JPH0820819A - Production of shape steel of low yield ratio for refractory use - Google Patents

Abstract

PURPOSE:To produce a shape steel having refractoriness and low yield ratio by forming a stock rolled into a shape steel while specifying the contents of C, Si, Mn, and Mo and rolling finishing temp., respectively, and then subjecting this shape steel to air cooling and to accelerated cooling under respectively specified conditions after the completion of rolling. CONSTITUTION:A stock, which has a composition containing, by weight, 0.03-0.18% C, 0.05-1.5% Si, 0.3-2.0% Mn, and 0.1-0.7% Mo and further containing, if necessary, one or >=2 kinds among 0.01-0.30% V, 0.003-0.030% Ti, 0.02-1.5% Cu, 0.02-1.5% Ni, 0.05-1.0% Cr, and 0.0005-0.0050% B and is rolled into a shape steel so that rolling finishing temp. (Tf) becomes not higher than the Ar3 point, is prepared. After rolling is finished, this stock is air-cooled for >=(Ar3-Tf)X0.8(sec) and then subjected to accelerated cooling down to 300-550 deg.C at a rate of (2 to 20) deg.C/sec. By this method, the shape steel with low yield ratio for refractory use, reduced in yield ratio at ordinary temp. and improved in high temp. strength, is obtained.

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 shaped steel having fire resistance and low yield ratio characteristics, which is mainly used as a structural member for construction.

[0002]

2. Description of the Related Art In the field of structural members for construction, along with the adoption of the limit state design method and the enactment of a new fireproof design method in April 1987, the low yield ratio and Higher strength at high temperature has been emphasized.

In response to such a trend, various methods of manufacturing a steel material having both a low yield ratio and fire resistance have been proposed. These are disclosed in JP-A-2-77523 and JP-A-2-1633.
No. 41 and Japanese Patent Laid-Open No. 3-271342, the rolling is mainly completed in the austenite high temperature region of Ar ₃ or more, and the increase of the yield ratio due to rolling in the γ + α2 phase region is avoided. There is.

On the other hand, in Japanese Unexamined Patent Publication (Kokai) No. 3-277715, rolling is performed in the γ + α2 phase region, immediately followed by accelerated cooling to yield a mixed structure of ferrite and bainite or martensite fixed by C and N. The strength is lowered and the uniform elongation is increased.

[0005]

In the shaped steel used for structural members for construction, the rolling finish temperature tends to decrease due to severe restrictions on the rolling shaping. Also series,
With the diversification of sizes, the rolling finishing temperature (defined in this application as the finishing temperature of a universal mill (R2) that substantially affects the material) changes over a wide range of 650 ° C to 850 ° C. In addition to such restrictions on rolling, in terms of material, a low yield ratio, high strength at high temperature, and low carbon equivalent are required.

In the above-mentioned conventional technique, in order to obtain a low yield ratio and a high strength at high temperature, a steel in which alloying elements such as Mo, Nb, and V are added together is employed, and austenite having an Ar ₃ point or more is used. Since the main focus is to finish rolling in the high temperature range, the requirement of low yield ratio is satisfied when the ratio of rolling finish temperature in the γ + α2 phase region below the Ar ₃ point is high, as in shaped steel. Is difficult.

The present invention has been made in view of the above circumstances, and it is possible to manufacture a fire resistant shaped steel having a low yield ratio by using a rolling finishing temperature of ₃ points or less of Ar, and a low yield ratio for fire resistant. An object is to provide a method for manufacturing a shaped steel.

[0008]

In order to solve the above-mentioned problems, according to the present invention, firstly, in% by weight, C: 0.0.
3 to 0.18%, Si: 0.05 to 1.5%, Mn:
0.3-2.0%, Mo: 0.1-0.7% is contained,
After the rolling is completed (Ar ₃ −T _f ) × for the material rolled into the shape steel so that the rolling finishing temperature (T _f ) is equal to or lower than the Ar ₃ point.
A method for producing a low-yield ratio steel for refractory, characterized by allowing to cool for 0.8 (seconds) or more and then performing accelerated cooling at a rate of 2 to 20 ° C / s to a temperature range of 300 to 550 ° C. I will provide a.

