JP2000239743A - Manufacture of high tensile steel of low yield ratio, reduced in difference in material quality in plate thickness direction - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for manufacturing a high tensile steel, capable of giving a stable material quality uniform in a plate-thickness direction and low yield ratio. SOLUTION: This process comprises steps of: producing a slab having a composition consisting of, by weight, 0.02-0.18% C, 0.05-0.5% Si, 0.6-1.7% Mn, <=0.08% Al and the balance Fe with inevitable impurities; heating the slab to >=1000 deg.C; hot rolling the heated slab at a temperature not lower than the Ar3 point at >=50% draft; cooling the hot rolled steel from a surface temperature of >=Ar3 down to <=(Ar3-150 deg.C) at a rate of (2 to 5) deg.C/s; cooling the cooled steel down to a temperature T deg.C between Ar3 and (Ar3-100 deg.C) at a rate of (2 to 15) deg.C/s; air cooling the steel of T deg.C for (t)s satisfying inequality 101.3-0.006×ΔT<=t<=150, wherein ΔT means (Ar3 transformation point -T)[ deg.C]; cooling the air cooled steel down to <400 deg.C at a rate of (2 to 15) deg.C/s; and tempering the cooled steel at a temperature between >500 deg.C and a temperature not higher than the Ac1 point.

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 high-strength steel material used for steel structures such as high-rise buildings, and more particularly to a method for producing a low-yield-ratio high-strength steel material having a small material difference in a thickness direction.

[0002]

2. Description of the Related Art In recent years, as buildings have become higher in height and larger in size, structural members used have been required to be thicker and have higher tensile strength. High tensile steel materials having a tensile strength of 490 N / mm ² or more have been used. It is becoming popular. In addition, today's high-rise buildings have a critical state design method that emphasizes human safety, which avoids large collapse by absorbing seismic energy by plastic deformation of columns and beam members when a huge earthquake is hit. Although it is applied, it is said that it is desirable to use a steel material having a low yield ratio for a column or a beam member.

[0003] Low yielding of a steel material is generally performed by a method of performing a complicated heat treatment in which a steel material after rolling is heated to a two-phase region of ferrite and austenite between quenching and tempering, or after hot rolling. Achieved by forming a mixed structure of ferrite + hard phase in which soft ferrite phase and hard bainite phase or martensite phase are mixed by air cooling the steel material to the two-phase region of ferrite + austenite and then accelerated cooling. Is known to be. However, in such a method, a complicated heat treatment or a long waiting time is required before the start of accelerated cooling, so that a reduction in productivity and an increase in manufacturing cost cannot be avoided.

[0004] As a method of higher productivity and lower cost, Japanese Patent Publication No. 7-74379 and Japanese Patent Application Laid-Open No. Hei 5-21761 disclose that a steel material after hot rolling is (Ar ₃ transformation point -100 ° C) or less. After pre-cooling to the temperature of (Ar ₃ transformation point −20 ° C.), the steel material is reheated until the temperature of the steel surface becomes (Ar ₃ transformation point −100 ° C.) or more, and the cooling rate again exceeds 15 ° C./sec. After cooling to a temperature of less than 400 ° C. by tempering at a temperature of 400 ° C. to the Ac ₁ transformation point, a method of lowering the yield ratio has been proposed.

However, Japanese Patent Publication No. 7-74379 and Japanese Patent Laid-Open No.
According to the method described in Japanese Patent No. 71761, there is a problem that a remarkable rise in strength occurs on the surface of the steel material, so that a uniform material cannot be obtained in the thickness direction, or that a low yield ratio cannot be obtained depending on a recuperation time after pre-cooling. There is.

On the other hand, Japanese Patent Application Laid-Open No. Hei 3-188216 discloses a method of achieving a uniform material in the thickness direction by water cooling a steel material rolled in an austenite recrystallization region from a temperature not lower than the Ar ₃ transformation point. Water cooling is stopped when the temperature reaches (Ar ₃ transformation point-150 ° C) or less, and water cooling is resumed after the surface temperature has returned to the temperature from the Ac ₁ transformation point to the Ar ₃ transformation point. Kaiping
No. 4-224623 discloses that steel having a specified carbon equivalent is 850-900.
After rolling at a temperature of ℃, from the temperature of (rolling temperature -50 ℃) or more to 40
How is cooled at 0 to 500 ° C. cooling rate 3 to 12 ° C. / sec until the temperature, also in JP-A-57-152430, after rolling in than the Ar ₃ transformation point temperature, the surface of the desired hard There is disclosed a method in which water is cooled at a water density that becomes small, and when the temperature reaches a temperature that determines the final hardness of the surface, the water density is increased to rapidly cool to make the hardness at the center almost equal to the hardness of the surface.

