JP7262288B2 - High-strength low-yield-ratio thick steel plate with excellent toughness of base metal and weld heat-affected zone and small acoustic anisotropy, and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength, low-yield-ratio steel plate with excellent toughness in the base metal and weld heat-affected zone and small acoustic anisotropy, and a method for producing the same.

For example, thick steel plates used for large structures such as building structures and bridges have high strength, toughness of welded joints formed by welding, and high safety and reliability in the event of an earthquake. is required.

For example, in Patent Document 1, the carbon equivalent Ceq calculated from the steel composition (% by mass) is 0.33% or more and 0.43% or less, the weld crack sensitivity composition _PCM is 0.15% or more and 0.24% or less, The weld heat affected zone toughness index f _HAZ is made of steel having a composition of 0.30% or more and 0.47% or less, and the yield strength in a tensile test at the original thickness of the steel plate is 325 to 505 MPa, and the tensile strength is 490 to 670 MPa. , the yield ratio is 80% or less, the uniform elongation is 13% or more with the original thickness of the steel plate as it is, and the 2 mm V notch Charpy value sampled from the front and back surfaces of the steel plate is 90 J/cm ² or more at -40 ° C. and -10 ° C. 180 J/cm ² or more for cold press forming square steel pipes.

In addition, in Patent Document 2, the applicant of the present application specifies the chemical composition, 90% by volume or more of the steel structure is bainite, the bainite block size is 15 μm or less, and the average aspect ratio of prior γ grains is 3.0. and the TA1 value defined by the formula (1): TA1=[Mn]+[Cr] is in the range of 2.00 to 4.00.

Furthermore, in Patent Document 3, the present applicant specifies the chemical composition and the average aspect ratio of the prior austenite grain size in the plate thickness cross section parallel to the main rolling direction (average grain size in the main rolling direction / thickness direction It proposes a high-strength and high-toughness bainite non-tempered steel sheet having an average grain size) of more than 1.8 and 5.3 or less and a tensile strength of 570 MPa or more.

JP 2016-011439 A JP 2006-299365 A Japanese Patent Application Laid-Open No. 2006-283126

Patent Document 1 attempts to secure a low yield ratio, HAZ toughness, and base material toughness, but it seems that further studies are required to ensure good base material toughness at lower temperatures. Moreover, Patent Document 1 does not attempt to reduce the acoustic anisotropy.

Patent Document 2 attempts to reduce the acoustic anisotropy and ensure the toughness of the base material, but does not attempt to achieve good toughness of the base material at lower temperatures. Patent Document 3 also attempts to achieve both reduction in acoustic anisotropy and improvement in base material toughness, but does not disclose achieving a low yield ratio while ensuring high strength. Moreover, it is not intended to improve the toughness of the base metal at lower temperatures.

There is a trade-off relationship between the toughness of the base material, especially the toughness of the base material at temperatures below -60°C, the reduction of the acoustic anisotropy, and the high strength and low yield ratio as the strength characteristics of the base material. was difficult to satisfy. The present invention has been made in view of this situation, and its object is to realize high strength and low yield ratio as the strength characteristics of the base material toughness at low temperatures, reduction of acoustic anisotropy, and Another object of the present invention is to provide a thick steel plate having excellent toughness in a heat affected zone (HAZ) formed by welding, and a method for manufacturing the thick steel plate.

In aspect 1 of the present invention, the component composition is
C: 0.11 to 0.18% by mass,
Si: 0.10 to 0.50% by mass,
Mn: 1.00 to 1.50% by mass,
P: more than 0% by mass, 0.020% by mass or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.005 to 0.060% by mass,
Nb: 0 to 0.004% by mass,
Ti: 0.005 to 0.020% by mass,
B: 0 to 0.0003% by mass,
Ca: 0.0003 to 0.0060% by mass, and N: 0.0010 to 0.0100% by mass
and the balance being iron and unavoidable impurities,
The carbon equivalent Ceq represented by the following formula (1) is 0.32 to 0.38% by mass,
The weld heat affected zone toughness index f _HAZ represented by the following formula (2) is 0.46% by mass or less,
The weld crack sensitivity composition _PCM represented by the following formula (3) is 0.15 to 0.24% by mass,
steel structure
Composed of a soft phase made of ferrite and a hard phase containing a total of 80 area% or more of one or more of bainite, pearlite, and martensite,
The ferrite fraction in the entire structure is 50 to 80 area%,
The aspect ratio of ferrite is 2.0 or less,
The fraction of island martensite (MA) in the entire structure is 1 area% or less,
The average grain size is 12.0 to 22.0 μm, and the standard deviation σ of the grain size is 13.0 μm or less. It is a yield specific thickness steel plate.
Ceq (% by mass)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 (1)
[C], [Mn], [Si], [Ni], [Cr], [Mo] and [V] in formula (1) are respectively C, Mn, Si, Ni, Contents of Cr, Mo and V are shown, and elements not included are set to zero.
f _HAZ (% by mass) = [C] + [Mn]/8 + 6 x ([P] + [S]) + 12 x [N] - 4 x [Ti] (2)
[C], [Mn], [P], [S], [N] and [Ti] in formula (2) are the mass % of C, Mn, P, S, N and Ti, respectively. The content is indicated, and the element not included is zero. When the Ti content is 0.005% by mass or less, [Ti]=0% by mass.
_PCM (% by mass)=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5[B]... (3)
[C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] in formula (3) are each expressed in % by mass. Contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are shown, and elements not included are zero.

Aspect 2 further comprises
Cu: more than 0% by mass, 0.50% by mass or less,
Ni: more than 0% by mass, 0.50% by mass or less,
Cr: more than 0% by mass, 0.50% by mass or less,
Mo: more than 0% by mass, 0.50% by mass or less,
V: more than 0% by mass, 0.1% by mass or less, and
REM: The high-strength, low-yield-ratio thick steel sheet according to aspect 1, containing one or more elements selected from the group consisting of more than 0% by mass and 0.1% by mass or less.

Aspect 3 is the high-strength, low-yield-ratio steel plate according to aspect 1 or 2, which is for cold-formed square steel pipes.

