JP7048378B2 - High strength and high ductility steel sheet - Google Patents

Description

The present invention relates to a high-strength, high-ductility steel plate having good tensile properties (yield strength, tensile strength, yield ratio) and not only excellent low-temperature toughness but also excellent elongation characteristics.

High-strength steel plates with a yield strength of 700 MPa or more, which are used as welded structural materials for bridges, ships, marine structures, pressure vessels, line pipes, etc., have excellent elongation characteristics and cold regions of about -40 ° C in addition to strength. Good low temperature toughness for use in (hereinafter, may be simply referred to as "toughness") may also be required. In particular, regarding the elongation characteristics, since steel sheets may be subjected to extremely severe cold bending such as a square steel pipe with an inner bending radius of 2.5 tons, it is an important characteristic for improving cold bending workability. be.

Various techniques have been conventionally studied for high-strength steel plates having a yield strength of 700 MPa or more.

For example, Patent Document 1 discloses a high-strength steel plate having a high tensile strength of 780 MPa or more and excellent base material toughness and HAZ toughness.
In the high-strength steel plate according to Patent Document 1, in addition to limiting the amount of C to an extremely low amount, the addition of Nb, V, and Mo, which adversely affect the base material toughness and HAZ toughness, is suppressed, and Mn, which is an element for improving hardenability, is used. , Ni, Cu are positively added to generate a structure mainly composed of bainitic ferrite at any of high and low cooling rates without particularly controlling the cooling rate after hot rolling the structure. At the same time, the bainite block is made finer by actively performing ultra-low temperature rolling.

Further, Patent Document 2 discloses a high-strength steel plate having excellent weldability (low temperature crack resistance and HAZ toughness) and also having good base metal toughness (particularly vE- ₁₀₀ ≧ 100J).
In the high-strength steel plate according to Patent Document 2, the amount of C is further reduced by using the formula expressed by KP in consideration of the steel structure, and preferably by further adding B, the low temperature crack resistance and the strength of the base metal are obtained. In addition to achieving both, the flatness ratio of the old γ grains is controlled to ensure high base metal toughness.

Japanese Unexamined Patent Publication No. 2005-54250 Japanese Unexamined Patent Publication No. 2001-220644

However, in Patent Document 1 and Patent Document 2, no study is made on the elongation characteristics, and the elongation characteristics may be insufficient.

The present invention has been made in view of such a situation, and an object of the present invention is to provide a high-strength high-ductility steel sheet having excellent yield strength, tensile strength, yield ratio and low-temperature toughness, as well as excellent elongation characteristics. It is something to do.

Aspect 1 of the present invention is
C: 0.035 to 0.070% by mass,
Si: 0.10 to 0.55% by mass,
Mn: 1.55 to 2.20% by mass,
P: 0.0100% by mass or less (not including 0% by mass),
S: 0.0050% by mass or less (not including 0% by mass),
Al: 0.015 to 0.050% by mass,
Ti: 0.005 to 0.030% by mass,
N: 0.0010 to 0.0060% by mass,
Ca: 0.0005 to 0.0040% by mass,
B: 0.0003 to 0.0030% by mass,
Cu: 0.20 to 0.70% by mass, Ni: 1.05 to 2.00% by mass, Cr: 0.55 to 1.00% by mass, and Mo: 0.20 to 0.60% by mass. Contains one or more selected from the group, the balance consisting of Fe and unavoidable impurities.
Pcm represented by the following formula (1) is 0.30 or less,
The DI represented by the following formula (2) is 7.0 or more, and the Ceq represented by the following formula (3) is more than 0.45.
The metallographic structure
The area ratio of bainite and martensite is 90% or more, and the area ratio of MA (Martensite-Austenite Constituent) is 5% or less.
The area ratio of carbide to the area of bainite and martensite is 5% or less, and the average circle-equivalent diameter of the carbide is 0.15 μm or less.
The aspect ratio, which is the value obtained by dividing the length of the old austenite grains in the rolling direction by the length in the plate thickness direction, is 3 or more and 20 or less.
In the plate width direction
Yield strength YP is 700MPa or more,
Tensile strength TS is 780 to 930 MPa,
Yield ratio YR is 85% or more,
The product TS × EL of tensile strength TS and elongation EL is 15000 MPa% or more, the Charpy absorption energy vE-40 at ₋₄₀ ° C. is 100 J or more, and the separation length in the fracture surface is 1 mm or more and 100 mm or less. It is a high-strength and high-ductility steel sheet.

Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (1) )

DI = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200) × [B] +1) ・ ・ ・ (2)

Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (3)

However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu, Ni, respectively. The content (% by mass) of Cr, Mo, V and B is shown.

Aspect 2 of the present invention is
The steel piece having the chemical composition according to the first aspect is heated so that the surface temperature becomes 1100 ° C. to 1400 ° C., then hot rolled so that the cumulative rolling reduction is 5% or more, and then the surface temperature is reached. The first rolling process, which cools the temperature to 300 ° C or less,
In the second rolling step, after the steel subjected to the first rolling step is heated so that the surface temperature becomes 950 ° C to 1250 ° C, the surface temperature is in the temperature range of 900 ° C to 1200 ° C. In the recrystallizing rolling step of hot rolling so that the cumulative rolling reduction in the above temperature is 5% or more, and in the temperature range where the surface temperature is 900 ° C. or less, the cumulative rolling reduction in the temperature range is 30% or more, 80% or less, and An unrecrystallized rolling process in which hot rolling is performed so that the rolling completion temperature is 680 to 900 ° C at the surface temperature, and 2 ° C / sec from the cooling start temperature of 500 ° C or higher to the cooling stop temperature of 250 ° C or lower. A cooling step of cooling at the above cooling rate, a second rolling step including this order, and a second rolling step.
A tempering step of tempering the steel subjected to the second rolling step at a tempering temperature of 560 ° C. or higher and 700 ° C. or lower.
1 is the method for producing a high-strength, high-ductility steel sheet according to the first aspect.

It is excellent in yield strength, tensile strength, yield ratio and low temperature toughness, and can also have excellent elongation characteristics.

As a result of diligent studies, the present inventor controlled the area ratio of carbides to the area of bainite and martensite to 5% or less and the average circle equivalent diameter of the carbides to 0.15 μm or less. It has been found that the elongation characteristics can be improved because ductile fracture starting from carbides is less likely to occur during processing.

Further, as a result of diligent studies by the present inventor, in order to control the area ratio of the charcoal to the area of bainite and martensite to 5% or less and the average circle equivalent diameter of the charcoal to 0.15 μm or less, the first step is first. Rolling under light pressure (first rolling step described later) is performed as heating and rolling, and then two-step rolling (second rolling step described later) is performed so as to obtain a desired plate thickness. I found it good. By performing the first stage heating and rolling under light rolling, the coarse carbides formed in the casting stage are sufficiently solid-dissolved, and the carbides reprecipitated in the subsequent steps are finely dispersed in the steel. Will be.

1. 1. Chemical Composition The chemical composition of the high-strength, high-ductility steel sheet of the present invention (hereinafter, may be simply referred to as “steel sheet”) will be described below.

[C: 0.035 to 0.070% by mass]
C is an element that contributes to increasing the strength of the steel sheet. If the C content is less than 0.035% by mass, the desired structure cannot be sufficiently obtained, and it becomes difficult to secure the required base metal strength. Therefore, the C content is 0.035% by mass or more. It is preferably 0.040% or more. On the other hand, C is an element that deteriorates HAZ toughness and is also an element that easily deteriorates weld crack resistance. When the C content exceeds 0.070% by mass, it becomes easy to secure the strength of the base metal, but the hardness of the surface portion of the steel sheet becomes large and the bending workability deteriorates. Further, if the C content is excessive, MA tends to remain, and it becomes difficult to obtain high strength and high toughness. In addition, the size of carbides deposited after tempering (diameter equivalent to the average circle of carbides) becomes large, and the elongation characteristics deteriorate. From this point of view, the upper limit of the C content is 0.070% by mass. It is preferably 0.065% by mass, more preferably 0.060% by mass.

