JPH0734123A - Method for manufacturing steel with excellent fire resistance and less ultrasonic anisotropy - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a method for manufacturing a steel material having high fire resistance and low acoustic anisotropy. [Structure] A steel piece is heated at 1100 ° C to 1250 ° C,
Rolling is carried out under the condition that the rolling end temperature is 850 ° C. or higher, followed by cooling at a cooling rate of 5 to 30 ° C./sec and the end temperature of 550 ° C. or higher, and then air cooling. [Effect] To provide a steel material having a fire resistance having a proof stress of 600 ° C. of ⅔ or more of a normal temperature proof stress and a sufficiently small acoustic anisotropy. Since the acoustic anisotropy is small, the position, size, etc. of the unhealthy portion can be correctly detected, and the reliability of the pass / fail judgment of the welded portion is greatly improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel material having excellent fire resistance and a low ultrasonic anisotropy, which is used in the fields of construction, civil engineering, marine structures and the like.

[0002]

2. Description of the Related Art Various rolled materials are specified by JIS G 3101 and others and are used as materials for construction structures. However, this type of steel is poor in fire resistance due to fire, and a fireproof coating must be applied. As a technique for improving this point, Japanese Patent Publication No. 4-50362 proposes a method for manufacturing a building steel material having excellent fire resistance. The characteristic of this method is that a steel slab obtained by adding a trace amount of Nb and an appropriate amount of Mo to a low C-low Mn steel is reheated at a high temperature and then heated to a relatively high temperature (800
Since the rolling ends at ~ 1000 ° C) and the microstructure has a relatively large ferrite-based composition, the yield strength at 600 ° C is sufficiently higher than that of the conventional fine-grained ferrite-based composition. It is a thing. In the above method, the steel material after rolling is air-cooled (natural cooling in the atmosphere), but this is "as-rolled" (meaning as-rolled).
It is often written as.

The steel material produced by the above method has excellent high-temperature characteristics, and has an uncoated or conventional refractory coating of 20-5.
Since the fireproof purpose can be achieved with a coating thickness of 0%, it is said that the cost required for fireproof construction can be significantly reduced.

[0004]

On the other hand, in a building structure in which refractory steel is mainly used, in order to guarantee the soundness of the welded part, the oblique angle ultrasonic wave of the welded part specified in JIS Z 3060 or the like is used. A flaw detection test (hereinafter referred to as "angled UST") is required. In the oblique angle UST, as is well known, an ultrasonic pulse wave is obliquely incident from the surface into the steel material from the oblique probe. If there is an unhealthy portion inside the material, sound waves will be reflected from the unhealthy portion. Presence of this reflected sound wave,
The oblique angle UST is to judge the existence and degree of the unhealthy portion based on the height and the spread.

By the way, it has been found that the refractory steel manufactured by the above manufacturing method has different sound wave propagation speeds, that is, sonic speeds, in the rolling direction and the direction orthogonal to the rolling. It is called "acoustic anisotropy" that the speed of sound is different between the rolling direction and the direction orthogonal to the rolling. If this acoustic anisotropy is large, the position and size of the unhealthy portion in the oblique angle UST is naturally wrong. , The reliability of the pass / fail judgment of the welded part is impaired, and the repair work of the welded defective part is hindered.

Therefore, an object of the present invention is to provide a manufacturing method capable of reducing acoustic anisotropy in refractory steel.

[0007]

The inventors of the present invention have found that the refractory steel is an as-rolled material, and that hot rolling that terminates at a relatively high temperature is an isotropic metallographic structure in the rolling direction and the direction perpendicular to the rolling direction. It was found that the acoustic anisotropy increased as a result. Then, as a result of continued earnest research, they succeeded in reducing acoustic anisotropy by performing an appropriate treatment in a cooling process subsequent to rolling.

Specifically, the present invention is a weight ratio of C: 0.
03-0.15%, Si: 0.05-0.6%, Mn:
0.3-1.6%, Mo: 0.3-0.7%, V: 0.
A steel slab consisting of 03 to 0.06%, Al: 0.005 to 0.10%, the balance: Fe and unavoidable impurities is
It is characterized in that it is heated at 0 ° C. to 1250 ° C., rolled at a rolling finish temperature of 850 ° C. or higher, and subsequently cooled at a cooling rate of 5 to 30 ° C./s and a finish temperature of 550 ° C. or higher, and then air cooled. It is a method for producing a steel material having excellent fire resistance and less ultrasonic anisotropy.

That is, the most important feature of the present invention is that the hot rolling is followed by the controlled moderate cooling. Moderate cooling means slower cooling than rapid cooling (quenching) for quenching and faster than natural cooling by the atmosphere.