Aspect 4 is a method for producing the high-strength, low-yield-ratio steel plate according to any one of aspects 1 to 3,
A steel slab having the chemical composition according to aspect 1 or 2 is heated to 1000 to 1200 ° C., and then hot rolled at an average temperature of 900 to 820 ° C. Cumulative rolling reduction: 10 to 50% , The average temperature is less than 820 ° C. Cumulative rolling reduction in the non-recrystallized γ region: 5% or less. After hot rolling, the average cooling rate is 5 to 30 ° C./s from the cooling start temperature of 730 ° C. is cooled to a cooling stop temperature of 350 to 650° C., and is a method for producing a high-strength, low-yield-ratio thick steel plate with excellent toughness in the base metal and weld heat-affected zone and small acoustic anisotropy.

Aspect 5 is the production method according to Aspect 4, wherein tempering is performed at 500 to 700° C. after the cooling.

Aspect 6 is a method for producing the high-strength, low-yield-ratio steel plate according to any one of aspects 1 to 3,
A steel slab having the chemical composition according to aspect 1 or 2 is heated to 1000 to 1200 ° C., and then hot rolled at an average temperature of 900 to 820 ° C. Cumulative rolling reduction: 10 to 70% , Cumulative rolling reduction in the non-recrystallized γ region with an average temperature of less than 820 ° C.: 5% or less. Cooling to room temperature at an average cooling rate of less than 5.0 ° C./s,
Next, the hot-rolled material is reheated to 720 to 850°C, quenched, and then tempered at 400 to 700°C. The base metal and weld heat affected zone have excellent toughness and small acoustic anisotropy. A method for producing a high-strength, low-yield-specific-thickness steel plate.

According to the present invention, it exhibits a high strength of 490 MPa or more and a low yield ratio, excellent toughness of the base material at low temperatures, reduced acoustic anisotropy, and excellent toughness of the HAZ formed by welding. It is possible to provide a thick steel plate and a method for manufacturing the thick steel plate.

When the present inventors reconsidered the prior art, most of the conventional techniques for improving toughness by grain size control depend on the regulation of the rolling reduction in the recrystallized γ region and the rolling reduction in the non-recrystallized γ region during hot rolling. The aim was to refine the average crystal grain size. However, it has been difficult to realize all of excellent base material toughness at lower temperatures, reduced acoustic anisotropy, and excellent strength characteristics of the base material. The recrystallized γ region is also called an austenite recrystallization temperature region, and the non-recrystallized γ region is also called an austenite non-recrystallization temperature region.

The present inventors have found (1) excellent low-temperature toughness of the base material, (2) reduction of acoustic anisotropy, (3) high strength and low YR as strength characteristics of the base material, and (4) excellent HAZ toughness. Intensive studies were conducted in order to combine the four characteristics. As a result, in order to satisfy the above (4) excellent HAZ toughness, the composition has a composition that satisfies the predetermined parameters, and in the manufacturing process, the reduction ratio of the recrystallized γ region and the non-recrystallized γ region is controlled, especially recrystallization In the γ region, the temperature range is further narrowed to control the rolling reduction, and the rolling reduction in the non-recrystallized γ region is suppressed as much as possible. It was found that it is effective to control the steel structure by performing quenching and tempering under certain conditions later. Hereinafter, the thick steel plate of the present invention may be referred to as "steel plate".

Hereinafter, the properties, steel structure, chemical composition, and production method of the steel sheet of the present invention will be described in order.

1. Properties The properties (1) to (4) of the steel sheet of the present invention are described in detail below. By combining these properties, it is possible to provide a steel plate with higher safety and reliability in the event of an earthquake, for example, for building structures.

(1) Excellent low-temperature toughness of the base material: vE-60°C (average) ≥ 70J, vE-60°C (minimum value) ≥ 49J, vTrs ≤ -60°C
When a steel plate is cold-worked to form, for example, a square steel pipe, the toughness of the corners of the square steel pipe deteriorates due to work hardening. In general, the radius of curvature of the corners of a square steel pipe is 3.5t±0.5t, where t is the plate thickness (mm) of the steel pipe. In order to ensure a high tensile strength, the steel sheet is required to satisfy vE-60°C (average) ≥ 70J, vE-60°C (minimum value) ≥ 49J, and vTrs ≤ -60°C. These measurement methods are as described in Examples described later.

In order to achieve this (1) excellent base material toughness,
(1-1) In the steel structure, it is effective to set the upper limit of the average grain size and suppress the standard deviation σ of the grain size to a certain value or less. For this purpose, it is effective to set the cumulative reduction rate in the temperature range of 900 to 820° C. in the hot rolling in the manufacturing process to a certain value or more.
(1-2) Furthermore, in the steel structure, it is effective to suppress the fraction of island martensite (MA, Martensite-Austenite constituent) below a certain level. It is effective to set it above a certain level.

(2) Reduction of acoustic anisotropy: VL/VC≤1.020
In the construction field, etc., it is necessary to guarantee the soundness of welded parts of structures. The soundness of welds is confirmed using ultrasonic testing. In order to accurately identify the defect position of the weld by ultrasonic flaw detection, the transverse wave sound velocity ratio between the rolling direction L and the direction perpendicular to the rolling direction C is suppressed to 1.020 or less as specified in JIS Z 3060. is required.

In order to reduce this (2) acoustic anisotropy, it is effective to keep the aspect ratio of ferrite below a certain level in the steel structure. It was found that it is effective to suppress the rate as much as possible.

(3) Base material strength characteristics (high strength and low YR): YP or 0.2% YS is 325 to 445 MPa, TS490 to 610 MPa, YR ≤ 80%
As a 490 MPa class low yield ratio steel plate that can improve safety in the event of an earthquake, strength characteristics equivalent to those of JIS standard SN490B are required. Specifically, the yield strength YP or 0.2% YS is 325 to 445 MPa, the tensile strength TS is 490 to 610 MPa, and the low yield ratio (YR: yield strength / tensile strength) ≤ 80% is satisfied. is required.

In order to achieve this (3) high strength and low YR,
(3-1) It is effective to keep the ferrite fraction within a certain range. It is effective to perform quenching and tempering by reheating to the temperature of the two-phase region after relatively slow cooling such as air cooling.
(3-2) Furthermore, it is effective to increase the average crystal grain size to a certain level or more, and for that purpose, in the hot rolling of the manufacturing process, the cumulative rolling reduction is set to a certain level or less in a predetermined temperature range in the recrystallized γ region. and suppressing the cumulative rolling reduction in the non-recrystallized γ region as much as possible.