[Si: 0.10 to 0.55% by mass]
Si is an effective element as a deoxidizing material. Further, Si is an element effective for improving the strength of the base material, and in order to exert these effects, Si is contained in an amount of 0.10% by mass or more. It is preferably contained in an amount of 0.15% by mass or more. However, when the Si content becomes excessive, MA is formed and it becomes difficult to secure the strength and toughness of the base metal. In addition, the Si content is set to 0.55% by mass or less because it tends to cause deterioration of HAZ toughness and weldability. The preferred upper limit is 0.50% by mass, more preferably 0.40% by mass.

[Mn: 1.55 to 2.20% by mass]
Mn is an element that stabilizes austenite and lowers the transformation temperature. Further, Mn is an element effective for ensuring impact characteristics due to the effect of refining the crystal grain size due to low temperature transformation. Further, Mn is effective in improving hardenability and strength. In order to exert these effects, Mn is contained in an amount of 1.55% by mass or more. It is preferably contained in an amount of 1.60% by mass or more. However, if Mn is excessively contained, the elongation characteristics, low temperature toughness and HAZ toughness deteriorate. Therefore, the upper limit of the Mn content is 2.20% by mass. The preferred upper limit is 2.10% by mass.

[P: 0.0100% by mass or less (not including 0% by mass)]
P is an element that adversely affects the impact characteristics (base metal toughness, toughness after bending) and HAZ toughness. Therefore, it is necessary to regulate the P content to 0.0100% by mass or less. It is preferably restricted to 0.0090% by mass or less.

[S: 0.0050% by mass or less (not including 0% by mass)]
S is an element that forms MnS and deteriorates impact characteristics, HAZ toughness, and base metal elongation. Therefore, the S content is restricted to 0.0050% by mass or less. It is preferably restricted to 0.0030% by mass or less.

[Al: 0.015 to 0.050% by mass]
Al is an element necessary for deoxidation and contains 0.015% by mass or more. It is preferably contained in an amount of 0.020% by mass or more. On the other hand, if Al is excessively contained, coarse alumina-based inclusions are formed and the impact characteristics are deteriorated. Therefore, the Al content is set to 0.050% by mass or less. It is preferably 0.040% by mass or less.

[Ti: 0.005 to 0.030% by mass]
Ti is an element that forms a nitride (TiN) with N to prevent coarsening of austenite grains (γ grains) during heating before hot rolling. Ti is an element that contributes to ensuring strength and improving toughness and HAZ toughness by refining the obtained structure. Further, Ti can be used in combination with B to form free B, thereby enhancing hardenability. In order to exert these effects, it is necessary to contain Ti in an amount of 0.005% by mass or more. It is preferably contained in an amount of 0.010% by mass or more. However, if the Ti content is excessive, TiC is precipitated in addition to TiN, and the toughness and HAZ toughness deteriorate. Therefore, the Ti content is 0.030% by mass or less, preferably 0.025% by mass or less.

[N: 0.0010 to 0.0060% by mass]
N is an element that produces TiN together with Ti, prevents coarsening of γ grains during heating and welding before hot rolling, and is effective in improving toughness and HAZ toughness. When the N content is less than 0.0010% by mass, TiN is insufficient, the γ grains become coarse, and the toughness and HAZ toughness deteriorate. Therefore, the N content is 0.0010% by mass or more, preferably 0.0020% by mass or more, and more preferably 0.0030% by mass or more. On the other hand, if the N content becomes excessive and exceeds 0.0060%, BN is formed and the strength, toughness and HAZ toughness deteriorate. Therefore, the upper limit of the N content is 0.0060% by mass, preferably 0.0055% by mass.

