JPH108192A - High toughness heat resistant steel for large heat input welding and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel excellent in high temp, proof stress and weldability by allowing the content of each element to satisfy specified character value and index value. SOLUTION: This steel contains, by weight, 0.05 to 0.15% C, <=0.3% Si, 1 to 1.8% Mn, 0.45 to 1% Mo, 0.05 to 0.08% V and <=0.04% Al. Also each element content satisfies the inequality, and the weld low temp. cracking sensitivity index PCM is regulated to 0.15 to 0.25%. Furthermore, the left side of the above inequality is called as the tenacity index, Moreover, PCM (%) is equal to C+(Si/30)+ (Mn+Cr+Cu)/20}+(Mo/15)+(Ni/60)+(V/10)+5B. In this way, the steel excellent in cold strength, small in deterioration in strength even if the temp. increases in the case of a fire and capable of large heat input welding without executing preheating can be obtd. Particularly, it is suitable for steel- frame buildings in which the coating of refractories can be reduced or eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel which is suitable for steel frame construction in which the strength does not decrease much even if the temperature of the steel material rises in a fire, for example, and a large heat input welding which improves welding workability during construction. The present invention relates to a high toughness heat-resistant steel for use and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, steel-framed buildings such as skyscrapers have been increasing more and more. Conventionally, steel materials for steel frame construction have been SS41 (JIS G 3101) or SM50 (JIS G 3106)
These steel materials are used as they are rolled or after normalizing after rolling. In recent years, steel produced by performing accelerated cooling after rolling has also been used. This accelerated cooling steel has improved weldability, especially low-temperature cracking resistance, as compared with conventional 50 kg class high-tensile steel (SM50R, SM50N, etc.), and work to standardize it as SM50TMC is currently underway.

However, the steel materials used for steel buildings are exposed to high temperatures at the time of fire, for example, at 600 ° C.
If the temperature becomes higher than the above, the strength is reduced, and the structural stability at the time of fire cannot be guaranteed. For this reason, in practice, it is necessary to coat the steel with a refractory, and by spraying rock wool or the like on the surface of the steel, an increase in the temperature of the steel during a fire is prevented.

Although the method of spraying rock wool has the advantage of being inexpensive, at the construction site, it is necessary to cover the sheet with a curing sheet in order to prevent scattering at the time of spraying. However, there is a problem that the surrounding environment is damaged.

[0005] For these reasons, there has been a demand for the development of a steel material having a small strength reduction even when exposed to a high temperature in the event of a fire, that is, a steel material excellent in high temperature proof stress.

As a heat-resistant steel (sometimes referred to as fire-resistant steel) having excellent high-temperature resistance, Mo steel plate (for example, JIS G 3103) or Cr-Mo steel (for example, JIS) widely used for boilers and pressure vessels is used. G 4109). For these steels, the high-temperature strength (proof strength) to be satisfied in the boiler construction standards is determined, and the steel can cope with the high-temperature environment at the time of fire.

However, since these steels have a high carbon (C) content, they are susceptible to low-temperature cracking in welding, and must be preheated or post-heated during welding.

On the other hand, the C content is kept low, and the 0.2% proof stress (hereinafter referred to as proof stress or P
S: Poof Stress is 2/3 of that at room temperature
The refractory steel described above and a method for producing the same have been proposed.
For example, as a steel material that guarantees strength at high temperatures and a method for producing the same, a low-C steel sheet containing Mo or V or both, and a method for producing the same have been disclosed (Japanese Patent Publication No. 5-79744, Japanese Patent Publication No. Hei 7-79744). Japanese Patent Publication No. 2968, Japanese Patent Publication No. 7-26158, Japanese Patent Publication No. 7-37648, Japanese Patent Publication No. 7-47777, Japanese Patent Publication No. 7-7737.
No. 3, JP-A-5-279735, JP-A-2-27935
163341, JP-A-3-277715,
JP-A-7-207338).

However, among these, Japanese Patent Publication No. 7-296
8, Japanese Patent Publication No. 7-47771, Japanese Patent Publication No. 7-7
No. 4373, JP-A-2-163341 and JP-A-3-277715 disclose a carbon equivalent (C _eq ).
However, other elements except Mo or V are not sufficiently limited. Therefore, good weldability and welding heat affected zone (HAZ: Heat Affected)
In addition to the fact that the toughness in d Zone) may not be ensured, sufficient fire resistance cannot always be obtained. In general, C _eq is a suitable index for estimating the low temperature weld cracking property of steel such as carbon steel which does not contain elements other than C, Si and Mn, for example, Mo, etc. It is said that it is not possible to accurately estimate the weld cracking property of steel containing an element that greatly increases penetration by C _eq .