(4) HAZ toughness: carbon equivalent Ceq ≤ 0.38 mass%, weld heat affected zone toughness index f _HAZ ≤ 0.46 mass%, weld crack susceptibility composition _PCM ≤ 0.24 mass%
A weld heat affected zone (HAZ) formed by welding generally generates a brittle structure, and there is concern that brittle fracture may occur during an earthquake. It is generally known that reducing the weld heat affected zone toughness index f _HAZ , the carbon equivalent Ceq, and the weld crack susceptibility composition _PCM are effective for improving the HAZ toughness. Also in the present invention, in order to ensure good HAZ toughness, it is premised that Ceq ≤ 0.38 mass%, f _HAZ ≤ 0.46 mass%, and _PCM ≤ 0.24 mass% are satisfied.

In the present invention, the manufacturing conditions are controlled after strictly controlling the carbon equivalent Ceq, the weld heat affected zone toughness index f _HAZ , and the weld crack susceptibility composition _PCM as well as the C content and Nb content as the chemical composition. We have found that all of the above properties (1) to (4) can be satisfied if the steel structure satisfies all of the following.

2. Steel structure The steel structure of the steel plate of the present invention consists of a soft phase composed of ferrite and a hard phase containing at least one of bainite, pearlite, and martensite in a total of 80 area % or more,
The ferrite fraction in the entire structure is 50 to 80 area%,
The aspect ratio of ferrite is 2.0 or less,
The fraction of island martensite (MA) in the entire structure is 1 area% or less,
An average crystal grain size of 12.0 to 22.0 μm and a standard deviation σ of the crystal grain size of 13.0 μm or less are satisfied.

As will be described later, the steel structure is observed at a quarter of the plate thickness t of the thick steel plate. Each requirement is explained below.

First, in the steel plate of the present invention, the steel structure is composed of a soft phase composed of ferrite and a hard phase, and the ferrite fraction of the soft phase is 50 to 80 area %. The "hard phase" in the present invention is a phase containing one or more of bainite, pearlite, and martensite in a total of 80 area % or more as a proportion of the hard phase. In the present invention, in order to ensure high strength, the fraction of ferrite in the entire structure is set to 80 area % or less. The ferrite fraction is preferably 75 area % or less. On the other hand, if the ferrite fraction is too low, the hard phase becomes relatively excessive and the strength becomes higher than necessary, exceeding, for example, the strength of SN490B, which is the JIS standard for rolled steel materials for building construction. Also, the yield ratio tends to be high. Therefore, the ferrite fraction in the entire structure is set to 50 area % or more.

Acoustic anisotropy can be reduced by setting the aspect ratio of ferrite, which is a soft phase, to 2.0 or less. The aspect ratio of ferrite is preferably 1.8 or less, more preferably 1.5 or less. The lower limit of the aspect ratio of ferrite is not particularly limited, and can be, for example, about 0.8.

The hard phase may also include island martensite (MA). By suppressing the proportion of MA to 1 area % or less in the entire structure, the toughness of the base material, especially at low temperatures, can be increased. The smaller the MA fraction, the more preferable, and the most preferable case is when the MA fraction is zero.

By setting the average grain size of the entire structure to 12.0 μm or more, high strength and low YR can be achieved, particularly low YR. The average grain size is preferably 12.5 μm or more, more preferably 13.0 μm or more. On the other hand, from the viewpoint of increasing the toughness of the base material, especially at low temperatures, the average grain size of the entire structure should be 22.0 μm or less and the standard deviation σ of the grain size should be 13.0 μm or less. The average grain size is preferably 21.0 μm or less, more preferably 20.0 μm or less. The standard deviation σ of the crystal grain size is preferably 12.0 μm or less, more preferably 11.0 μm or less. It should be noted that the smaller the standard deviation σ of the crystal grain size, the better, but the lower limit is about 3.0 μm in consideration of manufacturing conditions and the like.

3. Component composition In the present invention, after satisfying the range of each component described later, the parameters represented by formulas (1) to (3) are carbon equivalent Ceq, weld heat affected zone toughness index f _HAZ , weld crack sensitivity composition P _CM must also be within the following range. Each parameter will be described below.

Carbon equivalent Ceq represented by the following formula (1) is 0.32 to 0.38 mass%
Ceq (% by mass)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 (1)
[C], [Mn], [Si], [Ni], [Cr], [Mo] and [V] in formula (1) are respectively C, Mn, Si, Ni, Contents of Cr, Mo and V are shown, and elements not included are set to zero.

The higher the carbon equivalent Ceq, the higher the strength. With a low carbon equivalent of less than 0.32% by mass, the required yield strength YP or 0.2% YS and tensile strength TS are stably secured even if accelerated cooling or offline heat treatment is applied in the steel sheet manufacturing process. it becomes difficult to Therefore, the carbon equivalent Ceq is set to 0.32% by mass or more. The carbon equivalent Ceq is preferably 0.330% by mass or more, more preferably 0.340% by mass or more. On the other hand, if the carbon equivalent Ceq exceeds 0.38% by mass and is high, the risk of hardening of the base metal and HAZ and formation of martensite increases, and it is difficult to stably secure base metal toughness and HAZ toughness at a high level. become difficult. Therefore, the carbon equivalent Ceq is suppressed to 0.38% by mass or less. The carbon equivalent Ceq is preferably 0.370% by mass or less.

The weld heat affected zone toughness index f _HAZ represented by the following formula (2) is 0.46% by mass or less f _HAZ (mass%) = [C] + [Mn] / 8 + 6 x ([P] + [S]) +12×[N]−4×[Ti] (2)
[C], [Mn], [P], [S], [N] and [Ti] in formula (2) are the mass % of C, Mn, P, S, N and Ti, respectively. The content is indicated, and the element not included is zero. When the Ti content is 0.005% by mass or less, [Ti]=0% by mass.

The weld heat-affected zone toughness index f _HAZ is an index that gives a measure of the multi-layer weld joint toughness (HAZ toughness). The f _HAZ increases as the amount of elements such as C and Mn that improve hardenability increases, as the impurity elements P and S described later increase, and as the amount of Ti decreases. The lower the value of f _HAZ , the better the HAZ toughness. Therefore, in the present invention, f _HAZ is set to 0.46% by mass or less. The f _HAZ is preferably 0.43% by mass or less, more preferably 0.40% by mass or less. Although the lower limit is not particularly limited from the viewpoint of improving HAZ toughness, the lower limit of f _HAZ is approximately 0.30% by mass in consideration of the chemical composition of the present invention.