Secondly, C: 0.03% by weight.
0.18%, Si: 0.05 to 1.5%, Mn: 0.3
.About.2.0%, Mo: 0.1 to 0.7%, V: 0.01 to 0.30%, Ti: 0.003 to 0.0
30%, Cu: 0.02-1.5%, Ni: 0.02-
1.5%, Cr: 0.05 to 1.0%, B: 0.000
5 to 0.0050% of one or two or more of them are contained, and the rolled steel having a rolling finish temperature (T _f ) of Ar ₃ point or less is used after rolling (Ar ₃ −T _f). ) ×
A method for producing a low-yield ratio steel for refractory, characterized by allowing to cool for 0.8 (seconds) or more and then performing accelerated cooling at a rate of 2 to 20 ° C / s to a temperature range of 300 to 550 ° C. I will provide a.

In order to manufacture a fire-resistant shaped steel which requires excellent high-temperature strength and a low yield ratio at room temperature from the viewpoint of fire resistance, the inventors of the present invention have a rolling finish temperature of Ar ₃ due to restrictions in shaping.
As a result of investigating the manufacturing method of shaped steel having excellent high temperature strength and low yield ratio even when the temperature is below the point, the composition system and cooling conditions after rolling were optimized, and work hardening especially by rolling in the γ + α range The inventors have found that the above characteristics are satisfied by intentionally allowing the ferrite to cool, recover and recrystallize, and then perform accelerated cooling, and completed the present invention.

Therefore, although the technique disclosed in Japanese Patent Laid-Open No. 3-277715 is similar to the technique disclosed in Japanese Patent Laid-Open No. 3-277715 in that it is rolled and accelerated cooled in the γ + α region, the Japanese Patent Laid-Open No. 3-277715 discloses accelerated cooling immediately after rolling. In contrast to performing accelerated cooling from the state in which the dislocation density of ferrite is high, the present invention is fundamentally in the point of performing accelerated cooling from the state in which the dislocation density of ferrite is low by recovery and recrystallization by intentionally allowing it to cool. different.

Among the above characteristics, Mo, Nb, and V have been known to be effective in increasing the high temperature strength. Therefore, M which is effective in increasing the high temperature strength in this way
Based on the composition shown in Table 1 in which o, Nb, and V were added, an experiment was conducted from the viewpoint of obtaining shaped steel with a low yield ratio. In addition, Table 1
Among them, Steel 1 is a Mo-V-added steel, Steel 2 is a Mo-added steel, and Steel 3 is a Mo-Nb-added steel. The method for obtaining the value of Ar ₃ is shown in the margin of Table 1.

[0013]

[Table 1]

FIG. 1 shows the case where the flange thickness 12 mm manufactured from the steel 1 shown in Table 1 has a R2 finishing temperature of 730 ° C. and the time until the start of cooling is changed. The tensile properties when cooled to 500 ° C at 5 ° C / s are shown. As shown in FIG, 8 kg / mm ² higher degree than the air-cooled material is accelerated cooling tensile strength (T
S) is obtained. TS hardly changes with the cooling time until the start of cooling, but the yield strength (YS) decreases as the cooling time increases until the cooling time reaches 25 seconds, but it remains almost constant beyond that. Become. Therefore, the yield ratio (Y
R) is 79% when cooled immediately after rolling, whereas it is 6% when accelerated cooling is performed after cooling for 25 seconds or more.
It drops to 6% or less.

FIG. 2 shows that, in the case of a shaped steel having a flange thickness of 12 mm produced by using the steels 2 and 3 shown in Table 1 and having a flange thickness of 12 mm, the rolling is completed (R2 ) Change the time until the start of cooling after 4 ℃
The tensile properties when cooled to 450 ° C. at 1 / s are shown. M
For steel 2 containing o, as in the case of steel 1 in FIG.
Up to 25 seconds, the YR decreases with an increase in the cooling time, and YR decreases, but it becomes saturated when the cooling time exceeds 25 seconds. On the other hand, in the case of steel 3 containing Mo and Nb, the YS decrease amount is small even if the steel is left to cool for 50 seconds after the end of rolling (R2) and then accelerated cooling is performed, and it is difficult to achieve low YR. . Therefore, it is understood that Nb addition is not desirable from the viewpoint of lowering the yield ratio.

FIG. 3 shows cooling in the case where a flange height of 9 mm and a shaped steel manufactured using the steel 1 shown in Table 1 change the web height and the rolling finishing temperature changes from 660 ° C. to 730 ° C. Change the time to start 5 ℃ / s
The change in the yield ratio (YR) when cooled to 500 ° C is shown. The number in the white circle in the figure is the yield ratio. As is clear from FIG. 3, at any rolling finishing temperature, YR decreases as the time until the start of cooling increases, and becomes substantially constant over a certain time. This limit time is t = (Ar3- _Tf )
It is shown by × 0.8. Here, Ar ₃ is the transformation start point of the sample steel, and T _f is the rolling finishing temperature.