Japanese Patent Publication No. 5-79744, Japanese Patent Publication No.
Also in the steels disclosed in JP-A-26158, JP-B-7-37648 and JP-A-7-207338, welding workability and HAZ toughness are insufficient.

In this specification, "fire resistance" and "heat resistance" are used without distinction. Also, H
AZ indicates a portion in contact with the weld metal, that is, a heat affected zone including a so-called bond.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to obtain a steel material having excellent high-temperature yield strength and weldability, for example, suitable for steel building. Specifically, the objective is to satisfy the following performance.

[0013] 1. Base material (steel): 1) Room temperature strength: Yield strength (YS) ≧ 315 MPa 630 MPa ≧ Tensile strength (TS) ≧ 490 MPa 2) 600 ° C. strength: Yield strength ≧ 217 MPa 3) Toughness: Charpy test vE0 ≧ 60J Weldability 1) Toughness: 200 kJ / cm submerged arc welding (S
In a Charpy test of a test piece to which a reproducible heat cycle equivalent to HAZ of AW) was applied, vE0 ≧ 60J 2) Low temperature welding cracking property: y groove constraint cracking test (JIS Z3158)
Preheating temperature ≤ 25 ° C (normal temperature)

[0014]

The gist of the present invention is a steel containing the following alloy elements and a method for producing the same.

(1) In weight%, C: 0.05-0.1
5%, Si: 0.3% or less, Mn: 1 to 1.8%, M
o: 0.45~1%, V: 0.05~0.08 % and Al: includes 0.04% or less, and satisfies the following formula, weld cold cracking sensitivity index P _CM 0.15 ~ 0.25
% Of high toughness heat-resistant steel for large heat input welding (referred to as [Invention 1]).

V (%) / {Si (%) + 1.5Al (%)} ≧ 0.3, where the welding cold cracking susceptibility index P _CM (%) = C (%) + { S
i (%) / 30} + [{Mn (%) + Cr (%) + Cu (%)} / 2
0] + {Mo (%) / 15} + {Ni (%) / 60} + {V (%) / 1
0} + 5B (%) (2) For large heat input welding as described in (1) above, in which rolling is performed in a hot rolling at a temperature range of 900 ° C. or lower and at an Ar temperature of ₃ points or higher so that the cumulative draft is 25% or higher. A method for producing a high toughness heat-resistant steel (referred to as [Invention 2]).

The steels of the above [Invention 1] and [Invention 2] refer to thick steel plates, hot-rolled steel plates, section steels such as H-section steels and the like.

Further, V in the left side of the equation in [Invention 1]
(%) / {Si (%) + 1.5Al (%)} is referred to herein as a “strength and toughness balance index” or simply as a “toughness index”.

The reason why the toughness index is limited as described above in the present invention will be described.

In the construction of a high-rise building or the like, a welding heat input exceeding 100 kJ / cm is usually employed in order to improve the efficiency of welding work. Generally, as the welding heat input increases, the HAZ structure becomes coarser and the HAZ toughness deteriorates.

A steel containing Mo in the above range and securing strength at 600 ° C. has very low toughness, particularly HAZ toughness. V, which is added to supplement the high-temperature strength improving effect of Mo, contributes to the improvement of the strength, but the degree of deterioration of the HAZ toughness is small. On the other hand, the lower Si and Al, the higher the toughness, especially the higher the HAZ toughness.

The present inventors experimentally produced various types of steel and examined the toughness of a reproducible heat cycle HAZ which simulated welding with a proof stress of 600 ° C. and heat input of 200 kJ / cm. The chemical composition of the steel used is 0.11% C-1.2% M
A hot-rolled material whose toughness index is changed in the range of 0.16 to 1.6 by changing V, Si and Al with n-0.53% Mn-0.013% Ti as a central component. is there.

FIG. 1 shows the result of this test, V / (Si
+ 1.5Al), 60 of V and (Si + 1.5Al)
It is a drawing showing the effect on 0 ° C proof stress and HAZ toughness. According to the figure, by setting the toughness index to 0.3 or more and setting V to 0.05 to 0.08%, the HAZ toughness can be obtained even when welding with a heat input of 200 kJ / cm is performed. Is good, and the proof stress at 600 ° C. satisfies the target value.

The present invention has been completed by combining the above findings with a reduction in the welding cold cracking susceptibility index (P _CM ) so that welding can be performed without generating cracks without preheating. It is.