The weld crack sensitivity composition _PCM represented by the following formula (3) is 0.15 to 0.24 mass%
_PCM (% by mass)=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5[B]... (3)
[C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] in formula (3) are each expressed in % by mass. Contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are shown, and elements not included are zero.

When the weld crack susceptibility composition _PCM is as low as less than 0.15% by mass, as in the case where the carbon equivalent Ceq is less than 0.32% by mass, accelerated cooling or offline heat treatment is performed in the steel plate manufacturing process. , it is difficult to stably secure the required yield strength YP or 0.2% YS and tensile strength TS. Therefore, _PCM is made 0.15% by mass or more. _PCM is preferably 0.170 mass % or more, more preferably 0.190 mass % or more. On the other hand, when the _PCM is higher than 0.24% by mass, the risk of hardening of the base metal and HAZ and formation of martensite increases, and as a result, the base metal toughness and HAZ toughness are stably secured at a high level. becomes difficult. Therefore, _PCM is set to 0.24% by mass or less. _PCM is preferably 0.230 mass % or less, more preferably 0.225 mass % or less.

Next, the range of each component will be explained.

[C: 0.11 to 0.18% by mass]
C must be contained in an amount of 0.11% by mass or more in order to ensure the strength of the base material and the weld zone. The amount of C is preferably 0.12% by mass or more. However, if the amount of C is too large, the toughness of the base material and the weld heat affected zone is lowered and the weldability is deteriorated, so the upper limit is made 0.18% by mass. The amount of C is preferably 0.16% by mass or less.

[Si: 0.10 to 0.50% by mass]
Si is contained in steel for deoxidation. Therefore, the lower limit of the amount of Si is set to 0.10% by mass. The amount of Si is preferably 0.20% by mass or more, more preferably 0.25% by mass or more. However, if the Si content is too large, the weldability and HAZ toughness deteriorate, so the upper limit of the Si content is made 0.50% by mass. The amount of Si is preferably 0.45% by mass or less.

[Mn: 1.00 to 1.50% by mass]
Mn is essential for ensuring the strength and toughness of the base material and weld zone. Therefore, the lower limit was made 1.00% by mass. The Mn amount is preferably 1.10% by mass or more. However, too much Mn degrades HAZ toughness, promotes center segregation of the slab, and degrades weldability. Therefore, the upper limit of the amount of Mn is set to 1.50% by mass. The Mn content is preferably 1.40% by mass or less, more preferably 1.30% by mass or less.

[P: more than 0% by mass, 0.020% by mass or less]
P is an impurity element, and it is necessary to reduce the amount of P to 0.020% by mass or less in order to ensure good quality of the base material and HAZ. The amount of P is preferably 0.015% by mass or less, more preferably 0.010% by mass or less. Industrially, it is difficult to make the amount of P 0% by mass, so the lower limit of the amount of P is more than 0% by mass.

[S: more than 0% by mass, 0.005% by mass or less]
S is an impurity element. By reducing S, MnS is reduced, and the material properties of the base material and HAZ in the plate thickness direction can be improved. From this point of view, the amount of S is set to 0.005% by mass or less. The S content is preferably 0.003% by mass or less. Since it is industrially difficult to reduce the S content to 0% by mass, the lower limit of the S content is more than 0% by mass.

[Al: 0.005 to 0.060% by mass]
Al is an essential element for deoxidizing and reducing O (oxygen) to improve the cleanliness of steel. In order to exhibit the effect stably and reliably, the amount of Al must be 0.005% by mass or more. The amount of Al is preferably 0.010% by mass or more, more preferably 0.020% by mass or more. However, if it is contained in a large amount, the toughness of the base material and the toughness of the weld zone are lowered, so the upper limit of the Al content is made 0.060% by mass. The Al content is preferably 0.050% by mass or less, more preferably 0.040% by mass or less.

[Nb: 0 to 0.004% by mass]
When a large amount of Nb is added, island-shaped martensite tends to form in the HAZ, and the HAZ toughness greatly decreases. Also, by adding a large amount of Nb, the non-recrystallized γ region is pulled up to the high temperature side, leading to an increase in YR and acoustic anisotropy. Therefore, the Nb content is suppressed to 0.004% by mass or less. The Nb content is preferably 0.003% by mass or less, more preferably 0.002% by mass or less, and most preferably 0% by mass.

[Ti: 0.005 to 0.020% by mass]
Ti is an element that forms Ti-based oxides and TiN that are effective in refining the HAZ structure. In addition, when the slab is heated to 1350° C. or less in the manufacturing process of the steel sheet, by maximizing the use of TiN in addition to the Ti-based oxides, a strong pinning effect is exhibited and coarsening of γ grains is suppressed. In order to exhibit these effects, the amount of Ti must be 0.005% by mass or more. The amount of Ti is preferably 0.008% by mass or more. On the other hand, the upper limit of the amount of Ti is set to 0.020% by mass in order to prevent HAZ embrittlement due to excessive precipitation of TiC. The Ti content is preferably 0.018% by mass or less, more preferably 0.015% by mass or less.

[B: 0 to 0.0003% by mass]
B is sometimes added to improve hardenability and obtain strength, but it is not always necessary for a 490 MPa class steel plate. If the amount of B exceeds 0.0003% by mass, when thick steel plates of the same steel grade are mass-produced, the amount of dissolved B tends to increase in the base material and HAZ, and the variation in material properties of each product increases. Therefore, even when B is added, the upper limit is 0.0003% by mass.

[Ca: 0.0003 to 0.0060% by mass]
Ca is an element that contributes to the spheroidization of MnS and is effective in improving the toughness of the base material and the ductility in the plate thickness direction. In order to exhibit such an effect, the amount of Ca is made 0.0003% by mass or more. The amount of Ca is preferably 0.0010% by mass or more. However, if the amount of Ca exceeds 0.0060% by mass and becomes excessive, inclusions become coarse and the toughness of the base material deteriorates. Therefore, the amount of Ca is set to 0.0060% by mass or less. The amount of Ca is preferably 0.0040% by mass or less, more preferably 0.0030% by mass or less.

[N: 0.0010 to 0.0100% by mass]
N is an essential element for forming TiN and improving HAZ toughness. In order to secure a sufficient amount of TiN, the lower limit was made 0.0010% by mass. The amount of N is preferably 0.0020% by mass or more. On the other hand, in order to prevent embrittlement of the HAZ due to solute N, the amount of N is set to 0.0100% by mass or less. The N content is preferably 0.0080% by mass or less, more preferably 0.0060% by mass or less.