Next, the composition of the steel slab according to the present invention (% by weight)
Will be described. C (carbon) is necessary to secure the strength of the base material and the welded portion and to exert the effect of adding Mo. Since the effect is weakened at 0.03%, the lower limit is made 0.03%. Further, if the amount of C is too large, not only the low temperature toughness of HAZ (welding heat affected zone) is adversely affected, but also
Since the base material toughness and weldability are also reduced, the upper limit is 0.15%. Si (silicon) is an element contained in the deoxidized upper steel. Since the weldability and HAZ toughness decrease when the content of Si (silicon) increases, its upper limit is set to 0.6%.

Mn (manganese) is an essential element for ensuring strength and toughness, and its lower limit is 0.3%.
However, if the amount is too large, not only the hardenability becomes too high, but the weldability and HAZ toughness deteriorate, and it becomes impossible to obtain the base metal strength that meets the target specifications, so the upper limit is 1.6%. And Both Mo (molybdenum) and V (vanadium) form fine carbides and are effective in increasing the yield strength at high temperatures, but it is difficult to obtain yield strength of 60% or more at room temperature at 600 ° C alone. Therefore, Mo and V are added in pairs, and 0.3% and 0.03%, which are the amounts necessary for exhibiting the performance, are set as the respective lower limit values. Further, addition of a large amount may reduce weldability, so the upper limits are made 0.7% and 0.06%, respectively. Al (aluminum) is effective for refining crystal grains, and is 0.0
It is effective at more than 05%. Further, addition of a large amount forms inclusions and reduces toughness, so the upper limit is made 0.1%.

A steel slab having the above composition is heated in a steel slab heating furnace, rolled by a hot rolling mill to a predetermined plate thickness, and subsequently subjected to controlled cooling. These operating conditions will be described. The heating temperature of the billet is important, and in order to increase the high temperature yield strength of the steel, it is necessary to sufficiently dissolve the added Mo and V, which is the composite addition, and the lower limit of the heating temperature is 1100 ° C. Also, 1250
When heated above 0 ° C, the crystal grains become coarse and the low temperature toughness decreases, so the upper limit of the heating temperature is set to 1250 ° C.

A steel slab heated in the range of 1100 to 1250 ° C. is rolled by a hot rolling mill to a predetermined plate thickness, and the rolling end temperature is 850 ° C. or higher. If rolling is performed at less than 850 ° C, Mo carbide precipitates and a uniform bainite structure cannot be obtained in the controlled cooling step after rolling. Therefore, the rolling end temperature is set to 850 ° C or higher.

The cooling rate of the controlled cooling that follows rolling is extremely important. This cooling rate is 5 ° C /
If it is less than sec, a desired bainite structure cannot be obtained, and if it exceeds 30 ° C./sec, martensite is precipitated and a bainite-martensite mixed grain structure cannot be obtained and a uniform structure cannot be obtained. Therefore, the cooling rate of the controlled cooling is 5 to
Select from the range of 30 ° C / sec. By performing this controlled cooling, it is possible to obtain a uniform bainite structure, and as a result, it is possible to reduce the acoustic anisotropy to a predetermined level. Controlled cooling is stopped at 550 ° C or higher. This is because when cooled to 550 ° C. or lower, martensite and massive bainite are precipitated and a uniform bainite structure cannot be obtained.

[0015]

EXAMPLES Examples of the present invention will be shown below, but the present invention is not limited thereto. Examples 1 to 8 and Comparative Examples 1 to 8; Table 1 is a list of components (% by weight), processing conditions and plate thicknesses of Examples 1 to 8 and Comparative Examples 1 to 8 according to the present invention. is there. The heating temperature under the processing conditions is the extraction temperature of the billet heating furnace. The rolling start temperature of the billet is almost the same as the above heating temperature.
The cooling start temperature is a temperature at which controlled cooling is started, and this cooling is generally carried out by a water mist or shower cooling equipment provided at the exit of the rolling mill. The rolling end temperature is almost the same as the cooling start temperature. The cooling stop temperature is a temperature at which cooling with controlled cooling rate ends. After this, the steel plate is naturally cooled by the atmosphere (called air cooling or air cooling).
To be done.

[0016]

[Table 1]

The range of the present invention is shown in the first column of the table, and the components or processing conditions outside this range are marked with * for easy understanding. That is, in comparison with the present invention, V is too small in Comparative Example 1, Mo is too small in Comparative Example 2, V is too small in Comparative Example 3, cooling start temperature is too low in Comparative Example 4, and Comparative Example 5 is The cooling rate was too high, the cooling stop (end) temperature was too low in Comparative Example 6, and the cooling stop temperature was too low in Comparative Example 7, so tempering (700 ° C tempering) was applied, and Comparative Example 8 had a low heating temperature. Pass.