[Ca: 0.0005 to 0.0040% by mass]
Ca is an element that spheroidizes MnS and effectively acts to detoxify low temperature toughness and weld crack resistance. In order to effectively exert this effect, Ca is contained in an amount of 0.0005% by mass or more, more preferably 0.0010% by mass or more. However, if the Ca content is excessive, inclusions are coarsened and the toughness of the base metal is deteriorated. Therefore, the upper limit of the Ca content is 0.0040% by mass. The upper limit of the Ca content is preferably 0.0030% by mass.

[B: 0.0003 to 0.0030% by mass]
B exists as free B without forming BN by being combined with Ti, and is an element effective for improving hardenability and increasing strength. Therefore, B is contained in an amount of 0.0003% by mass or more. It is preferably contained in an amount of 0.0008% by mass or more. However, if the B content is excessive, coarse precipitates are formed, which rather lowers the hardenability. Therefore, the upper limit of the B content is 0.0030% by mass. A more preferable upper limit is 0.0025% by mass.

[Cu: 0.20 to 0.70% by mass]
Cu is an element effective for improving the strength and toughness of the base metal without significantly adversely affecting the weldability and HAZ toughness. In order to effectively exert these effects, Cu is contained in an amount of 0.20% by mass or more, more preferably 0.30% by mass or more. However, from the viewpoint of reducing the raw material cost, it is better that the amount of Cu is small. Therefore, Cu is contained in an amount of 0.70% by mass or less, more preferably 0.60% by mass or less.

[One or more selected from the group consisting of Ni: 1.05 to 2.00% by mass, Cr: 0.55 to 1.00% by mass and Mo: 0.20 to 0.60% by mass]
The steel sheet of the present invention contains at least one selected from the group consisting of Ni, Cr and Mo. These elements will be described below.
Ni is an element effective for improving the strength and toughness of the base metal without significantly adversely affecting the weldability and HAZ toughness. When Ni is contained, in order to effectively exert this effect, Ni is contained in an amount of 1.05% by mass or more, preferably 1.10% by mass or more. However, from the viewpoint of reducing the raw material cost, it is better that the amount of Ni is small. Therefore, when Ni is contained, Ni is contained in an amount of 2.00% by mass or less, preferably 1.90% by mass or less.

Cr is an element that contributes to high strength. In addition, Cr forms and stabilizes alloy carbides, which has the effect of reducing the size of the carbides. When Cr is contained, in order to effectively obtain these effects, Cr is contained in an amount of 0.55% by mass or more, preferably 0.60% by mass or more. On the other hand, from the viewpoint of reducing the raw material cost, when Cr is contained, Cr is contained in an amount of 1.00% by mass or less, preferably 0.95% by mass or less.

Mo is an element that contributes to high strength. In addition, Mo forms and stabilizes alloy carbides, which has the effect of reducing the size of the carbides. Mo is an element that suppresses the formation of hokka products and improves hardenability. When Mo is contained, in order to effectively obtain these effects, Mo is contained in an amount of 0.20% by mass or more, preferably 0.25% by mass or more. On the other hand, from the viewpoint of reducing the cost of raw materials, when Mo is contained, it is contained in an amount of 0.60% by mass or less, preferably 0.55% by mass or less.

[Remaining]
In one preferred embodiment, the balance is iron and unavoidable impurities. As unavoidable impurities, trace elements (for example, As, Sb, Sn, V, etc.) brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. are allowed to be mixed. In addition, for example, there are elements such as P and S, which are usually preferable as the content is smaller and are therefore unavoidable impurities, but the composition range thereof is separately defined as described above. Therefore, in the present specification, the term "unavoidable impurities" constituting the balance is a concept excluding the elements whose composition range is separately defined.

Further, the chemical composition of the steel sheet according to the present invention satisfies Pcm of 0.30 or less, DI of 7.0 or more, and Ceq of more than 0.45, which will be described in detail below.

