JPH05279735A - Manufacture of building fire resistant steel plate excellent in toughness in high heat input weld heat-affected zone - Google Patents

Abstract

PURPOSE:To manufacture a building fire resistant steel plate excellent in toughness in the high heat input weld heat-affected zone. CONSTITUTION:Steel contg., by weight, 0.05 to 0.12% C <=0.6% Si, 0.8 to 1.6% Mn, <=0.03% P, <=0.005% S, 0.35 to 0.80% Mo, 0.005 to 0.025% Ti, <=0.005% Al, 0.001 to 0.004% N and 0.001 to 0.006% O and substantially contg. no Al is reheated to the temp. range of 1000 to 1150 deg.C, is thereafter rolled so as to regulate the cumulative draft at <=900 deg.C to >=30%, is subsequently cooled from >=750 deg.C to the temp. range of 350 to 650 deg.C at 3 to 40 deg.C/sec cooling velocity and is thereafter air-cooled, by which the objective fire resistant 50kgf/mm<2> class steel plate excellent in toughness in the high heat input weld heat-affected zone can be manufactured.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has a thickness of 50 mm used for various structures in the fields of construction, civil engineering and marine structures.
The present invention relates to a method for producing a refractory steel sheet having excellent toughness in the heat affected zone (HAZ) particularly in large heat input welding (welding heat input = 500 to 1500 kJ / cm) such as electroslag welding.

[0002]

2. Description of the Related Art Generally, HAZ toughness of low alloy steel is (1)
Grain size, (2) dispersed state of high carbon island martensite, hardened phase such as upper bainite (Bu), (3)
It is governed by various metallurgical factors such as grain boundary embrittlement and (4) elemental microsegregation. In particular, it is known that the size of the crystal grains of HAZ has a great influence on the low temperature toughness, and many techniques have been developed and put to practical use for refining the HAZ structure. Finely disperse nitrides such as TiN, which are relatively stable even at high temperatures, in the steel, which results in HAZ
The technology for suppressing the coarsening of austenite (γ) grains is particularly famous. However, in the region where HAZ is heated to 1400 ° C. or higher, TiN coarsens or dissolves, and the ability to suppress coarsening of γ grains disappears. For this reason, the toughness is largely deteriorated in the vicinity of the melting line, and it is not possible to stably obtain high toughness in the entire HAZ. That is, in a Charpy test in which a notch is formed near the fusion line, the frequency is low, but a low value appears, which is not preferable from the viewpoint of safety of the welded structure. On the other hand, in the steel in which Ti oxide (mainly Ti ₂ O ₃ ) is finely dispersed (Japanese Patent Application No. 59-203099), intragranular acicular ferrite (hereinafter referred to as IGF) is generated even in the vicinity of the melting line to generate HAZ. Tissue can be made smaller and T
Excellent low temperature toughness is obtained as compared with iN steel. But,
The refractory steel sheet disclosed in JP-A-2-77523 has a high Mo content, and particularly large heat input welding (welding heat input =
In the case of 500 to 1500 kJ / cm), IGF is hard to be generated, and sufficient HAZ toughness cannot be obtained even by this method.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a technique for inexpensively producing a refractory steel sheet having extremely excellent HAZ toughness in high heat input welding. The steel produced by the method of the present invention has a fine HAZ structure even in the vicinity of the fusion line during high heat input welding, and exhibits excellent low temperature toughness throughout the HAZ.

[0004]