The basic components of the steel sheet of the present invention are as described above, and the balance is iron and unavoidable impurities. Unavoidable impurities are elements brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like. For example, there are elements, such as P and S, whose content is usually preferably as low as possible and thus are unavoidable impurities, but whose composition range is separately defined as described above. For this reason, the above-mentioned "inevitable impurities" in this specification mean those excluding elements whose composition range is separately defined.

The steel sheet of the present invention may contain the above elements in its chemical composition, and the parameters represented by the formulas (1) to (3) are within a predetermined range. The optional elements described below may not be contained, but by containing them together with the above elements as necessary, they contribute to further improvement of base material toughness and the like.

[Cu: more than 0% by mass, 0.50% by mass or less, Ni: more than 0% by mass, 0.50% by mass or less, Cr: more than 0% by mass, 0.50% by mass or less, Mo: more than 0% by mass, One or more elements selected from the group consisting of 0.50% by mass or less, V: more than 0% by mass and 0.1% by mass or less, and REM: more than 0% by mass and 0.1% by mass or less]

Cu, Ni, Cr, Mo, V, and REM (rare earth metal) are effective in further improving properties such as strength and toughness of the base material without impairing the excellent characteristics of the steel sheet of the present invention. element. When these elements are contained, it is preferable that each element exceeds 0% by mass, more preferably 0.01% by mass or more. On the other hand, considering the cost and the like to fully exhibit the characteristics of the steel sheet of the present invention, the content of Cu, Ni, Cr, and Mo is preferably 0.50% by mass or less, more preferably 0.30% by mass. % or less, more preferably 0.10 mass % or less. The contents of V and REM are each preferably 0.1% by mass or less, more preferably 0.05% by mass or less, and still more preferably 0.03% by mass or less. In the present invention, REM means lanthanoid elements (15 elements from La to Lu), Sc (scandium) and Y.

4. Manufacturing Method Next, a manufacturing method of the high-strength, low-yield-ratio steel plate according to the present invention will be described. The present inventors performed hot rolling under the conditions described later on a steel billet having a predetermined chemical composition, and then subjected to accelerated cooling after hot rolling, or air cooling after hot rolling, and then reheating and It has been found that the above-described desired steel structure can be obtained by quenching from the phase temperature and then tempering, and as a result, a steel plate having the above-described desired properties can be obtained.

The production method of the present invention is according to any one of the following production methods I to III.
(Manufacturing method I)
A steel slab that satisfies the above chemical composition is heated to 1000 to 1200 ° C., and then hot-rolled at an average temperature of 900 to 820 ° C. Cumulative reduction rate in a temperature range of 900 to 820 ° C.: 10 to 50%, average temperature is 820 ° C. Cumulative rolling reduction in the non-recrystallized γ region of less than 5%: 350 to 650°C at an average cooling rate of 5 to 30°C/s from a cooling start temperature of 730°C or higher after hot rolling. Cool down to the cooling stop temperature.
(Manufacturing method II)
After cooling in manufacturing method I, tempering is further performed at 500 to 700°C.
(Manufacturing method III)
A steel slab that satisfies the above chemical composition is heated to 1000 to 1200 ° C., and then hot-rolled at an average temperature of 900 to 820 ° C. Cumulative reduction rate in a temperature range of 900 to 820 ° C.: 10 to 70%, average temperature is 820 ° C. Cumulative rolling reduction in the non-recrystallized γ region of less than: 5% or less. After cooling at a rate of less than 5.0°C/s, the hot-rolled material is reheated to 720-850°C prior to quenching, followed by tempering at 400-700°C.

First, the conditions of manufacturing method I will be described.

[Heating temperature: 1000 to 1200°C]
If the heating temperature is low when heating the billet, it is difficult for the elements to form a solid solution, making it difficult to obtain the desired structure during rolling and subsequent heat treatment. Therefore, the heating temperature was set to 1000° C. or higher. It is preferably 1050° C. or higher. On the other hand, if the heating temperature is too high, γ becomes coarse, hardenability increases, and it becomes difficult to secure the soft phase. Therefore, the heating temperature is set to 1200° C. or less. It is preferably 1150° C. or less.

[Cumulative reduction rate in the temperature range where the average temperature is 900 to 820 ° C.: 10 to 50%]
[Cumulative rolling reduction in the non-recrystallized γ region with an average temperature of less than 820°C: 5% or less]
In the prior art, only the rolling reduction in the non-recrystallized γ region and the rolling reduction in the recrystallized γ region were controlled as the control of the steel structure. On the other hand, in the present invention, the upper and lower limits of the temperature are further defined in the recrystallization .gamma. region, and the upper and lower limits of the reduction ratio are set in that temperature region. In other words, the inventors have found that the above structure can be achieved by controlling the rolling conditions more strictly than in the prior art.

Specifically, by setting the cumulative reduction ratio in the temperature range of 900 to 820° C. to 10% or more, the average crystal grain size is 22.0 μm or less and the standard deviation σ of the crystal grain size is 13.0 μm. Crystal grains can be made uniform and fine as follows. As a result, it is possible to secure the base material toughness at low temperatures. The cumulative rolling reduction in the temperature range of 900 to 820° C. is preferably 15.0% or more, more preferably 20.0% or more. On the other hand, the cumulative rolling reduction in the temperature range of 900 to 820 ° C. is set to 50% or less, and the cumulative rolling reduction in the non-recrystallized γ region with an average temperature of less than 820 ° C. is suppressed to 5% or less, whereby the average crystal The grain size can be 12.0 μm or more, and as a result, high strength and low YR, especially low YR can be achieved. Furthermore, by suppressing the cumulative rolling reduction in the non-recrystallized γ region to 5% or less, the aspect ratio of ferrite can be made 2.0 or less, and as a result, the acoustic anisotropy can be reduced. The cumulative rolling reduction in the temperature range of 900 to 820° C. is preferably 45% or less. Also, the cumulative rolling reduction in the non-recrystallized γ region is preferably 2% or less, and most preferably 0%. Incidentally, in Patent Document 1, since rolling is applied in the non-recrystallized γ region, it is considered that the acoustic anisotropy is not improved.

The "average temperature" refers to the average temperature of the steel sheet in the thickness direction, and is the temperature obtained by the method described in Examples below.