[Pcm: 0.30 or less]
The Pcm represented by the following formula (1) is called a weld crack sensitive composition, and it is necessary to set it to 0.30 or less in order to stably suppress weld cracks even in a steel sheet having a thick wall and a large degree of restraint. In the present invention, in addition to improving the strength and elongation characteristics, welding cracks can be stably suppressed by setting the Pcm to 0.30 or less. The Pcm is preferably 0.29 or less. The smaller the value of Pcm is, the more preferable it is, and the lower limit is not particularly limited. However, in the chemical composition of the present invention, the lower limit of Pcm is about 0.24.

Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (1) )
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu, Ni, respectively. The content (% by mass) of Cr, Mo, V and B is shown.
If there is an element not contained in the steel sheet in the above formula, the content of the element not included is calculated as zero.

[DI: 7.0 or higher]
The DI represented by the following equation (2) is called a hardenability multiple, and it secures a stable structure (specifically, the area ratio of bainite and martensite is 90% or more) even in a thick steel plate and is high. In order to achieve strength, it needs to be 7.0 or higher. It is preferably 7.5 or more. The upper limit is not particularly limited, but is about 15.0.

DI = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200) × [B] +1) ・ ・ ・ (2)
If there is an element not contained in the steel sheet in the above formula, the content of the element not included is calculated as zero.

[Ceq: over 0.45]
Ceq represented by the following equation (3) is called carbon equivalent, and it is necessary to make it more than 0.45 in order to secure a stable structure. It is preferably 0.50 or more. The upper limit is not particularly limited, but is about 0.80 from the viewpoint of weldability.

Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (3)
If there is an element not contained in the steel sheet in the above formula, the content of the element not included is calculated as zero.

2. 2. Steel Structure Next, the details of the steel structure of the steel sheet of the present invention will be described.
The following description of the steel structure may describe a mechanism by which having such a structure can improve various properties. It should be noted that these are the mechanisms considered by the present inventor based on the findings obtained at present, but do not limit the technical scope of the present invention.

[Area ratio of bainite and martensite: 90% or more]
In the present invention, in order to secure the tensile properties and toughness of the base metal, in addition to the optimization of the chemical composition and the optimization of the hot rolling conditions, the accelerated cooling process described later is adopted to obtain the steel. Utilizes transformation strengthening and carbide precipitation strengthening. Here, when transformation is started at a high temperature and the number of soft ferrite phases increases, it becomes difficult to satisfy the tensile properties, particularly the yield strength of 700 MPa or more. Therefore, in order to secure the tensile properties, it is necessary to use bainite and martensite as the main structures. Specifically, the area ratio of bainite and martensite needs to be 90% or more with respect to the total structure of the steel. If it is less than 90%, ferrite as a structure increases, and it becomes difficult to secure tensile properties as described above. The area ratio of bainite and martensite is preferably 92% or more. The higher the area ratio of bainite and martensite, the better, and the upper limit is not particularly limited, and most preferably 100%.

[MA area ratio: 5% or less]
The steel sheet of the present invention can secure high tensile strength and achieve high yield strength. For that purpose, the area ratio of MA needs to be 5% or less with respect to the total structure of steel. MA is an abbreviation for martensite-austenite constituent, and is a complex (complex tissue) of martensite and austenite. When the area ratio of MA exceeds 5%, the yield strength is lowered due to the effect of reducing the yield ratio by the hard MA, and the high yield strength cannot be satisfied. In addition, when the hard MA is dispersed in the steel structure, cracks are generated starting from the MA and the impact characteristics cannot be obtained satisfactorily. The area ratio of MA is preferably 1 area% or less. The smaller the area ratio of MA, the better, and the lower limit is not particularly limited, and most preferably 0%.