As described above, when a rolling finishing temperature of Ar ₃ or less is adopted, Mo, which is effective for securing high temperature strength,
Among alloy elements such as Nb and V, Nb, which does not decrease the yield ratio little even when the yield ratio is increased and then the material is allowed to cool, is used, and steel mainly composed of Mo or Mo-V is used.
After the rolling is finished, t = (Ar ₃ −T _f ) × 0.8 seconds or more is intentionally allowed to cool and then accelerated cooling is performed, so that the yield ratio is significantly lower than that in the case of performing accelerated cooling immediately after rolling. And a low yield ratio steel for fireproofing having a tensile strength equal to or higher than that.

That is, M which is effective in increasing the high temperature strength
All of o, Nb, and V ensure high-temperature strength by precipitation strengthening, but when the rolling finishing temperature is below the Ar ₃ point, dislocations are introduced into the ferrite, and the above-mentioned precipitation-type elements fix such dislocations. Therefore, recovery / recrystallization of ferrite is suppressed, and as a result, it is difficult to obtain a low yield ratio. In particular, when accelerated cooling is performed after rolling in order to increase tensile strength, work-hardened ferrite exists even at room temperature because cooling is performed immediately after completion of rolling, and the yield ratio increases. Of these elements, Nb has the greatest effect of suppressing the recovery and recrystallization of such ferrite. Therefore, in the present invention, among the above-mentioned elements that are effective in increasing the high temperature strength, Nb, which has the largest effect of suppressing ferrite recovery and recrystallization, is not added, and the specific time determined by the Ar ₃ point and the rolling finishing temperature is not less than By allowing the material to cool for a period of time to recover and recrystallize the ferrite and then perform accelerated cooling, it has excellent high-temperature strength and low yield ratio even when the rolling is completed at the Ar ₃ point or less, and it is an air-cooled material. It is possible to manufacture shaped steel with improved tensile strength compared to.

Next, the reasons for limiting the content of each element in the present invention will be shown. In addition, all percentages are by weight.
C is an element effective for stably ensuring the room temperature strength and high temperature strength of steel. However, if it is less than 0.03%, it is difficult to obtain sufficient strength, and if it exceeds 0.18%, the weldability deteriorates. Therefore, the C content is 0.03 to 0.18.
% C.

Si is an element effective in deoxidizing and increasing the strength. For that purpose, it is necessary to add 0.05% or more, but if it exceeds 1.5%, the weldability is impaired. Therefore, the amount of Si is set to 0.05 to 1.5%.

Mn is an element effective in securing strength,
For that purpose, it is necessary to add 0.3% or more. on the other hand,
Addition exceeding 2.0% impairs weldability. Therefore, the Mn amount is set to 0.3 to 2.0%.

Mo is effective in increasing the strength of steel by improving hardenability and precipitation strengthening, and is particularly effective for medium and high temperature strength. 0.1% to exert such effect
Although the above is required, addition of more than 0.7% causes an increase in cost and deteriorates weldability. Therefore, the amount of Mo is set to 0.1 to 0.7%.

In the present invention, in addition to the above essential components, one or two of the following can be added, if desired.
V is effective for increasing the strength at ordinary temperature and high temperature even when added in a small amount, and for this purpose, addition of 0.01% or more is necessary. On the other hand, addition of more than 0.3% deteriorates weldability.
Therefore, the V amount is set to 0.01 to 0.30%.

Ti has the effect of forming TiN and refining the austenite grains, and is effective in improving the toughness. To achieve this effect, 0.003% or more is necessary.
When it exceeds 0.03%, TiC is formed, and like the above-mentioned Nb, the recovery of ferrite during rolling at the Ar ₃ point or less.
Recrystallization is significantly suppressed. Therefore, the Ti content is 0.003
Was set to 0.030%.

Cu is an element effective for increasing the strength, and for that purpose 0.02% or more is necessary. However, addition of more than 1.5% causes an increase in cost and a surface flaw problem.
It was set to 0.02 to 1.5%.

Ni is effective not only for strengthening but also for improving the low temperature toughness. For that purpose, 0.02% or more is required, but since it is expensive, it is 0.02% to 1.5%. .

