JP3468072B2 - Manufacturing method of low yield ratio section steel - Google Patents

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 low yield ratio section steel used for steel structures such as high-rise buildings.

[0002]

2. Description of the Related Art In today's high-rise buildings, when a huge earthquake strikes, the plastic energy of the columns and beams absorbs the seismic energy and avoids a major collapse. Design method is applied. Therefore, the column / beam members used in the limit state design method should have a low yield ratio (YR) as a measure of high plastic deformability, that is, a low yield ratio is desired. Is said to be excellent.

From the viewpoint of plastic deformation, it is preferable that YR from the design side of this building structure is as low as possible within the range in which strength can be secured, and 590 N / mm.
^In the case of the ^second class, the allowable range of YR is 80% or less, but 75% is listed as the target value. Regarding the low yield ratio, generally, as represented by a method of performing an intermediate heat treatment of heating in a two-phase region during quenching and tempering, ferrite as a soft phase and bainite or martensite as a hard phase are generally used. It is known that this is achieved by a ferrite + hard phase structure in which a mixture of As a conventional technique for obtaining this ferrite + hard phase structure, the above-mentioned method of quenching-two-phase region quenching-tempering, a method of accelerating cooling after waiting until the two-phase region of ferrite and austenite after hot rolling, etc. However, these techniques inevitably reduce productivity due to the need for complicated heat treatment steps and the long waiting time before the start of quenching.

As a method of avoiding this, Japanese Patent Publication No. 7-74379 and Japanese Unexamined Patent Publication No. 5-27176 disclose a steel sheet.
The technique of Japanese Patent No. 1 is disclosed. Japanese Patent Fair 7-7437
In both No. 9 and JP-A-5-271761, after hot rolling, pre-cooling is performed to Ar ₃ −20 ° C. or lower and Ar ₃ −100 ° C. or higher, and then the steel sheet surface is reheated to Ar ₃ −100 ° C. or higher. , Again at a cooling rate of over 15 ° C./sec.
It is to cool to 600 ° C.

[0005]

However, Japanese Patent Publication No. 7-74379 and Japanese Unexamined Patent Publication No. 5-271761 do not specify the reheat time during cooling, so that the structure is controlled to a suitable low yield ratio. It is difficult to do so, and a decrease in the manufacturing stability of low yield ratio steel is inevitable. Further, the water cooling equipment in the production of shaped steel generally has a lower cooling capacity than the water cooling equipment for steel plates, and it is difficult to cool at a cooling rate exceeding 15 ° C / sec. Therefore, there is a need for a technique capable of producing a low yield ratio section steel inexpensively and in large quantities without impairing the productivity without using such a high cooling rate. The purpose of the present invention is to
It is an object of the present invention to provide a method for producing a low yield ratio shaped steel, which can inexpensively and stably produce a large amount of shaped steel having a yield ratio of 75% or less used for high-rise buildings and the like.

[0006]

In order to solve the above problems and achieve the object, the present invention uses the following means. (1) In the manufacturing method of the present invention, C: 0.04% by weight.
0.18%, Si: 0.05 to 0.5%, Mn:
0.6-1.7%, P ≦ 0.05%, S ≦ 0.01
%, Al ≤ 0.08%, N ≤ 0.008%, and the balance is Fe and inevitable impurities.
_A step of performing hot rolling with a surface reduction rate of 50% or more in a temperature range of ₃ or more, and a hot rolled steel material from Ar ₃ or more to A
Accelerated cooling to a temperature range of r ₃ -100 ° C to Ar ₃ -20 ° C at a cooling rate of 1 ° C / sec or more, standby temperature: T (° C) = Ar
₃ -100 ℃ _{~Ar 3} -20 below in ° C. (1) satisfies the formula waiting time: the step of performing standby in t (seconds), the flange
A steel material having a thickness: h (mm) is acceleratedly cooled to a temperature range of 400 to 600 ° C. at a cooling rate satisfying the following expression (2): v (° C./sec), and a tensile strength is 490 N / mm ² or more, Katsuri
And a step of forming a low yield ratio shaped steel having a yield ratio of 75% or less, and a method for producing a low yield ratio shaped steel.