[Area ratio of carbides to the area of bainite and martensite: 5% or less]
In order to exhibit good elongation characteristics, it is necessary to uniformly deform the steel sheet, for example, during bending. This can be achieved by finely dispersing the carbides in the steel. When the carbide is finely dispersed in the steel, ductile fracture starting from the carbide is less likely to occur, for example, during bending. Specifically, the area ratio of carbide to the area of bainite and martensite is 5% or less, and the average circle-equivalent diameter of carbide described later is 0.15 μm or less. When the area ratio is controlled to 5% or less and the average circle-equivalent diameter of the carbide is controlled to 0.15 μm or less, the precipitation of coarse carbide is suppressed and the carbide is finely dispersed. The area ratio is preferably 4% or less, more preferably 3% or less. The lower limit of the area ratio is not particularly limited, but is about 2% in consideration of the range of the C content of the present invention. When carbides are present in bainite and martensite, the area of the bainite and martensite is the area including the area of the carbides. The carbides targeted in the present invention are cementite, alloy carbides M ₂₃ C ₆ , M ₇ C ₃ (M is an alloy element such as Fe, Cr, Mo) and the like.

[Average circle-equivalent diameter of carbide: 0.15 μm or less]
By controlling the area ratio of the carbide to the area of bainite and martensite to 5% or less and controlling the average circle equivalent diameter of the carbide to 0.15 μm or less, the carbide can be finely dispersed. Thereby, good elongation characteristics can be exhibited. The diameter equivalent to the average circle is preferably 0.13 μm or less. The lower limit of the diameter corresponding to the average circle is not particularly limited, but is about 0.01 to 0.05 μm in consideration of the range of the C content of the present invention.

[Aspect ratio of old austenite grains: 3 or more, 20 or less]
In the present invention, it is necessary to secure good low temperature toughness (vE- ₄₀ ≧ 100J) in the base metal so that the toughness after bending is also excellent. For that purpose, it is good to make the steel structure mainly bainite and martensite as described above, and to increase the aspect ratio, which is the value obtained by dividing the length of the old austenite grains in the rolling direction by the length in the plate thickness direction. The energy absorbed by the rolling steel is secured. Specifically, the aspect ratio is set to 3 or more and 20 or less. In order to effectively exert the above effect, it is necessary to set the aspect ratio to 3 or more, and if it is excessive, the Charpy absorption energy becomes small, so it is set to 20 or less. The preferred upper limit of the aspect ratio is 10, and the preferred lower limit is 4.

3. 3. Characteristics As described above, the steel sheet of the present invention has either YP (YS), TS, YR, TS × EL, absorption energy (Charpy absorption energy) in the Charpy impact test at -40 ° C, and separation length in the fracture surface. Is also at a high level. These characteristics will be described below.

(1) Yield strength (YP)
YP in the plate width direction (C direction) is 700 MPa or more.

(2) Tensile strength (TS)
The TS in the plate width direction (C direction) is 780 MPa or more. The higher the tensile strength, the more preferable, but in consideration of the chemical composition and manufacturing conditions of the steel sheet of the present invention, the upper limit of the tensile strength is 930 MPa.

(3) Yield ratio (YR)
The YR in the plate width direction (C direction) is 85% or more. It is preferably 88% or more. The upper limit of YR is not particularly limited, but is preferably about 97% from the viewpoint of safety.

(4) Tensile strength x elongation (TS x EL)
Product of tensile strength TS and elongation (total elongation) EL A (= TS × EL) (in the case of JIS4 test piece), A (= TS × EL × 2.48 / t ^0.5 , but t: plate thickness (Mm)) (in the case of JIS No. 5 test piece) satisfies 15,000 MPa% or more. It is preferably 15500 MPa% or more. By having the product A of high tensile strength TS and elongation (total elongation) EL, it is possible to obtain a high level of strength ductility balance having both high strength and high elongation at the same time.

(5) Charpy absorption energy at -40 ° C (vE- ₄₀ )
The vE- ₄₀ in the plate width direction (C direction) is 100 J or more.

(6) Separation length In order to set vE- ₄₀ to 100J or more, separation in the plate width direction (C direction) is generated in the fracture surface during the Charpy impact test at −40 ° C. By generating the separation, the energy of destruction can be dispersed in the length direction of the separation. If the separation length is less than 1 mm, a brittle fracture surface is generated and the Charpy absorption energy becomes low. On the other hand, when the separation length exceeds 100 mm, the energy is relaxed by the separation, so that the value becomes low. Therefore, the separation length is set to 1 mm or more and 100 mm or less. The lower limit of the preferred separation length is 5 mm. Further, the upper limit of the preferable separation length is 90 mm.