Cr is effective for increasing the strength at ordinary temperature and high temperature, and for that purpose, it is necessary to add 0.05% or more, but if it exceeds 1.0%, the weldability deteriorates. Therefore Cr
The amount was 0.05-1.0%.

B is an element that improves the hardenability, and from this viewpoint, 0.0005% or more is necessary, but 0.00
If it exceeds 5%, the weldability is deteriorated. Therefore, the B content is set to 0.0005 to 0.005%.

Next, the reasons for limiting the manufacturing conditions will be described. Of the manufacturing conditions, what is important in the present invention is that after the rolling is completed, it is assumed that the rolling finishing temperature is at or below the Ar ₃ point.
The cooling time before the accelerated cooling and the accelerated cooling conditions are other conditions, and other conditions are not particularly specified.

The cooling time after completion of rolling in the present invention is
Determined by the value of Ar ₃ and the rolling end temperature T _f , (A
r ₃ −T _f ) × 0.8 seconds or more. Within this range, ferrite recovery / recrystallization sufficiently occurs before the start of accelerated cooling, and a sufficiently low yield ratio can be obtained.

The purpose of accelerated cooling is to achieve higher strength in steel of the same composition as compared with the cold-release material. Therefore, the cooling rate is required to be 2 ° C./s or more. Further, at a cooling rate exceeding 20 ° C / s, cooling strain becomes remarkable, so the range was set to 2 to 20 ° C / s. The cooling stop temperature is 300 to 5
When the temperature is out of the range of 50 ° C., the effect of accelerated cooling cannot be obtained, and the stability and distortion of the characteristics also increase, so the above range was set.

As described above, other conditions are not specifically defined as general conditions are adopted, but the heating temperature for rolling is preferably 1000 to 1350 ° C. If the temperature is lower than 1000 ° C., the rolling finishing temperature becomes extremely low, so that it is difficult to perform rolling to the final shape. On the other hand, if the temperature exceeds 1350 ° C., the heating cost remarkably increases.

In the present invention, the case where the rolling finishing temperature is below the Ar ₃ point is targeted, but the temperature is 600.
It is preferable that the temperature exceeds ℃. Rolling finish temperature is 600 ℃
This is because in the following, the cooling time for causing recovery and recrystallization of the processed ferrite becomes extremely long.

[0034]

Embodiments of the present invention will be described below. (Example 1) Using steels 4 and 5 shown in Table 2, section steels were produced under various production conditions shown in Table 3 when rolling was completed at an Ar ₃ point or less. Then, the strength characteristics of these shaped steels were evaluated. The results are also shown in Table 3.

In Table 3, symbols A to E are steels 1250, which are shaped steels having a flange thickness of 25 mm and a web height of 750 mm.
It is the result when it was heated to ℃ and rolled. The rolling finish temperature in this case is 770 ° C. A is the case where accelerated cooling was performed immediately after the completion of rolling, and the cooling time until the start of accelerated cooling was 10 seconds, and the cooling time specified in the present invention was:
Since (Ar ₃ −T _f ) × 0.8 seconds or more (25 seconds or more in this case) is not satisfied, the yield ratio is as high as 78%.

On the other hand, B and C are intentionally left to cool for 25 seconds or longer after completion of rolling to accelerate ferrite recovery / recrystallization and then accelerated cooling, so that the yield ratio is 72%. And low. Further, D, which was allowed to cool after rolling, and E, which was subjected to accelerated cooling but completed accelerated cooling at 650 ° C., which is higher than the condition range of the present invention, are TS at room temperature and higher temperature regions than B and C, which are the ranges of the present invention. YS is low.

The symbols F, G, and H are the results when the steel 4 was also used and a shaped steel having a Franci thickness of 25 mm and a web height of 700 mm was heated to 1200 ° C. and rolled,
The rolling finishing temperature is 740 ° C. F, G, although H is then the cooled either after it is completed rolling intentional, F is allowed to cool time specified in the present invention: (Ar3 -T _f) × 0.8 seconds or more (in this case 49 The yield ratio is as high as 81% because it does not satisfy (sec or more), while the yield ratio of G and H is as low as 73% because it satisfies the conditions of the present invention.

The symbols I, J, K, and L are made of steel 5, and 125 shaped steel having a flange thickness of 16 mm and a web height of 650 mm is used.
It is a result when it is heated to 0 ° C. and rolled. In this case, the rolling finishing temperature is 730 ° C. Compared to J and K satisfying the conditions of the present invention, they are the cooling conditions until accelerated cooling.
I which does not satisfy the second or more has a high yield ratio. Further, in L that does not satisfy the accelerated cooling conditions, TS at room temperature and YS at high temperature are low.