Means for Solving the Problems The present invention overcomes the above-mentioned problems and achieves the object, and its concrete means are shown in (1) and (2) below. (1) C 0.05 to 0.12% by weight and Si 0.
6% or less, Mn 0.8 to 1.6%, P 0.03% or less, S 0.005% or less, Mo 0.35 to 0.80
%, Ti 0.005 to 0.025%, Al 0.00
5% or less, N 0.001 to 0.004%, O 0.0
A steel containing 0.1 to 0.006% and the balance of iron and unavoidable impurities and containing substantially no Al is 1000
After reheating in the temperature range of ˜1150 ° C., rolling is performed so that the cumulative rolling reduction of 900 ° C. or less is 30% or more, and then 75
350 at a cooling rate of 3 to 40 ° C / sec from a temperature of 0 ° C or higher
To a temperature range of 650 ° C., and then air-cooling, a thickness of 50 mm or more (preferably 50 to 10).
0mm) High heat input welding Heat-affected zone Manufacturing method for building fireproof 50kgf / mm ² grade steel sheet with excellent toughness. (2) C 0.05 to 0.12% by weight ratio and Si 0.
6% or less, Mn 0.8 to 1.6%, P 0.03% or less, S 0.005% or less, Mo 0.35 to 0.80
%, Ti 0.005 to 0.025%, Al 0.00
5% or less, N 0.001 to 0.004%, O 0.0
01-0.006%, Nb 0.003-0.0
1%, V 0.003 to 0.03%, Ni 0.05 to
1.0%, Cu 0.05 to 1.0%, Cr 0.05
.About.1.0%, Ca 0.001 to 0.006%, and the balance consisting essentially of iron and inevitable impurities.
After reheating the steel not containing 1 in the temperature range of 1000 to 1150 ° C., it is rolled so that the cumulative rolling reduction of 900 ° C. or less is 30% or more, and then 3 to 4 from the temperature of 750 ° C. or more.
50 mm thickness characterized by cooling to a temperature range of 350 to 650 ° C. at a cooling rate of 0 ° C./sec and then air cooling
The above (preferably 50 to 100 mm) high heat input welding heat-affected zone is excellent in toughness for construction, and is a manufacturing method of fireproof 50 kgf / mm ² grade steel sheet for construction.

[0005]

The present invention will be described below. Performance specified by rolled steel for welded structure (JIS G3106) and 600
In order to maintain a high yield strength at a high temperature of ℃ and to improve the toughness of the high heat input welding heat affected zone, it is important to refine the structure together with the steel components. The feature of the present invention is to add an appropriate amount of Mo, and further reheat a billet in which Ti ₂ O ₃ and TiN are finely dispersed in Ti, N, and O in a temperature range of 1000 to 1150 ° C., and then 900 ° C. After rolling so that the cumulative rolling reduction below becomes 50% or more, it is cooled to a temperature range of 350 to 650 ° C. at a cooling rate of 3 to 40 ° C./sec from a temperature of 750 ° C. or higher and fine ferrite-bainite. The structure is to simultaneously obtain fire resistance and excellent high heat input welding HAZ toughness.

Mo is an essential element for ensuring fire resistance because it forms fine carbonitrides and further increases high temperature strength by solid solution strengthening. However, Mo
If the amount is too high, the weldability and HAZ toughness will deteriorate, so
The upper limit of its content needs to be 0.80%. However, it is difficult to obtain sufficient yield strength in a high temperature region of 600 ° C. only by adding Mo alone. Therefore, making the structure a ferrite-bainite structure is effective in increasing the yield strength in the high temperature region. Therefore, in order to obtain high high temperature strength, low temperature toughness and excellent HAZ toughness,
It is also necessary to make the manufacturing conditions for steel sheets appropriate. In order to increase the high temperature strength by adding Mo, it is necessary to solutionize Mo during reheating. Therefore, the lower limit of the reheating temperature is 100
Set to 0 ° C. If the reheating temperature is too high, the crystal grains become large and the low temperature toughness deteriorates, so the upper limit must be 1150 ° C.

Subsequently, in rolling, it is essential to set the cumulative rolling reduction at 900 ° C. or less to 30% or more. This is to refine the γ grains and obtain excellent low temperature toughness. The cooling conditions of the steel sheet following rolling are from 750 ° C or higher to 3-40 ° C /
It is necessary to perform accelerated cooling to a temperature range of 350 to 650 ° C. at a cooling rate of 2 seconds. This is because when cooled from a temperature lower than 750 ° C., a ferrite-based structure is formed and sufficient high-temperature strength cannot be obtained. The cooling rate of 3 ° C./sec or more is the minimum cooling rate necessary for making the structure ferrite-bainite. Further, when the cooling rate is higher than 40 ° C./sec, a large amount of martensite is generated and sufficient high temperature strength cannot be secured. The reason for setting the lower limit of the cooling stop temperature to 350 ° C. is that if accelerated cooling is performed to a temperature lower than that, toughness is deteriorated by low temperature transformation products such as island martensite. Further, the upper limit of the cooling stop temperature is set to 650 ° C. because an appropriate amount of bainite structure cannot be obtained at a temperature higher than that. The lower limit of the amount of Mo added must be 0.35% in order to secure sufficient yield strength at 600 ° C.