[0007]

However, according to the method described in Japanese Patent Application Laid-Open No. Hei 3-188216, the gas is generated by the first controlled cooling in the process of regaining the temperature from the Ac ₁ transformation point to the Ar ₃ transformation point. After reverse transformation of part of the hard bainite structure,
Reverse cooling austenite is transformed into hard martensite or bainite structure again due to rapid cooling at a water density of 0.6m ³ / (m ²・ min) or more, and the hardness difference in the sheet thickness direction cannot be reduced stably. There is. Further, in the method described in JP-A-4-224623, it is difficult to simultaneously control the hardness of the steel sheet surface and the hardness of the inside of the steel sheet at the same time, and the difference in hardness in the thickness direction increases as the cooling rate approaches the upper limit and becomes higher. That is inevitable. Furthermore, in the method described in JP-A-57-152430, the cooling stop temperature is not specified, and when the stop temperature is low, not only the hardness inside the steel sheet rather increases but also the continuous cooling. Therefore, it is difficult to obtain a low yield ratio steel.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for producing a high-strength steel material stably having a uniform material in a thickness direction and a low yield ratio. And

[0009]

Means for Solving the Problems The above problems are expressed in terms of% by weight,
C: 0.02-0.18%, Si: 0.05-0.5%, Mn: 0.6-1.7%, A
l: a step of producing a slab containing 0.08% or less, with the balance being Fe and unavoidable impurities;
Step of heating to a temperature of not less than ° C., the step of hot-rolling the heated slab, at a temperature of not less than the Ar ₃ transformation point, at a rolling reduction of 50% or more, the steel material after the hot rolling, the surface thereof temperature
A step of cooling at a cooling rate of 2 to 5 ° C./sec from a temperature of Ar ₃ transformation point or higher to a temperature of (Ar ₃ transformation point −150 ° C.) or less,
The cooled steel material is changed from the Ar ₃ transformation point to (Ar ₃ transformation point −100).
(C) at a cooling rate of 2 to 15 ° C / sec until the temperature reaches T ° C, and a step of allowing the steel material at T ° C to cool for a time t seconds satisfying the following equation (1). , The steel material after the holding, 400 ℃
A step of cooling at a cooling rate of 2 to 15 ° C./sec until a temperature of less than, and a step of tempering the cooled steel material at a temperature higher than 500 ° C. and lower than the Ac ₁ transformation point. The problem is solved by a method for manufacturing a high yield strength low tensile steel material having a small difference in material in the thickness direction.

[0010] ^{10 1.3-0.006 × Δ T ≦ t ≦} 150 ... (1) However, [Delta] T is (Ar ₃ transformation point -T) [° C.] Note that the above temperature, the calculation of the steel plate thickness direction unless otherwise stated Represents the average temperature determined by

The reasons for limiting the components and the production conditions will be described below. C: To obtain a tensile strength of 490 N / mm ² or more, it is necessary to make it 0.02% or more. However, if it exceeds 0.18%, toughness and weldability deteriorate, so it is made 0.02 to 0.18%.

Si: 0.05% or more must be used for deoxidation, but if it exceeds 0.5%, the toughness and weldability of the welded HAZ are deteriorated.

Mn: To secure strength, improve toughness and suppress generation of FeS, it is necessary to increase the content to 0.6% or more. However, if it exceeds 1.7%, hardenability increases, a hardened layer is formed during welding, and cracking sensitivity is high. Therefore, it is set to 0.6 to 1.7%.

Al: an element necessary for deoxidation,
If the content exceeds 08%, the cleanliness deteriorates and the toughness of the welded portion is deteriorated. Therefore, the content is set to 0.08% or less.

When hot-rolling a slab consisting of these components and the balance consisting of Fe and unavoidable impurities,
It is necessary to heat the slab to a temperature of 1000 ° C. or higher to ensure good hot workability.

In hot rolling, when rolling is performed at a temperature lower than the Ar ₃ transformation point, acoustic anisotropy occurs during ultrasonic flaw detection of a welded joint,
If the rolling reduction is less than 50%, the ferrite grains after rolling become coarse and the toughness deteriorates. Therefore, it is necessary to perform the rolling at a temperature higher than the Ar ₃ transformation point and at a rolling reduction of 50% or higher.

The rolled steel material needs to be cooled under the following various cooling conditions in order to achieve a uniform material in the thickness direction and a low yield ratio.