The calculation method of the cumulative rolling reduction was calculated by the following formula.
Cumulative reduction rate in the temperature range of 900 to 820 ° C = (H1 - H2) / H1 x 100
Cumulative rolling reduction in non-recrystallized γ region = (H3-t)/H3 x 100
In the above, H1 is the plate thickness at the start of rolling in the temperature range of 900 to 820°C, H2 is the plate thickness at the end of rolling in the temperature range of 900 to 820°C, and H3 is the start of rolling in the non-recrystallized γ region. The plate thickness at time and t is the finished thickness, both in mm.

[Average cooling rate during cooling after hot rolling: 5 to 30 ° C./s]
[Cooling stop temperature: 350 to 650°C]
After hot rolling, cooling is performed from a cooling start temperature of 730°C or higher to a cooling stop temperature of 350 to 650°C at an average cooling rate of 5 to 30°C/s. This cooling allows the ferrite fraction to fall within the range of 50 to 80 area %, and as a result, high strength and low YR can be achieved. In production method I, the average cooling rate is set to 5° C./s or more in order to suppress the ferrite fraction and ensure high strength. The average cooling rate is preferably 10° C./s or higher, more preferably 15° C./s or higher. On the other hand, if the average cooling rate is too high, the ferrite will be insufficient, the hard phase will be excessive, the strength will increase more than necessary, and the yield ratio will also increase. The average cooling rate is preferably 28° C./s or less, more preferably 25.0° C./s or less.

If the cooling at the average cooling rate described above ends in a temperature range higher than 650° C., the ferrite fraction increases, resulting in the problem that high strength cannot be ensured. Therefore, the cooling stop temperature is set to 650° C. or less. The cooling stop temperature is preferably 630° C. or lower. On the other hand, if cooling at the above average cooling rate is performed to a temperature lower than 350° C., the ferrite will be insufficient and the hard phase will increase, resulting in an unnecessarily high strength. In addition, a large amount of MA is generated, making it difficult to achieve excellent base material toughness. Therefore, the cooling stop temperature is set to 350° C. or higher. The cooling stop temperature is preferably 400°C or higher, more preferably 450°C or higher.

Cooling from the cooling stop temperature to room temperature can be air cooling, for example.

Next, manufacturing method II and manufacturing method III will be described. In the explanation of the production method II and the production method III below, the explanation of the same steps as in the production method I is omitted.

In manufacturing method II, the same method as in manufacturing method I is performed until cooling after hot rolling, and after cooling, tempering is further performed. Strength can be adjusted by performing this tempering. The tempering temperature is preferably 500°C or higher, more preferably 600°C or higher. Also, the tempering temperature is preferably 700° C. or lower. From the viewpoint of obtaining the effect of tempering, the tempering time can be more than 0 minutes and 60 minutes or less. After tempering, it may be air-cooled to room temperature.

In the production method III, the slab is heated in the same manner as in the production method I, hot rolled under conditions slightly different from those in the production method I, and then cooled at a slower rate than in the production method I. In production method III, after cooling, quenching and tempering (tempering) are performed from the two-phase region temperature. Hot rolling, cooling, and quenching/tempering from the two-phase region temperature in production method III will be described below.

[Cumulative rolling reduction in the temperature range where the average temperature is 900 to 820 ° C.: 10 to 70%]
[Cumulative rolling reduction in the non-recrystallized γ region with an average temperature of less than 820°C: 5% or less]
By setting the cumulative reduction rate in the temperature range of 900 to 820 ° C. at an average temperature of 10% or more, the average crystal grain size is 22.0 μm or less and the standard deviation σ of the crystal grain size is 13.0 μm or less. can be made uniform and fine. As a result, it is possible to secure the base material toughness at low temperatures. The cumulative rolling reduction in the temperature range of 900 to 820° C. is preferably 15.0% or more, more preferably 20.0% or more. On the other hand, the cumulative rolling reduction in the temperature range of 900 to 820 ° C. is set to 70% or less, and the cumulative rolling reduction in the non-recrystallized γ region with an average temperature of less than 820 ° C. is suppressed to 5% or less, whereby the average crystal The grain size can be 12.0 μm or more, and as a result, high strength and low YR, especially low YR can be achieved. Furthermore, by suppressing the cumulative rolling reduction in the non-recrystallized γ region to 5% or less, the aspect ratio of ferrite can be made 2.0 or less, and as a result, the acoustic anisotropy can be reduced. The cumulative rolling reduction in the temperature range of 900 to 820° C. is preferably 60% or less. Also, the cumulative rolling reduction in the non-recrystallized γ region is preferably 2% or less, and most preferably 0%.

[Average cooling rate during cooling after hot rolling: less than 5.0 ° C./s]
After hot rolling, the steel is cooled at an average cooling rate of less than 5.0°C/s from the cooling start temperature of finish rolling temperature to (finish rolling temperature - 150°C) to room temperature. Air cooling is mentioned as said cooling. The lower limit of the average cooling rate is preferably 0.5° C./s or more from the viewpoint of easily ensuring high yield strength.

[Quenching temperature: 720 to 850°C]
The quenching temperature corresponds to the temperature in the two-phase region. If the quenching temperature is low, the reverse transformation fraction is insufficient, and the hard phase fraction is insufficient to achieve high strength. Therefore, the hardening temperature is set to 720° C. or higher. Preferably, it is 750°C or higher. On the other hand, if the quenching temperature is high, although the reverse transformation fraction increases, the concentration of components in the hard phase portion becomes insufficient, and the hardness of the hard phase decreases. Therefore, the hardening temperature is set to 850° C. or lower. It is preferably 840° C. or less. In addition, if the holding time at the quenching temperature is short, the elements are difficult to concentrate and the hardness cannot be ensured. Therefore, the quenching heating time is preferably 5 minutes or longer. More preferably, it is 10 minutes or longer. On the other hand, if the quenching heating time is long, the productivity is lowered, so the heating time is preferably set to 60 minutes or less.

After holding at the quenching temperature, quenching is performed to approximately room temperature. Quenching can be performed by water quenching, oil quenching, or the like. C, Mn, etc. are concentrated in the austenite that has been reverse-transformed by the two-phase region heating, and is in a state of high hardenability. Therefore, even if the average cooling rate during quenching is slow, the hard phase can be formed. However, in order to reliably increase the hardness of the hard phase, it is preferable to cool at an average cooling rate of 1° C./s or more during quenching.