10 ^{1.2-0.01 × ΔT} ≦ t ≦ 150 seconds (1) where ΔT = Ar ₃ (° C.) − T (° C.) 0.9-0.4 logh ≦ logv ≦ 2.6-0.9 l
oh ... (2) However, h: Flange thickness (mm) (2) In the manufacturing method of the present invention, as a steel component, further, by weight%, Cu: 0.05 to 1%, Ni: 0.05 to 0. .8
%, Cr: 0.05 to 1%, Mo: 0.01 to 1%, N
b: 0.005-0.1%, V: 0.005-0.1%
And Ti: one or two or more selected from the group of 0.005 to 0.03% is contained, and the method for producing a low yield ratio section steel according to the above (1) is characterized.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In order to obtain a technique for producing a low yield ratio section steel without waiting for a long time and heat treatment after rolling, the present inventors have made a study on the standby temperature and the standby temperature that affect the strength and yield ratio of steel materials. As a result of diligent examination of the influence of time and the cooling rate after waiting, the following findings have been obtained. Figure 1
0.14C-0.25Si-1.2Mn steel at 1250 ° C
And reduce the surface area by 80% to reduce the Ar ³ point (771
Tensile strength (tensile strength, yield ratio) when rolled into 900 mm higher H-section steel with a flange thickness of 20 mm, cooled to a predetermined temperature at 2 ° C./sec and held, and further cooled to 550 ° C. (YR)) is shown. As shown in FIG. 1, 10 ^{1.2-0.01 × ΔT} ≦ t ≦ 150 seconds (ΔT = Ar) at standby temperature: T (° C.) = Ar ₃ −100 ° C. to Ar ₃ −20 ° C.
_3- T) Waiting for tensile strength of 490 N / mm ²
It is clear that above (strength is secured) and the yield ratio is 75% or less.

FIG. 2 shows that 0.14C-0.25Si-1.
4Mn-0.04V steel is heated to 1280 ° C, and the flange thickness (h) is 950 ° C higher than Ar ₃ point (757 ° C).
The tensile properties are shown when H-section steels of 16 mm, 40 mm, and 100 mm are rolled, then cooled to 700 ° C. at 1.2 ° C./sec, held for 20 seconds, and then cooled to 500 ° C. at various cooling rates. As shown in FIG. 2, log in shaped steel
It is clear that the yield ratio becomes 75% or less by setting v ≧ 0.9−0.4logh.

Based on the above findings, the present inventors have found that hot rolling conditions and accelerated cooling conditions (standby temperature during second stage cooling, standby time and second stage cooling) to be applied to steel having a specific amount of chemical composition. (Including speed) is controlled within a certain range, and the steel structure has a mixed structure of ferrite and bainite, and low yield that can achieve tensile strength of 490 N / mm ² or more and low yield ratio of 75% or less as rolled. Finding a manufacturing method for ratio steel,
The present invention has been completed.

That is, according to the present invention, by limiting the steel composition and the manufacturing conditions to the following ranges, it is possible to inexpensively and stably manufacture a large amount of shaped steel having a yield ratio of 75% or less, which is used for high-rise buildings, etc. A method for manufacturing a yield ratio section steel can be provided. Reasons for adding the components of the present invention, reasons for limiting the components,
The reason for limiting the manufacturing conditions will be described.

(1) Component composition range C: 0.04 to 0.18% C is added in an amount of 0.04% or more in order to secure the strength of the steel, but is contained in a large amount exceeding 0.18%. Since the toughness or weldability deteriorates, the range is 0.04 to 0.
18%.

Si: 0.05 to 0.5% Si is an element necessarily contained in steel for deoxidation, and its content is 0.05% or more from the viewpoint of ensuring a low yield ratio. However, if the Si content exceeds 0.5%, HAZ
(Heat-affected zone) Since it has an unfavorable influence from the viewpoint of toughness and weldability, its range is 0.05 to 0.5%.

Mn: 0.6 to 1.7% Mn is required to be 0.6% or more for improving the strength and toughness of steel and suppressing FeS formation, but a large amount of addition exceeding 1.7%. Causes an increase in the hardenability of the steel, and a hardened layer appears during welding to increase the crack susceptibility, so the range is 0.6 to 1.7%. P ≦ 0.05%, S ≦ 0.01% P and S inevitably exist as impurities mixed in the steel, but reduction of P is effective in preventing intergranular fracture, and reduction of S is Since it is effective in preventing hydrogen cracking in the weld heat affected zone,
The P and S content ranges are 0.05% or less and 0.01, respectively.
% Or less.

Al ≦ 0.08% Al is an element contained in the deoxidized upper steel, but if contained in a large amount, the cleanliness of the steel is deteriorated and the toughness of the welded portion is deteriorated, so the range is 0. It is 0.08% or less.