First cooling: In cooling the rolled steel material, when the surface of the steel material is cooled to a temperature lower than the Ar ₃ transformation point, coarse ferrite grains are formed and the toughness is deteriorated. It is necessary to start cooling the surface at a temperature equal to or higher than the Ar ₃ transformation point. If the cooling rate at this time is less than 2 ° C / sec, coarse ferrite grains are formed, toughness is deteriorated, and 5 ° C /
If the time is longer than 2 seconds, the surface is excessively hardened and a material difference occurs in the thickness direction. When cooling at this rate, if the surface of the steel material is finished at a temperature higher than (Ar ₃ transformation point -150 ° C.), the surface is excessively hardened by the second cooling and a material difference occurs in the sheet thickness direction. The surface is (Ar ₃ transformation point-
It is necessary to terminate when the temperature reaches 150 ° C or lower.

Second cooling: In the first cooling, the cooling condition of the surface of the steel material is defined, but the average temperature in the thickness direction does not decrease in conjunction with the surface temperature. Therefore, the purpose of this cooling is to perform cooling following the first cooling to reduce the average temperature. At a cooling rate of less than 2 ° C / sec, coarse ferrite grains are formed and the toughness deteriorates, and at 15 ° C / sec or more, the surface hardens too much and a material difference occurs in the thickness direction.
It needs to be 2 to 15 ° C / sec. Further, as described above, a mixed structure of ferrite and a hard phase is required to reduce the yield, so that a predetermined amount of ferrite must be precipitated. Therefore, the second cooling is performed in a temperature range where the ferrite is easily precipitated in a short time, from the Ar ₃ transformation point to the (Ar ₃ transformation point −1).
(00 ° C.).

Third cooling: The purpose of this cooling is to allow the steel material at a temperature of T ° C. to cool and slowly cool to precipitate a predetermined amount of ferrite. The relationship between the temperature T ° C at which the tensile strength TS of 490 N / mm ² or more was obtained and the yield ratio YR was 80% or less and the cooling time t seconds was investigated. Thus, it was clarified that it is sufficient to allow the cooling to be performed for a time t seconds satisfying the above expression (1). All other conditions for obtaining the result of FIG. 1 are within the scope of the present invention. Further, in consideration of productivity, the upper limit of the cooling time t is set to 150 seconds. Fourth cooling: After a predetermined amount of ferrite is precipitated by the third cooling, the steel material after cooling is cooled to less than 400 ° C. in order to transform untransformed austenite into a hard phase of bainite or martensite. It is necessary to cool at a cooling rate of 2 to 15 ° C./sec until the temperature reaches the temperature of

Since the as-transformed hard phase formed by the fourth cooling has remarkably poor toughness, 500%
It is necessary to perform a tempering treatment at a temperature exceeding ℃. However, if the temperature exceeds the Ac ₁ transformation point, the hard phase undergoes reverse transformation to become soft and a high-strength steel material cannot be obtained, so that tempering must be performed at the Ac ₁ transformation point or less. In addition, this tempering treatment is also effective in making the material uniform in the thickness direction.

As described above, by controlling the components, hot rolling, cooling, and tempering, a low-yield-ratio high-strength steel material having a small difference in material in the thickness direction can be stably manufactured.

When a predetermined amount of ferrite is precipitated in the third step and then cooled to a temperature lower than 400 ° C., if the cooling rate is higher than 15 ° C./sec, the lower limit temperature of the tempering treatment is set to “630 ° C. Beyond.

By weight%, Cu: 0.05-1.0%, Ni: 0.05-0.8
%, Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.10
%, V: 0.005 to 0.1%, Ti: 0.005 to 0.03% Addition of one or more elements selected from the group is effective in increasing strength, improving toughness, improving hardenability, and preventing tempering softening. It is. The reason for limiting the amount of addition will be described below.

When Cu is added in an amount of 0.05% or more, high strength can be achieved and toughness is improved. However, if it is added in excess of 1.0%, precipitation hardening becomes remarkable and surface cracks easily occur.

When Ni: 0.05% or more is added, high strength can be achieved and toughness is improved. However, adding more than 0.8% significantly increases the cost.

[0027] When Cr is added in an amount of 0.05% or more, hardenability is improved. However, if added over 1.0%, the weldability and the toughness of the welded HAZ deteriorate.

Mo: When added in an amount of 0.05% or more, hardenability is improved and softening during tempering is prevented. But,
If added in excess of 1.0%, the weldability deteriorates and the yield ratio increases due to the precipitation of carbides.

When Nb: 0.005% or more is added, high strength can be achieved by formation of fine carbonitrides and toughness is improved. However, when added in excess of 0.10%, the yield ratio increases due to precipitation hardening.