The symbols M, N, and O use steel 5 and form steel having a flange thickness of 19 mm and a web height of 750 mm at 1200 ° C.
It is the result when it is heated and rolled. The rolling finishing temperature in this case is 680 ° C. Compared with O satisfying the conditions of the present invention, the yield ratios are higher in M and N which do not satisfy the cooling condition of 66 seconds or longer until accelerated cooling.

[0040]

[Table 2]

[0041]

[Table 3]

(Example 2) Using the steels 6 to 16 shown in Table 4, section steels were produced under various production conditions shown in Table 5 when rolling was completed at an Ar ₃ point or less. Then, the strength characteristics of these shaped steels were evaluated. The results are also shown in Table 5.

In Table 5, symbols P to U are steels 6 to 11,
This is a result when a shaped steel having a flange thickness of 19 mm and a web height of 700 mm was heated to 1280 ° C., rolled, allowed to cool for 40 seconds, and then accelerated cooling to 450 ° C. at 10 ° C./s. P, Q, and R satisfying the conditions of the present invention are TS of 70 kgf / mm ² or more at room temperature, YR of about 70%, and Y of about 30 kgf / mm ^{2 at} 600 ° C.
In contrast to S, which contains S in an excess amount of Ti and T which contains Nb, since the recovery of ferrite does not proceed sufficiently by cooling for 40 seconds until cooling, Y
R is as high as 76 to 77%. Further, U containing no Mo has a significantly lower YS at 600 ° C. as compared with the method of the present invention.

Symbols V to Z are steels 12 to 16 and 128 shaped steel having a flange thickness of 12 mm and a web height of 550 mm.
After heating to 0 ℃ and rolling, let it cool for 50 seconds, then 5 ℃
It is a result when accelerated cooling was performed to 540 ° C. at / s. V, W, X, and Y satisfying the conditions of the present invention are all TS of 50 kgf / mm ² or more at room temperature, YR of about 70%, YS of about 20 kgf / mm ^{2 at} 600 ° C.
However, Z containing no Mo has a significantly low YS at 600 ° C.

[0045]

[Table 4]

[0046]

[Table 5]

[0047]

As described above, according to the present invention, in the shaped steel rolling in which the rolling finishing temperature is at or below the Ar ₃ point, cooling is allowed to cool for a predetermined time after the rolling finishing, and recovery of ferrite and recrystallization proceeded. By performing the post-acceleration cooling, it becomes possible to manufacture a low-yield ratio steel for refractory having a low yield ratio at room temperature and a high strength at high temperature.

[Brief description of drawings]

FIG. 1 is a diagram showing a change in normal temperature tensile properties with a cooling time after rolling until accelerated cooling.

FIG. 2 is a diagram showing a change in normal temperature tensile properties with a cooling time after rolling until accelerated cooling.

FIG. 3 is a diagram showing a cooling time until accelerated cooling and a value of yield strength when Ar ₃ −T _f is changed.

Claims (2)

[Claims] 1. C: 0.03 to 0.18% by weight,
Si: 0.05 to 1.5%, Mn: 0.3 to 2.0%,
Mo: 0.1-0.7% is contained, and rolling finish temperature (T
_f ) is left to cool for a period of (Ar ₃ −T _f ) × 0.8 (seconds) or more after the material is rolled into a shape steel so that Ar ₃ points or less, and then 2 to 20 ° C. / 300 ~ 5 at s speed
A method for producing a low-yield ratio steel for refractory, which comprises performing accelerated cooling to a temperature range of 50 ° C. 2. C: 0.03 to 0.18% by weight,
Si: 0.05 to 1.5%, Mn: 0.3 to 2.0%,
Contains Mo: 0.1 to 0.7%, and further V: 0.01
~ 0.30%, Ti: 0.003 to 0.030%, C
u: 0.02-1.5%, Ni: 0.02-1.5%,
Cr: 0.05-1.0%, B: 0.0005-0.0
After completion of rolling (Ar ₃ −T _f ) × for a material that contains one or more of 050% and is shaped steel rolled so that the rolling finishing temperature (T _f ) is at or below the Ar ₃ point. 0.8
(Sec) Allowed to cool for a second or more, and then accelerated cooling to a temperature range of 300 to 550 ° C. at a rate of 2 to 20 ° C./s, and a method for producing a low yield ratio section steel for fireproofing.