N ≦ 0.008% N is an unavoidable impurity element contained in steel,
If the amount of N is large, the deterioration of the HAZ toughness and the occurrence of continuous cast slab scratches are promoted, so the range is 0.008% or less.

In the present invention, the above is the basic component, and one or more of the following selective component groups may be added. (Selected component group) Cu: 0.05 to 1% Cu is an element that is very effective in increasing strength and improving toughness, but if the content is less than 0.05%, sufficient effect is not exhibited and 1%. When the content of Cu is more than 0.03%, the precipitation hardening is remarkable and the surface of the steel material is apt to crack.

Ni: 0.05 to 0.8% Ni has the effect of improving the strength and toughness of the base material, but if its content is less than 0.05%, a sufficient effect cannot be obtained. Since the addition of more than 8% leads to cost increase, the range is 0.0 when Ni is added.
5 to 0.8%. Cr: 0.05 to 1% Cr is an element effective for improving hardenability, but if its content is less than 0.05%, the effect is small, and if it exceeds 1%, weldability and HAZ toughness are deteriorated. Therefore, when Cr is added, the range is 0.05 to 1%. Mo: 0.01 to 1% Mo enhances hardenability and temper softening resistance, and is effective in increasing strength, but its content is 0.01%.
If it is less than 1%, the effect is not sufficiently exhibited, and if it exceeds 1%, the weldability is deteriorated and the yield ratio increases due to the precipitation of carbides. Therefore, when Mo is added, the range is 0.01 to 1%. is there.

Nb: 0.005 to 0.1% Nb is an element that effectively acts to increase strength and toughness due to the precipitation effect of fine carbonitrides, but its content is 0.
If it is less than 005%, the effect is not exhibited, and if it exceeds 0.1%, the precipitation ratio is hindered by the excessive precipitation effect. Therefore, when Nb is added, the range is 0.005 to 0.005.
It is 0.1%.

V: 0.005 to 0.1% V is an element that improves the hardenability and the temper softening resistance by adding a small amount, but its content is 0.005%.
If less than 0.1%, the effect is not sufficiently exhibited, and if added over 0.1%, the weldability deteriorates. Therefore, when V is added, the range is 0.005 to 0.1%.

Ti: 0.005-0.03% Ti is an element that suppresses the coarsening of the structure of the welded HAZ part of TiN and contributes to the improvement of HAZ toughness. 0.005%
If the amount of Ti is less than the above, the effect of improving the HAZ toughness will not be exhibited. If added in excess of 0.03%, TiC precipitates in the cooling process of welding, leading to deterioration of HAZ toughness. Therefore, when Ti is added, the range is 0.005-0.03%.

By adjusting the compositional range to the above range, shaped steel having a yield ratio of 75% or less, which is used for high-rise buildings and the like, can be stably obtained in large quantities at low cost without the need for plate heat treatment after rolling. It becomes possible.

The steel material having such characteristics can be manufactured by the following manufacturing method. (2) Steel material manufacturing process (manufacturing method) After the steel adjusted to the above component composition range is melted and the steel material obtained by continuous casting is heated to 1000 ° C. or higher, the area reduction rate is increased in the temperature range of Ar ₃ or higher. subjected to hot rolling 50% or more, then Ar ₃ -100 ℃ ~Ar ₃ -2
Accelerated cooling to a temperature range of 0 ° C at a cooling rate of 1 ° C / sec or more. Subsequently, the steel material that has been accelerated and cooled has a standby temperature of T (° C).
= Ar ₃ −100 ° C. to Ar ₃ −20 ° C., after waiting for time t seconds satisfying the following formula (1), 400 to
Accelerated cooling is performed up to a temperature range of 600 ° C. at a cooling rate v (° C./sec) that satisfies the following expression (2).

10 ^{1.2-0.01 × ΔT} ≦ t ≦ 150 seconds (1) where ΔT = Ar ₃ (° C.) − T (° C.) 0.9-0.4 logh ≦ logv ≦ 2.6-0.9 l
oh ... (2) where h: flange thickness (mm) a. Steel heating temperature: 1000 ° C. or higher The steel is heated to 1000 ° C. or higher in order to obtain good hot workability.

B. Hot rolling end temperature: to terminate the rolling at Ar ₃ or more than the Ar ₃ temperature zone, to suppress the development of texture, in order to eliminate the adverse effect sound anisotropy measurement accuracy of the ultrasonic flaw detection is there.