4. Manufacturing Method Next, a manufacturing method for a steel sheet according to the present invention will be described.
The present inventor obtains the above-mentioned desired steel structure by performing hot rolling in two stages of a first rolling step and a second rolling step, the details of which will be described later, on a steel piece (slab) having a predetermined chemical composition. As a result, it has been found that a high-strength, high-ductility steel sheet having the above-mentioned desired characteristics can be obtained.
The details will be described below. The "temperature" in the first rolling step, the second rolling step, and the tempering step described below is the temperature on the surface of the steel sheet. Further, the surface temperature in the heating stage is almost the same temperature in the inside (center of the plate thickness) in the heating in a general heating furnace.

[First rolling process]
First, a first-stage rolling (first rolling step) is performed on a steel piece having the above-mentioned predetermined chemical composition and obtained by a conventional casting method such as continuous casting. That is, in the first rolling step, the steel pieces are heated to 1100 ° C. to 1400 ° C., then hot rolled so that the cumulative rolling reduction ratio is 5% or more, and then cooled to 300 ° C. or lower. Although the first rolling step has not been performed conventionally, by performing the first rolling step, the coarse carbide formed in the casting step is sufficiently solidified to reduce segregation, and the carbide reprecipitated in the subsequent steps. Enables miniaturization of. The cumulative reduction rate in the first rolling process is the reduction rate achieved by breakdown (BD), which may be conventionally performed in the final stage of the casting process (for example, continuous casting), in the first rolling process. It may be realized.
The lower limit of the heating temperature is preferably 1150 ° C. The upper limit of the heating temperature is preferably 1350 ° C. The lower limit of the cumulative rolling reduction in the first rolling step is preferably 8%, more preferably 10%. The upper limit of the cumulative rolling reduction in the first rolling step is not particularly limited, but is preferably 65% from the viewpoint of ensuring the rolling reduction in the second rolling step.

[Second rolling process]
Subsequently, the second rolling step is performed on the steel subjected to the first rolling step. The second rolling step includes a recrystallization rolling step, an unrecrystallized rolling step, and a cooling step in this order, as will be described in detail below.

-Recrystallization Rolling Step In the recrystallization rolling step, first, the steel subjected to the first rolling step is heated to 950 ° C to 1250 ° C. When the heating temperature is low, the solid solution of the element is small, the carbide does not dissolve again, and it becomes coarse in rolling and subsequent heat treatment. Therefore, the temperature was set to 950 ° C. or higher. It is preferably 1000 ° C. or higher. However, if the temperature is too high, γ becomes coarse and it becomes difficult to secure the impact characteristics. Therefore, the temperature is set to 1250 ° C or lower. It is preferably 1200 ° C. or lower.
Subsequently, hot rolling is performed in a temperature range of 900 ° C. to 1200 ° C. so that the cumulative rolling reduction in the temperature range is 5% or more. As a result, the austenite grains are repeatedly recrystallized and made finer, and the strength and toughness of the steel sheet can be achieved at the same time. The cumulative reduction rate in the temperature range of 900 ° C to 1200 ° C is preferably 30% or more. The above effect is saturated when the cumulative reduction rate in the temperature range of 900 ° C to 1200 ° C is 30% or more.

Unrecrystallized rolling step In the unrecrystallized rolling step, austenite does not recrystallize, that is, in the so-called unrecrystallized region of 900 ° C. or lower, the cumulative rolling reduction rate in the temperature region is 30% or more, 80% or less, and Hot rolling is performed so that the rolling completion temperature FRT (Finishing Rolling Temperature) is 680 to 900 ° C. In order to secure excellent impact characteristics and desired yield strength, austenite grains are repeatedly recrystallized in the above recrystallization rolling step, and then a cumulative reduction rate of 30% or more is secured in this unrecrystallized region. It is necessary to. It is preferably 40% or more. On the other hand, when the cumulative reduction rate of the unrecrystallized region exceeds 80%, the aspect ratio of γ (austenite) becomes too large and the toughness decreases. Therefore, the cumulative reduction rate of the unrecrystallized region is set to 80% or less. It is preferably 70% or less. Further, when the rolling completion temperature FRT is lower than 680 to 900 ° C., the productivity is lowered. Therefore, the rolling completion temperature FRT is set to 680 to 900 ° C.