[Tempering temperature: 400 to 700°C]
After quenching from the two-phase region temperature, tempering is performed at 400 to 700°C. If the tempering temperature is low, the hard phase becomes too hard and the strength becomes higher than necessary. Therefore, the lower limit of tempering temperature is set to 400° C. or higher. The tempering temperature is preferably 450°C or higher. On the other hand, when the tempering temperature is high, the hardness of the hard phase is lowered, the strength tends to be insufficient, the hardness ratio between the soft phase and the hard phase is lowered, and the yield ratio is increased. Therefore, the tempering temperature should be 700° C. or lower. The tempering temperature is preferably 600° C. or lower. Moreover, in order to obtain the effect of tempering, the tempering time is preferably 5 minutes or longer. More preferably, it is 10 minutes or more. On the other hand, from the viewpoint of productivity, the tempering time is preferably 60 minutes or less.

In production method III, the quenching and tempering from the two-phase temperature can be performed as off-line heat treatment.

The thick steel plate of the present invention has a thickness of, for example, 16 to 40 mm.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can match the spirit described above and below. subsumed in

A steel billet (slab) satisfying the chemical composition shown in Table 1 was obtained by a conventional method. After heating the steel slab to the heating temperature shown in Table 2, hot rolling and cooling immediately after hot rolling were performed under the conditions shown in Table 2. In addition, "recrystallized γ" in "rolling reduction" in Table 2 indicates the cumulative rolling reduction in the temperature range of 900 to 820°C, and "non-recrystallized γ" indicates the non-recrystallized γ region below 820°C. shows the cumulative reduction rate of In Table 2, FRT, SCT, and FCT indicate the finish rolling temperature and the cooling start and stop temperatures at the average cooling rate shown in Table 2, respectively. Air cooling was performed from the cooling stop temperature shown in Table 2 to room temperature. Then test no. 4, 7 to 9, and 11 to 13 were further tempered at the tempering temperature shown in Table 2 for a tempering time of 15 minutes. Test no. 6, 14 and 15 were quenched after being reheated from room temperature to the quenching temperature shown in Table 2, and then tempered at the tempering temperature shown in Table 2 for a tempering time of 10 minutes. Steel sheets having thicknesses shown in Table 2 as finished thicknesses were obtained by these manufacturing methods.

Each temperature in the hot rolling, quenching temperature, and tempering temperature in the manufacturing process is an average temperature calculated from the surface temperature of the steel sheet using parameters such as thickness and thermal conductivity. The cooling start temperature and cooling stop temperature are surface temperatures. The cooling rate is the rate determined from the surface cooling start temperature, cooling stop temperature, and cooling time.

The obtained steel sheets were evaluated for steel structure, tensile test, low-temperature toughness of the base material, and acoustic anisotropy in the following manner.

[Observation of steel structure]
[Ferrite Fraction, Ferrite Aspect Ratio]
A sample was taken from the steel sheet so that a thickness section parallel to the rolling direction and perpendicular to the steel sheet surface could be observed. The cross section was polished and subjected to nital corrosion to reveal a soft phase composed of ferrite. The structure was photographed at a position of 1/4 of the plate thickness using an optical microscope at a magnification of 400, and the fraction of ferrite, which is the soft phase, was obtained. Furthermore, the grain size of ferrite, which is a soft phase, was measured in the rolling direction and plate thickness direction, and the aspect ratio of ferrite was calculated from the ratio thereof. In the present invention, the fraction of the hard phase is obtained by subtracting the fraction of ferrite, which is the soft phase, from the total structure. It was confirmed that the hard phase contained at least one of bainite, pearlite, and martensite in a total of 80 area % or more.

[Average grain size and standard deviation of grain size]
A plate thickness cross section parallel to the rolling direction and perpendicular to the steel plate surface is polished, and at the 1/4 position of the plate thickness, EBSD (Electron Back Scatter Diffraction, electron beam backscatter diffraction) method measuring equipment (EDAX) The measurement area was 200 μm×200 μm, and the measurement step was 0.5 μm intervals.

[MA fraction]
A sample was taken from the steel sheet so that a thickness section parallel to the rolling direction and perpendicular to the steel sheet surface could be observed. The cross section was polished and etched with Repeller reagent to reveal the MA phase. Using an optical microscope, the structure was photographed at a magnification of 1000 times at a position of 1/4 of the plate thickness, and the fraction of MA was calculated.

[Tensile test (evaluation of strength characteristics)]
A flat tensile test piece (same as JIS 1A or JIS 5 test piece) was taken from the full thickness perpendicular to the rolling direction, and a tensile test was performed according to JIS Z 2241 to obtain a yield strength YP or 0.2% YS. The tensile strength TS was measured to obtain the yield ratio YR. A steel with a yield strength YP or 0.2% YS of 325 to 445 MPa, a tensile strength TS of 490 to 610 MPa, and a yield ratio YR of 80% or less was evaluated as showing high strength and low yield ratio.

[Evaluation of low temperature toughness of base material]
Three JIS No. 4 test pieces were taken such that the longitudinal direction of the test piece was in the L direction, that is, the rolling direction, at a 1/4 portion of the plate thickness. Then, a V-notch Charpy impact test was carried out according to the method specified in JIS Z 2242. In the present invention, as an index of low-temperature toughness, the energy values of the three test pieces were measured at a test temperature of -60°C. Also, the impact test was performed, and the brittle fracture surface transition temperature was obtained from the curve showing the relationship between the test temperature and the brittle fracture surface ratio. The minimum value of the energy values of the three test pieces is 49 J or more, that is, the energy values of the three test pieces are all 49 J or more, the average value is 70 J or more, and the brittle fracture surface transition temperature When vTrs satisfies −60° C. or lower, it was evaluated that the base material stably exhibits excellent low-temperature toughness.

[Evaluation of acoustic anisotropy]
As specified in JIS Z 3060, the shear wave sound velocity value VL when the vibration direction of the shear wave is matched with the main rolling direction (L direction) and the shear wave sound velocity when matched with the direction perpendicular to the L direction (C direction) The value VC was measured to obtain the transverse wave speed ratio VL/VC. Then, when the sound velocity ratio was 1.020 or less, the acoustic anisotropy was evaluated as small. These results are shown in Table 3.

Consider the results in Tables 1-3.

Test no. Nos. 6 to 15 are examples of production under the manufacturing conditions specified in the present invention using steel grades satisfying the chemical compositions specified in the present invention. They exhibit high strength and low yield ratio, excellent low-temperature toughness of the base material, and small acoustic anisotropy. Furthermore, it can be seen that the HAZ formed when welding satisfies all of the ranges of f _HAZ , Ceq, and _PCM specified in the present invention can exhibit excellent toughness. On the other hand, Test No. 1 to 5, the component composition is within the range specified by the present invention, but at least one of the manufacturing conditions specified by the present invention is not satisfied, and at least one of strength characteristics, toughness, and acoustic anisotropy is inferior. became.