C. Ar ₃ or more reduction of area: the 50% or Ar ₃ or more reduction of area is 50% or more, the tissue was recrystallized with coarsened by heating, in order to improve the strength and toughness. d. It accelerated cooling speed before waiting: 1 ° C. / sec or more after the end of rolling to the accelerated cooling rate to Ar ₃ -100 ℃ ~Ar ₃ -20 ℃ and 1 ° C. / sec or more, Ar ₃ -100 from after rolling This is to prevent precipitation of a large amount of ferrite in the accelerated cooling process from ℃ to Ar ₃ -20 ℃. As a result, the structure is controlled mainly by the subsequent isothermal transformation during standby, and the accuracy of structure control can be improved. e. Standby temperature T (° C): Ar ₃ -100 ° C to Ar ₃ -2
0 ° C, standby time t (second): 10 ^{1.2-0.01 × ΔT ≤t≤1}
50 seconds, where ΔT = Ar ₃ (° C.) − T (° C.) As described in FIG. 1, the standby temperature T (° C.) is such that ferrite precipitates in a relatively short time Ar ₃ −100 ° C. to Ar
₃ -20 ℃, standby time 10 ^{1.2-0.01 × ΔT} ≤ t ≤
By controlling to a predetermined ferrite fraction by setting it to 150 seconds (ΔT = Ar ₃ −T), the remaining austenite is made hard bainite by further accelerated cooling, and finally the ferrite + bainite structure yield ratio ≦ 75. Achieve%. The upper limit of the waiting time is 150 seconds so as not to impair the productivity. When the standby temperature is higher than Ar ₃ −20 ° C., the rate of ferrite formation is slow, so a long standby time is required to obtain sufficient ferrite effective for a low yield ratio, and productivity is impaired. On the other hand, Ar ₃ -10
If the temperature is lower than 0 ° C., ferrite is excessively generated even in a short standby time, so that it becomes difficult to secure the strength.

Therefore, strength is secured and a low yield ratio (YR
≦ 75%), the standby temperature T (° C.) is Ar ₃ −1.
00 ° C to Ar ₃ -20 ° C, standby time t (second) is 10
^{1.2-0.01 × ΔT} ≦ t ≦ 150 seconds (where ΔT = Ar ₃
(° C) -T (° C)). f. Accelerated cooling rate v (° C / sec) after standby: 0.9-0.
4logh ≤ logv ≤ 2.6-0.9logh where : h: flange thickness (mm) As described in Fig. 2, considering the flange thickness in the above component range, the untransformed austenite after waiting becomes hard. To obtain bainite, logv ≧ 0.9−0.
A cooling rate satisfying 4 log is required. With water-cooling equipment generally used industrially, it becomes difficult to obtain a high cooling rate as the thickness increases. As a result of diligent study by the present inventors, 0.9-0.4logh≤logv≤2.6-0.
9 logh (here, h: thickness of steel material (flange) (m
m)), a yield ratio of 75% or less was obtained. Therefore, the accelerated cooling rate after waiting was 0.9-0.4logh≤logv≤2.6-0.9l.
and oh (here, h: flange thickness (mm) ).

G. Accelerated cooling stop temperature after standby: 400 ~
When the accelerated cooling stop temperature after standing by at 600 ° C. is less than 400 ° C., the island martensite generated during accelerated cooling remains without being decomposed, so that the toughness deteriorates. On the other hand, when the accelerated cooling stop temperature exceeds 600 ° C., it becomes difficult to secure the strength because the bainite transformation does not proceed sufficiently.

As described above, by adopting the above component system and the rolling / accelerated cooling conditions, the productivity is 490 N / m without impairing the productivity.
It is possible to secure a strength of m ² or more and a yield ratio of 75% or less. Examples of the present invention will be given below to prove the effects of the present invention.

[0030]

[Example] (Example 1) 490 MPa whose components are shown in Table 1
Rolling as shown in Table 2 using the present invention steels A and B,
Shaped steel manufactured under different cooling conditions (Example of the present invention: No. 1,
2, 6-8, 10, 11, 13-16, Comparative Example: No.
The mechanical properties of 3-5, 9, 12) were investigated. Table 2 shows the tensile properties (YS, TS, Y according to JIS Z 2241).
R) is also described.