[0025]

Next, the reason for limiting the present invention will be described.

1. Chemical composition of steel (base material) C: 0.05% to 0.15% C is an element necessary for obtaining strength. To obtain the desired strength, 0.05% or more is required. However, if the content exceeds 0.15%, the susceptibility to weld cracking is increased, so that the content is limited to 0.1%.
15% or less. In order to obtain sufficient strength after further reducing the weld cracking susceptibility, the content is preferably set to 0.07 to 0.13%.

Si: 0.3% or less Si is used not only as a deoxidizing agent at the time of steel making but also to improve the strength at room temperature. However, in the present invention, relying on Si to improve the strength is minimized.
If the content of Si exceeds 0.3%, the toughness of the base material and the HAZ deteriorates. H exceeds 0.3%
In AZ, toughness is reduced by forming island martensite. In order to obtain better HAZ toughness, the content is desirably 0.1% or less. In the present invention, Si may not be substantially contained in the steel after deoxidation.

Mn: 1 to 1.8% Mn is an element effective for increasing the strength. However 1%
If it is less than 1.8%, the effect is small, and if it exceeds 1.8%, the effect of improving strength and toughness is saturated, and at the same time, the susceptibility to weld cracking becomes high.

Mo: 0.45% to 1% Mo has an effect of increasing the strength at high temperatures by precipitating fine carbides using the microstructure of the base material as bainite. However, when the content is less than 0.45%, the effect is small. On the other hand, when it exceeds 1%, the weldability is reduced.
1%.

V: 0.05-0.08% V added to supplement Mo forms V carbide when exposed to a temperature range exceeding 400 ° C., and has the effect of increasing the strength at high temperatures. For that purpose, the content must be 0.05% or more. On the other hand, if it exceeds 0.08%, the base metal toughness deteriorates.

Al: 0.04% or less Al is the sum of both Al as an oxide remaining in the steel acting as a deoxidizing agent and other Al. As long as deoxidation has been performed, both of Al need not be contained. However, the latter Al is included in the case where the toughness of the base material is further improved because it has the effect of generating AlN and improving the toughness of the base material through the refinement of the structure of the steel material.
However, if the sum of both exceeds 0.04%, the HAZ toughness is greatly degraded, so the content is set to 0.04% or less. HA more
In order to improve the Z toughness, the content is desirably 0.02% or less.

V (%) / ｛Si (%) + 1.5 Al
(%)｝: 0.3 or more When the above-mentioned toughness index is less than 0.3, even if V is in the range of 0.05 to 0.08%, the HAZ toughness is improved and 600 Since it is not possible to secure the proof stress in ° C, it is set to 0.3 or more. In order to obtain more excellent HAZ toughness and 600 ° C proof stress, the above-mentioned toughness index is desirably 0.35 or more. The upper limit does not need to be particularly set as long as the individual elements of V, Si and Al are limited to the above ranges.

P _CM : 0.15 to 0.25% The lower the P _CM, the more the welding can be performed without causing cracking even if the welding preheating temperature is lowered. Preheating at the time of welding at the construction site of a steel building structure significantly impedes the shortening of the construction period. Therefore, in order to weld the steel without preheating in the field, it is necessary that the P _CM below 0.25%. P _CM is improved weld cold cracking resistance as low as possible, but that there can not therefore 0.15% or more to secure the strength at 600 ° C. is less than 0.15%.

Other than the above elements, Cr: 0
1%, Ni: 0 to 0.75%, Cu: 0 to 1%, Nb:
0 to 0.05%, Ti: 0 to 0.05%, Ca: 0 to 0%
0.008%, Zr: 0-0.05% and B: 0
It may contain 0.005%. REM: La, C
Other elements such as e may be included.

All of these elements further improve the HAZ toughness when large heat input welding is performed, or
It is added to further improve the 0 ° C proof stress, but is limited to a certain range for reasons such as securing base metal toughness or HAZ toughness.

2. Manufacturing conditions The steel according to the present invention is generally manufactured under the following conditions. The smelting is performed by a converter or an electric furnace, and is made into a slab by a continuous casting method or the like, and then manufactured through hot rolling.
After this, heat treatment may be performed in some cases.

Next, the heating and rolling conditions in [Invention 2] will be specifically described.

2-1) Heating temperature: The heating temperature of steel (slab) is desirably 1000 to 1250 ° C. this is,
These carbides are present in steels containing Mo and V,
If the temperature is lower than 0 ° C., these solid solutions cannot be obtained, and it is difficult to strengthen the precipitation. However, if the heating temperature is 1
If the temperature exceeds 200 ° C., the austenite grains become coarse, which has an adverse effect on the base material toughness. Therefore, the heating temperature is desirably 1200 ° C. or lower.

