temperature strength did not meet the standard values, and comparative steel M had a low rolling reduction of 40% below 900°C.
Base material has poor toughness. Furthermore, comparative steel N has a rolling finish temperature of 7
Since the temperature is as low as 60°C, the yield ratio exceeds 80%. On the other hand, comparative steel O has a high rolling end temperature of 880°C, so the base material toughness is poor.

Figure 1 shows inventive steel A with T1 addition and comparative steel G without Ti addition.
The microscopic structure diagram after the simulated thermal cycle test is shown. As shown in the figure, the steel A of the present invention, which has good high heat input weld joint toughness, has finer austenite grains and intragranular ferrite grains than comparative steel G.

It goes without saying that the above-mentioned embodiments relate to a method for producing thick steel plates, but the present invention can also be applied to the production of other steel products, such as long steel products and shaped steel products.

(Effects of the Invention) As explained above, the method for manufacturing 50 kgf/'mm grade 2 fire-resistant steel for construction with excellent high heat input weld joint toughness according to the present invention uses chemical components, particularly Cr-Mo system as a basic component. , Nb precipitation strengthening significantly improves high-temperature yield strength without impairing weldability, and the use of TiN ensures excellent high heat input weld joint toughness.
Combines high yield strength and good weldability at 600℃,
In addition, it is possible to manufacture steel with a low yield ratio, and it is possible to significantly reduce or omit the fire-resistant coating that was previously required. This has the excellent effect of increasing the safety of objects.

[Brief explanation of the drawing]

FIG. 1 shows a metallurgical microstructure diagram of a comparison w4G of Ti-added LIO, Honkome Steel A and T1-free '10, after a simulated thermal cycle test. Patent applicant: Kobe Steel Oriyoshi Co., Ltd. Patent attorney: Akira Kanemaru

Claims (1)

[Claims] (1) C: 0.05-0.15%, Si: 0.05-0
．． 60%, Mn: 0.50-1.50%, P: 0.02
0% or less, S: 0.005% or less, Cr: 0.10-0
．． 40%, Mo: 0.10-0.40%, Nb: 0.0
05-0.060%, Ti: 0.005-0.030%
, N: 0.0020 to 0.0070%, the balance being F
A steel billet consisting of e and unavoidable impurities and having a P_C_M value of 0.24% or less as defined by the following formula [1] is heated to a temperature of 1050°C or higher, and then the rolling reduction rate of 900°C or lower is 50%. % or more, rolling is completed in the temperature range of 850 to 780°C, and the yield strength at 600°C is 22 kg.
A method for producing a 50 kgf/mm^2 fire-resistant steel for construction use having excellent high heat input weld joint toughness characterized by f/mm^2 or more. SiMn+Cu+CrN1MoV P_C_M=C+(Si)/(30)+(Mn+Cu+
Cr)/(20)+(Ni)/(15)+V/(10)
+5B(%)...[1](2)V:0.005~0
．． 060%, Cu: 0.05-0.50%, Ni: 0.060%, Cu: 0.05-0.50%, Ni: 0.
05-0.50%, Ca: 0.0005-0.0050
%, REM: 50kgf for architectural use with excellent toughness for high heat input welded joints according to claim [1], characterized in that it contains one or more selected from 0.001 to 0.020%.
/mm^ Method for producing second class fireproof steel.