As shown in Table 2, Comparative Example No. Since No. 3 does not satisfy the standby temperature of the present invention, ferrite is excessively generated and the target strength (490 MPa) cannot be secured. In addition, Comparative Example No. In Comparative Example No. 4, the cooling rate after waiting was slower than that of the conditions of the present invention. In No. 5, the standby time is shorter than that of the conditions of the present invention, so the YR (yield ratio) is high. Furthermore, Comparative Example N
o. Since No. 9 was left to cool after rolling, the target strength (490MP
Comparative example No. In No. 12, since the standby temperature is higher than that of the conditions of the present invention, ferrite is not sufficiently generated and YR is high.

On the other hand, the present invention example: No. 1, 2, 6
.About.8,10,11,13 to 16 all satisfy the conditions of the present invention, a sufficient tensile strength (T) of 490 MPa or more (T
S) and a yield ratio (YR) of 75% or less.

[0033]

[Table 1]

[0034]

[Table 2]

(Example 2) 490MP showing the components in Table 1
a-class invention steels C and D, 590 MPa-class invention steel E
The mechanical properties of shaped steels (Examples of the present invention: Nos. 17 to 28) produced by changing the rolling and cooling conditions as shown in Table 3 were investigated using. Table 3 also shows tensile properties (YS, TS, YR according to JIS Z2241).

As shown in Table 3, the invention sample No. 17-
24 (invention steels C and D of 490 MPa class) all satisfy the present invention conditions, so that they show sufficient tensile strength (TS) of 490 MPa or more and yield ratio (YR) of 75% or less, and invention example No. ． All of 25 to 28 (invention steel E of 590 MPa class) satisfy the requirements of the present invention, and thus 590MP
It shows a sufficient tensile strength (TS) of a or more and a yield ratio (YR) of 75% or less.

[0037]

[Table 3]

[0038]

According to the present invention, a low yield ratio shaped steel having a yield ratio of 75% or less, which is used for high-rise buildings and the like, can be produced as it is by specifying the steel composition and production conditions. Since there is no need for heat treatment, productivity and economic efficiency can be significantly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the effects of standby temperature and standby time on the strength and yield ratio of a steel material according to an embodiment of the present invention.

FIG. 2 is a diagram showing an influence of a cooling rate after standby on a yield ratio of a steel material according to an embodiment of the present invention.

─────────────────────────────────────────────────── --- Continuation of front page (56) References JP-A 8-20819 (JP, A) JP-A 6-240350 (JP, A) JP-A 3-191020 (JP, A) JP-A 52- 14517 (JP, A) Japanese Patent Publication 7-74379 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/00-8 / 10,9 / 00 B21B 1/08 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.04 to 0.18% by weight
And Si: 0.05 to 0.5% and Mn: 0.6 to 1.
7%, P ≦ 0.05%, S ≦ 0.01%, Al ≦
0.08% and N ≦ 0.008% with the balance being F
In the method for producing a shaped steel consisting of e and unavoidable impurities, a step of heating the steel to 1000 ° C. or higher and then hot rolling at a surface reduction rate of 50% or higher in a temperature range of Ar ₃ or higher, and hot rolling. The steel material that has been subjected to Ar ₃ or more to Ar ₃ -100 ° C.
Accelerated cooling to a temperature range of Ar ₃ -20 ° C. at a cooling rate of 1 ° C./sec or more, standby temperature: T (° C.) = Ar ₃ -100 ° C.
A waiting time for satisfying the following formula (1) at Ar ₃ -20 ° C: waiting time t (seconds), and a steel material having a flange thickness h (mm) up to a temperature range of 400 to 600 ° C as described below (2 Cooling rate satisfying the formula: v (° C /
Seconds) accelerated cooling , tensile strength 490N / mm ² or more
And a step of producing a low yield ratio shaped steel having a yield ratio of 75% or less, and a method for producing a low yield ratio shaped steel. 10 ^{1.2-0.01 × ΔT} ≦ t ≦ 150 seconds (1) However, ΔT = Ar ₃ (° C.)-T (° C.) 0.9-0.4 logh ≦ log v ≦ 2.6-0.9 l
oh ... (2) where h: flange thickness (mm) 2. As a steel component, in addition to Cu:
0.05 to 1%, Ni: 0.05 to 0.8%, Cr:
0.05-1%, Mo: 0.01-1%, Nb: 0.0
05-0.1%, V: 0.005-0.1% and Ti:
The method for producing a low yield ratio section steel according to claim 1, characterized by containing one or more selected from the group of 0.005 to 0.03%.