V: When added in an amount of 0.005% or more, hardenability is improved and softening during tempering is prevented. The formation of fine carbonitrides can increase the strength and improve the toughness. However, if it exceeds 0.1%, the weldability deteriorates.

When Ti: 0.005% or more is added, coarsening of the structure of the welded HAZ is suppressed by the formation of nitride, and the toughness of the HAZ is improved. However, if it is added in excess of 0.03%, carbides precipitate during cooling after welding and the toughness of the HAZ deteriorates.

[0032]

EXAMPLES Steels A to N (A to N) having components and transformation points shown in Table 1
L: Inventive steel, M, N: Comparative steel) slabs were manufactured and subjected to hot rolling conditions, cooling conditions, and tempering conditions shown in Table 2 under a thickness of 25 to 130 m.
m samples 1 to 24 were prepared. And mechanical properties (yield strength YS, tensile strength TS, yield ratio YR, Charpy impact value vE ₀ ),
The Vickers hardness difference ΔHv between the surface and the center of the thickness was investigated. The first to fourth coolings in Table 2 correspond to the first to fourth coolings described above.

The results are shown in Table 3. Samples 1 to 12 produced under the production conditions of the present invention were all TS of 490 N / mm ² or more, 80%
It shows the following YR and ΔHv of 24 or less, indicating that it is a low-yield-ratio high-tensile steel material with a small difference in material in the thickness direction suitable for high-rise buildings and the like. In addition, vE ₀ is 260 J or more, and it has sufficiently good toughness for use in high-rise buildings and the like.

On the other hand, in Samples 14, 18, 22, and 23 manufactured under manufacturing conditions outside the present invention, the YR exceeded 80% and a low yield ratio was not obtained, and in Samples 13, 15, 17, 21, and 24, ΔHv is 40 or more, resulting in inferior material uniformity in the plate thickness direction. In samples 16 and 19, TS is less than 490 N / mm ² , resulting in insufficient strength. In samples 17 and 20, vE ₀ is significantly inferior in toughness as compared with the steel of the present invention.

[0035]

【table 1】

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

Since the present invention is constructed as described above, it is possible to stably provide a uniform material in the thickness direction and a low yield ratio, and provide a method for producing a high-tensile steel material.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between TS, YR, ΔT, and cooling time t.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Ryuji Muraoka 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. F-term (reference) 4K032 AA01 AA04 AA05 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CB02 CC03 CD02 CD03 CF01 CF02

Claims (3)

[Claims] C .: 0.02 to 0.18% by weight, Si: 0.05 to 1% by weight
A step of producing a slab containing 0.5%, Mn: 0.6 to 1.7%, and Al: 0.08% or less, with the balance being Fe and unavoidable impurities; and heating the slab to a temperature of 1000 ° C or higher, The heated slab is heated to a temperature not lower than the Ar ₃ transformation point, 5
Hot rolling at a rolling reduction of 0% or more, and subjecting the steel material after the hot rolling to a surface temperature of the Ar ₃ transformation point or higher (Ar
₍₃ transformation point -150 ° C.) step of cooling at a cooling rate of 2-5 ° C. / second until the temperature is not more than, the cooled steel material, Ar ₃ transformation point ~ (Ar ₃ transformation point -100 ℃) 2 ~ 15 ℃ until temperature T ℃
Cooling at a cooling rate of / sec, and a step of allowing the steel material at T ° C to cool for a time t seconds that satisfies the following formula (1); and 2 ~ 15 ℃ / until temperature
A step of cooling at a cooling rate of seconds, and a step of tempering the cooled steel material at a temperature of 500 ° C. or higher and an Ac ₁ transformation point or lower,
A method for producing a low-yield-ratio high-tensile steel material having a small material difference in the thickness direction, comprising: ^{10 1.3-0.006 × Δ T ≦ t ≦} 150 ... (1) However, [Delta] T is (Ar ₃ transformation point -T) [° C.] 2. The steel material after cooling is cooled at a cooling rate exceeding 15 ° C./sec until the temperature becomes less than 400 ° C., and then tempered at a temperature exceeding 630 ° C. and equal to or lower than an Ac ₁ transformation point. The method for producing a low-yield-ratio high-tensile steel material having a small difference in material in the thickness direction according to the above item. 3. Cu: 0.05-1.0%, Ni: 0.05-% by weight.
0.8%, Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-
3. The material in the thickness direction according to claim 1 or 2, wherein a slab containing one or more elements selected from 0.10%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.03% is used. A method for producing a low-yield-ratio high-strength steel material with a small difference.