Table 2 summarizes the room temperature strength, YS, YS, YS / YS and evaluation of Examples 1 to 8 and Comparative Examples 1 to 8. The YS is JIS G 31
It is the lower limit of YS (proof strength) of SM490B specified in 06, specifically 315 when the plate thickness is 16 or more and less than 40 mm.
N / mm ² , 295 N / m when plate thickness exceeds 40 and 100 mm or less
m ² . 315 or 295 is described in the YS column depending on the plate thickness specified in Table 1. YS is a measured value of yield strength at 600 ° C.

[0019]

[Table 2]

Since it is a refractory steel, the yield strength at 600 ° C. must be at least 2/3 of the room temperature yield strength. Therefore,
Check the ratio of YS that is 600 ° C proof stress and YS that is room temperature proof and enter it in the YS / YS column, and in the evaluation column, ○ (66.7% or more), × (less than 66.7%)
Was stated and it was clarified whether or not it is compatible with refractory steel.

Table 3 summarizes the sound velocity ratios of Examples 1 to 8 and Comparative Examples 1 to 8 and their evaluations.

[0022]

[Table 3]

Acoustic anisotropy means that the speed of sound in the rolling direction and the speed of sound in the direction perpendicular to the rolling are different, and it is important whether or not the difference (or ratio) between them is within an allowable range. Architectural Institute of Japan Ultrasonic flaw detection standard for steel structure welded parts ・ Sound velocity ratio control value stipulated in the manual is plate thickness 25mm
For the following, the range of 0.990 to 1.020 (this is described as "A" in the table), and for the plate thickness exceeding 25 mm and 75 mm or less, the range of 0.995 to 1.015 (this is It is described as "B" in the table). On the other hand, V _STB is the sound velocity of the JIS Z 2347 A1 standard test piece for ultrasonic oblique flaw detection (STB-A1), V _L is the sound velocity in the rolling direction of the test material, and V _C is the sound velocity in the direction perpendicular to the rolling of the test material. is there. _VL
/ V _STB or V _C / V _STB is listed in the table, and these are compared with the sound velocity ratio control values A and B, respectively, and those within that range are marked with a circle in the evaluation column. The bran was represented by x.

If both V _L / V _STB and V _C / V _STB are ◯, the acoustic anisotropy is sufficiently small and there is no problem. It was found that Comparative Examples 4, 5, 6, and 7 had x on one side and large acoustic anisotropy, which was not acceptable.

Thus, from Table 3, Comparative Examples 4, 5, 6, 7
Is unacceptable in terms of acoustic anisotropy, and from Table 2, Comparative Examples 1, 2, 3 and Comparative Example 8 are unacceptable in terms of fire resistance.
In this respect, since Examples 1 to 8 pass both the fire resistance and the acoustic anisotropy, the present invention based on the Examples provides a steel material having excellent fire resistance and less ultrasonic anisotropy.

[0026]

As described above, the present invention has a weight ratio of
C: 0.03 to 0.15%, Si: 0.05 to 0.6
%, Mn: 0.3 to 1.6%, Mo: 0.3 to 0.7
%, V: 0.03-0.06%, Al: 0.005-
A steel slab consisting of 0.10%, balance: Fe and unavoidable impurities is heated at 1100 ° C to 1250 ° C, rolled at a rolling end temperature of 850 ° C or higher, and subsequently cooled at a cooling rate of 5 to 3.
Cooling under conditions of 0 ° C./sec and ending temperature of 550 ° C. or higher, and then air-cooling provide 600 ° C. proof stress that is 2/3 or more of room temperature proof stress and sufficiently small acoustic anisotropy. Since the acoustic anisotropy is small, the position, size, etc. of the unhealthy portion can be correctly detected, and the reliability of the weld pass / fail judgment is greatly improved.

Claims (1)

[Claims] 1. A weight ratio of C: 0.03 to 0.15%,
Si: 0.05-0.6%, Mn: 0.3-1.6%,
Mo: 0.3-0.7%, V: 0.03-0.06%,
Al: 0.005 to 0.10%, balance: Fe and steel inevitable impurities are heated at 1100 ° C to 1250 ° C, rolled at a rolling end temperature of 850 ° C or higher, and then continuously cooled at a cooling rate of 5 to 5. 30 ° C / sec and end temperature 55
A method for producing a steel material having excellent fire resistance and little ultrasonic anisotropy, which is characterized by cooling under conditions of 0 ° C. or higher and then air cooling.