2-2) Rolling temperature: The purpose of hot rolling is to form a steel into a required shape and at the same time to eliminate the cast structure by utilizing recrystallization by rolling and to transform from unrecrystallized austenite. To obtain a fine structure. For this purpose, it is necessary to perform rolling so that the cumulative draft at 900 ° C. or less and at _three or more Ar points is 25% or more. Rolling finishing temperature may become lower than Ar ₃ point but, Ar ₃
If the temperature is lower than the point, rolling anisotropy of mechanical properties appears, so that the rolling finishing temperature is desirably set to the Ar ₃ point or higher.

[0040]

Next, the effects of the present invention will be described based on examples.

Table 1 shows the chemical composition of the steel used in the practice. A slab having the chemical composition shown in Table 1 obtained by melting the converter was cut into a size that could be rolled by a rolling mill in a laboratory and used as a raw material for rolling.

[0042]

[Table 1]

Table 2 shows the rolling and cooling conditions for these rolling stocks. The thickness of the obtained steel plate is 1
There are two types, 2 mm and 28 mm.

[0044]

[Table 2]

The Charpy impact test of a welding heat cycle reproduction HAZ simulating the strength (YS, TS), toughness (vE0), proof stress at 600 ° C., and heat input of 200 kJ / cm at room temperature of the steel plate manufactured in this manner. The presence or absence of cracks in a y-type welding crack test (JIS Z3158: environmental temperature 25 ° C., humidity 60%) was examined.

Table 2 shows the results. Examples of the present invention are as follows: YS at room temperature: 315 MPa or more;
630 MPa, vE0: 60 J or more, proof stress at 600 ° C .:
217MPa or more, vE0: 60 in reproducible heat cycle test
J or more, all of the characteristics without cracks are satisfied in the y-type welding crack test without preheating.

On the other hand, Test No. 10 has a low C, and Test No. 11 has a high C, conversely, the room temperature strength is too high and the toughness of the base material and HAZ is low.

Test No. 12 has low room temperature strength due to low Mn. Test No. 13 has high Si and high strength due to low toughness index, but low HAZ toughness. Test No. 14 has low HAZ toughness due to high Al.

Test No. 15 has a low Mo and test No. 16 has a low V, so that both have a low 600 ° C. proof stress.

In Test No. 17, the base metal toughness was low because V was high. Further, Test Nos 18 to P _CM is excessive, is generating cracks in the y groove restraint cracking test.

Test Nos. 19 and 20 had low HAZ toughness due to low toughness index, test No. 21 had insufficient high temperature strength due to low V, and test No. 22 had low toughness due to excessive V. It is insufficient.

From these results, it is apparent that only in the range of the present invention, the steel satisfies the performance of the base material at ordinary temperature and 600 ° C. and has good weldability.

[0053]

According to the present invention, it is possible to obtain a steel material which is excellent in normal-temperature strength, has a small decrease in strength even when the temperature rises in the event of a fire, and can perform large heat input welding without preheating. This steel material is very useful as a steel material for a steel frame building that can reduce or omit the coating of the refractory.

[Brief description of the drawings]

FIG. 1 shows the effect of V / (S) on 600 ° C. proof stress and HAZ toughness.
3 is a drawing showing the effects of (i + 1.5Al), V and (Si + 1.5Al).

Claims (2)

[Claims] (1) C: 0.05 to 0.15% by weight%
Si: 0.3% or less, Mn: 1 to 1.8%, Mo: 0.
45-1%, V: 0.05-0.08% and Al:
Comprises 0.04% or less, and satisfies the following formula, high toughness heat-resistant steel for high heat input welding, wherein the weld cold cracking sensitivity index P _CM is 0.15 to 0.25%. V (%) / {Si ( %) + 1.5Al (%)} ≧ 0.3 ······· However, weld cold cracking sensitivity index _{P CM (%) = C (} %) + {S
i (%) / 30} + [{Mn (%) + Cr (%) + Cu (%)} / 2
0] + {Mo (%) / 15} + {Ni (%) / 60} + {V (%) / 1
0} + 5B (%) 2. The steel sheet according to claim 1, wherein the hot rolling of the steel is performed at a temperature of 900 ° C. or less.
_{2. The} method for producing a high toughness heat-resistant steel for large heat input welding according to claim 1, wherein rolling is performed so that the cumulative draft in _three or more temperature ranges is 25% or more.