Next, the reasons for limiting the components other than Mo in the present invention will be explained. C is an important element for high temperature strength in a ferrite-bainite structure such as the steel of the present invention, and the addition of 0.05% or more increases the high temperature strength. Therefore, the lower limit is 0.05%. If the amount of C is too large, IGF will not be generated in the HAZ portion of the high heat input weld, so 0.12% is made the upper limit. Since Si is an element contained in the deoxidized upper steel, the weldability and HAZ toughness deteriorate when the amount of Si increases, so the upper limit was made 0.6%. Mn is an essential element for ensuring strength and toughness, and the lower limit is 0.
8%. However, if the amount of Mn is too large, the hardenability increases and the weldability and HAZ toughness deteriorate, so the upper limit of Mn is 1.
It was 6%. Al is an element generally contained in deoxidized upper steel, but it is an element not preferred in the present invention and is 0.005%.
Limited to: This is because if Al is contained in the steel, it will combine with oxygen and Ti oxide will not be generated.
Since deoxidation is also carried out by Si and Ti, the less Al in the steel of the present invention, the better, and 0.003% or less is desirable.

[0009] Ti has a content of 0.
005% or more is required, and if it exceeds 0.025%, T
Since the HAZ toughness is deteriorated due to the formation of iC, 0.0
It is limited to 05 to 0.025%. N is an element necessary to secure TiN, and 0.001 to secure the minimum amount.
% Is required, and when the amount of N increases, H due to solid solution N
Since the AZ toughness is deteriorated, the range is 0.001 to
It was 0.004%. O is an element necessary for producing Ti ₂ O ₃ , and the minimum necessary amount is 0.001%,
If it exceeds 0.006%, the cleanliness and toughness of steel will be deteriorated, so the content is limited to 0.001 to 0.006%. The steel of the present invention contains P and S as unavoidable impurities.
Since the effects of P and S on high temperature strength are small, the amounts thereof are not particularly limited, but generally, the properties of steel such as toughness and strength in the plate thickness direction are improved as the amounts of P and S are decreased. Desirable P and S contents are 0.03% and 0.0, respectively.
It is 05% or less. The basic components of the steel of the present invention are as described above, and the object can be sufficiently achieved. However, in order to further improve the characteristics for the object, the elements described below, namely Nb, V, N
By selectively adding i, Cu, Cr, and Ca, more preferable results can be obtained with respect to improvement in strength and toughness.

Next, the above-mentioned additional element and its addition amount will be explained. Nb forms fine carbonitrides and increases high temperature strength. However, if it is less than 0.003%, it has no effect, and if it exceeds 0.01%, it has an unfavorable effect on the high heat input welding HAZ toughness. V is an element having almost the same effect as Nb, and its effect on high temperature proof stress is smaller than that of Nb, but it is not effective at 0.03% or less, and has an unfavorable effect on HAZ toughness if it exceeds 0.03%. Next, Ni improves the strength and toughness of the base metal without adversely affecting weldability and HAZ toughness, but the effect is weak at 0.005% or less and extremely expensive at 1.0% or more, which is economical. Therefore, the upper limit was made 1.0%.

Cu has almost the same effect as Ni,
It is also effective in increasing the high temperature strength and improving the corrosion resistance and weather resistance due to the Cu precipitates. In this case, when the amount of Cu is 0.5% or more, the effect is remarkable. However, if the Cu content exceeds 1.0%, Cu cracking occurs during hot rolling, which makes production difficult, and if it is 0.05% or less, there is no effect, so the Cu content is 0.05 to 1.0%. limit. Cr is an element that enhances the strength of the base material and the welded portion. When the amount of Cr is 0.5% or more, the weather resistance is also improved, but when it exceeds 1.0%, the weldability and HA are improved.
The Z toughness is deteriorated, and if it is less than 0.05%, the effect is small. Therefore, the Cr content is 0.05% to 1.0%. Ca
Has the effect of controlling the morphology of sulfide (MnS), increasing Charpy absorbed energy, and improving low temperature toughness.
However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it exceeds 0.006%, a large amount of CaO and CaS are formed and become large inclusions, which not only deteriorates the toughness of the steel but also the cleanliness, resulting in weldability, As it also has a negative effect on lamella tear resistance,
The range of the amount of Ca added is 0.001 to 0.006%.

[0012]

EXAMPLE A steel plate was manufactured by a well-known converter, continuous casting, and thick plate process, and the strength at room temperature and 600 ° C. and the low temperature toughness of the base material were investigated. Tables 1 to 10 show the chemical compositions of the present invention steels and 11 to 25 are comparative steels. Table 2 shows the steel plate manufacturing conditions and the mechanical properties of the present invention steel and the comparative steel. Inventive Steels 1 to 1 in Table 2
No. 10 has a yield strength at 600 ° C of 7 which is the standard yield strength at room temperature.
In addition to having 0% or more, excellent low temperature toughness of the base material is obtained and excellent large heat input welding HAZ toughness is obtained.