Test no. No. 1 is an example in which accelerated cooling was performed after hot rolling, but since the cooling stop temperature was low, the ferrite fraction became insufficient and the MA fraction increased. As described above, the ferrite fraction was insufficient and the hard phase was relatively excessive, resulting in an unnecessarily high tensile strength. In addition, since the MA fraction was large, the base metal did not have excellent low-temperature toughness.

Test no. In No. 2, the cumulative rolling reduction in the temperature range of 900 to 820° C. was large, so the average crystal grain size was excessively refined, and the YR exceeded the upper limit. In addition, since the rolling reduction in the non-recrystallized γ region was large, the aspect ratio of ferrite increased and the acoustic anisotropy increased.

Test no. 3 is an example in which a steel sheet was obtained by performing air cooling after hot rolling, and is an example in which two-phase region quenching and tempering were not performed after air cooling. In the case of this example, high strength could not be ensured due to the large ferrite fraction.

Test no. In No. 4, since rolling was not performed in the temperature range of 900 to 820° C., the average grain size was large, the standard deviation σ of the grain size was large, and the low-temperature toughness of the base metal was lowered.

Test no. In No. 5, the average cooling rate during cooling after hot rolling was high, and the ferrite fraction was insufficient, so the yield strength and tensile strength increased more than necessary, and the YR exceeded the upper limit.

The steel plate of the present invention is a 490 MPa class low yield ratio steel plate that has excellent base material toughness at low temperatures, small acoustic anisotropy, and excellent toughness in the HAZ formed by welding. The thick steel plate of the present invention is suitable for building structural members that require ensuring safety and reliability in the event of an earthquake. In particular, it is suitable for cold-formed rectangular steel pipes, especially cold-press-formed rectangular steel pipes, which are used for columns of steel frame structures, for example.

Claims (6)

Ingredient composition
C: 0.11 to 0.18% by mass,
Si: 0.10 to 0.50% by mass,
Mn: 1.00 to 1.50% by mass,
P: more than 0% by mass, 0.020% by mass or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.005 to 0.060% by mass,
Nb: 0 to 0.004% by mass,
Ti: 0.005 to 0.020% by mass,
B: 0 to 0.0003% by mass,
Ca: 0.0003 to 0.0060% by mass, and N: 0.0010 to 0.0100% by mass
and the balance being iron and unavoidable impurities,
The carbon equivalent Ceq represented by the following formula (1) is 0.32 to 0.38% by mass,
The weld heat affected zone toughness index f _HAZ represented by the following formula (2) is 0.46% by mass or less,
The weld crack sensitivity composition _PCM represented by the following formula (3) is 0.15 to 0.24% by mass,
The steel structure at the 1/4 position of the plate thickness is
Composed of a soft phase made of ferrite and a hard phase containing a total of 80 area% or more of one or more of bainite, pearlite, and martensite,
The ferrite fraction in the entire structure is 50 to 80 area%,
The aspect ratio of ferrite is 2.0 or less,
The fraction of island martensite (MA) in the entire structure is 1 area% or less,
The average grain size is 12.0 to 22.0 μm, and the standard deviation σ of the grain size is 13.0 μm or less. Yield specific thickness steel plate.
Ceq (% by mass)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 (1)
[C], [Mn], [Si], [Ni], [Cr], [Mo] and [V] in formula (1) are respectively C, Mn, Si, Ni, Contents of Cr, Mo and V are shown, and elements not included are set to zero.
f _HAZ (% by mass) = [C] + [Mn]/8 + 6 x ([P] + [S]) + 12 x [N] - 4 x [Ti] (2)
[C], [Mn], [P], [S], [N] and [Ti] in formula (2) are the mass % of C, Mn, P, S, N and Ti, respectively. The content is indicated, and the element not included is zero. When the Ti content is 0.005% by mass or less, [Ti]=0% by mass.
_PCM (% by mass)=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5[B]... (3)
[C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] in formula (3) are each expressed in % by mass. Contents of C, Si, Mn, Cu, Ni, Cr, Mo, V and B are shown, and elements not included are zero. Furthermore,
Cu: more than 0% by mass, 0.50% by mass or less,
Ni: more than 0% by mass, 0.50% by mass or less,
Cr: more than 0% by mass, 0.50% by mass or less,
Mo: more than 0% by mass, 0.50% by mass or less,
V: more than 0% by mass, 0.1% by mass or less, and
REM: The high-strength, low-yield-ratio thick steel sheet according to claim 1, containing one or more elements selected from the group consisting of more than 0% by mass and 0.03 % by mass or less. The high-strength, low-yield-ratio steel plate according to claim 1 or 2, which is used for cold-forming square steel pipes. A method for producing a high-strength, low-yield-ratio thick steel plate according to any one of claims 1 to 3,
A steel slab having the chemical composition according to claim 1 or 2 is heated to an average temperature of 1000 to 1200 ° C., and then hot rolled at an average temperature of 900 to 820 ° C. Cumulative reduction rate: 10 ~50%, cumulative rolling reduction in the non-recrystallized γ region with an average temperature of less than 820°C: 5% or less. Manufacture of high-strength, low-yield-ratio thick steel plates that are cooled at a rate of 5 to 30°C/s to a cooling stop temperature with a surface temperature of 350 to 650°C. Method. The manufacturing method according to claim 4, wherein tempering is performed at an average temperature of 500 to 700°C after the cooling. A method for producing a high-strength, low-yield-ratio thick steel plate according to any one of claims 1 to 3,
A steel slab having the chemical composition according to claim 1 or 2 is heated to an average temperature of 1000 to 1200 ° C., and then hot rolled at an average temperature of 900 to 820 ° C. Cumulative reduction rate: 10 up to 70%, and the cumulative rolling reduction in the non-recrystallized γ region with an average temperature of less than 820°C: 5% or less. ) from the cooling start temperature to room temperature at an average cooling rate of less than 5.0 ° C./s,
Next, the hot-rolled material is reheated to an average temperature of 720 to 850°C and then quenched, and then tempered at an average temperature of 400 to 700°C. A method for producing a high-strength, low-yield-ratio thick steel plate with a small orientation.