On the other hand, in Comparative Steel 11, since the C content is small, the yield strength at 600 ° C. is low, and the ratio of the yield strength at 600 ° C. to the standard yield strength at room temperature does not reach 70%. Comparative Steel 12 has a large amount of C, and thus has a low HAZ toughness. Comparative Steel 13 has a small amount of Mo, so 600 ° C.
The yield strength is low and does not reach 70% of the standard yield strength at room temperature. In Comparative Steel 14, the HAZ toughness is low due to the large amount of Mo. Comparative steel 15 has a small amount of Ti, and comparative steel 16 has a large amount of Ti, and both have low HAZ toughness. In Comparative Steel 17, since the amount of Al was large, Ti oxide was not formed, and the HAZ toughness was low. In Comparative Steel 18, since the reheating temperature was low and Mo was not sufficiently solid-dissolved,
The yield strength at 600 ° C does not reach 70% of the standard yield strength at room temperature. In Comparative Steel 19, the reheating temperature was too high, causing the structure to become coarse, and the low temperature toughness of the base material deteriorated and the HAZ toughness decreased. In Comparative Steel 20, the cumulative rolling reduction at 900 ° C. or lower is low, the grain refinement of the structure is not sufficiently performed by rolling, and the low temperature toughness of the base material and the HAZ toughness are reduced.

In Comparative Steel 21, the water cooling start temperature is low and the amount of ferrite becomes too large, so that the yield strength at 600 ° C. does not reach 70% of the standard yield strength at room temperature. Comparative steel 22 has a low water-cooling stop temperature and island martensite is formed, and the low temperature toughness of the base material is deteriorated. In Comparative Steel 23, since the water cooling stop temperature is high, the amount of bainite is small, and the yield strength at 600 ° C. does not reach 70% of the standard yield strength at room temperature. In Comparative Steel 24, the cooling rate is low, a bainite structure is not formed, and the yield strength at 600 ° C. does not reach 70% of the standard yield strength at room temperature. In Comparative Steel 25, the cooling rate is high and a large amount of martensite is formed, and the low temperature toughness of the base material is deteriorated.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

Industrial Applicability The steel materials such as thick steel plate, shaped steel, and steel bar manufactured by the chemical composition and manufacturing method of the present invention are steels having high yield strength of 600 ° C., excellent low temperature toughness, and excellent high heat input welding HAZ toughness. Therefore, the safety of architecture, civil engineering, and marine structures can be greatly improved.

Claims (2)

[Claims] 1. A weight ratio of C: 0.05 to 0.12%, Si: 0.6% or less, Mn: 0.8 to 1.6%, P: 0.03% or less, S: 0.0. 005% or less, Mo: 0.35 to 0.80%, Ti: 0.005 to 0.025%, Al: 0.005% or less, N: 0.001 to 0.004%, O: 0.001 ~ 0.006% and the balance of iron and unavoidable impurities and substantially Al-free steel is reheated in the temperature range of 1000 to 1150 ° C, and the cumulative rolling reduction at 900 ° C or less is 30% or more. High heat input welding heat, characterized by cooling from 750 ° C. or higher to a temperature range of 350 to 650 ° C. at a cooling rate of 3 to 40 ° C./sec, and then air cooling. Affected zone A manufacturing method of fire-resistant steel plates for construction with excellent toughness. 2. By weight ratio, C: 0.05 to 0.12%, Si: 0.6% or less, Mn: 0.8 to 1.6%, P: 0.03% or less, S: 0.0. 005% or less, Mo: 0.35 to 0.80%, Ti: 0.005 to 0.025%, Al: 0.005% or less, N: 0.001 to 0.004%, O: 0.001 -0.006% Further Nb: 0.003-0.01%, V: 0.003-0.03%, Ni: 0.05-1.0%, Cu: 0.05-1.0%, Cr: 0.05 to 1.0%, Ca: 0.001 to 0.006%, one or more kinds of steel, with the balance being iron and unavoidable impurities and containing substantially no Al. 100
After reheating in the temperature range of 0 to 1150 ° C, rolling is performed so that the cumulative rolling reduction of 900 ° C or less is 30% or more, and then 7
35 at a cooling rate of 3 to 40 ° C / sec from a temperature of 50 ° C or higher
A method for producing a refractory steel sheet for construction having excellent toughness in a heat-affected zone with a high heat input welding, which comprises cooling to a temperature range of 0 to 650 ° C. and then air cooling.