-Cooling step In the cooling step, cooling (accelerated cooling) is performed at a cooling rate of 2 ° C / sec or higher from a cooling start temperature SCT (Starting Cooling Temperature) of 500 ° C or higher to a cooling stop temperature FCT (Finishing Cooling Temperature) of 250 ° C or lower. .. The cooling means is not particularly limited as long as a cooling rate of 2 ° C./sec or higher can be achieved. For example, it is water-cooled, preferably air-cooled. When the SCT is lower than 500 ° C., soft polygonal ferrite is formed, which causes a decrease in the strength of the base metal. Therefore, accelerated cooling needs to be started from a temperature of 500 ° C. or higher. Further, if the cooling is stopped at a temperature exceeding 250 ° C., the transformation is not completed, MA is excessively contained in the structure, and the strength of the steel sheet is lowered. Therefore, cooling is stopped at 250 ° C. or lower. Further, when the cooling rate is slow, ferrite precipitates and the area ratio of bainite and martensite decreases. Therefore, the cooling rate is set to 2 ° C./sec or more. The upper limit of the cooling rate is not particularly limited, but is about 80 ° C./sec.

[Tempering process]
Subsequently, the steel subjected to the second rolling step is tempered at a tempering temperature of 560 ° C. or higher and 700 ° C. or lower. Tempering reduces MA and makes it possible to achieve both strength and toughness. The lower limit of the preferred tempering temperature is 580 ° C, and the upper limit of the preferred tempering temperature is 650 ° C.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples as well as the present invention, and appropriate modifications are made to the extent that it can meet the purposes of the preceding and the following. Of course, it is possible to carry out, and all of them are included in the technical scope of the present invention.

1. 1. Sample Preparation Using the steel pieces having the chemical composition shown in Table 1, a sample was prepared under the production conditions shown in Table 2.
As for the temperature of each step shown in Table 2, the surface temperature was measured using a radiation thermometer. After hot rolling in the first rolling step, the mixture was cooled to 300 ° C. or lower.
Further, in Tables 1 to 3, the underlined numerical values indicate that the values are outside the scope of the embodiment of the present invention. Further, in Table 2, for example, the column described as "1003-962" means a numerical value in the range of 962 to 1003.

2. 2. Steel structure The steel structure was observed as follows.
(1) A sample is taken from the steel sheet so that the sheet thickness cross section including the front and back surfaces of the steel sheet, which is parallel to the rolling direction and perpendicular to the surface of the steel sheet, can be observed.
(2) The observation surface is mirror-finished by polishing with wet emery polishing paper (# 150 to # 1000) or by a polishing method having the same function (polishing with a polishing agent such as diamond slurry).
(3) The polished sample is corroded with a 3% nital solution and a repeller solution according to the purpose, and grain boundaries and MA are exposed.
(4) At t (plate thickness) / 4 sites, the exposed structure was observed with an optical microscope (observation magnification: 400 times, observation area: about 200 μm × about 160 μm), and polygonal ferrite, bainite, and martensite. , MA, as well as the microstructural fraction of the carbide, the aspect ratio of the old γ grains, and the average circle-equivalent diameter of the carbide were calculated. Based on the calculated tissue fraction, the area ratio of bainite and martensite to the total tissue, the area ratio of MA to the total tissue, and the area ratio of carbide to the area of bainite and martensite were calculated. The results of these measurements are shown in Table 3. The carbides have floated as granules on the observation surface due to the above corrosion, and it has been confirmed by composition analysis that these granules are carbides. Therefore, the observed granules were judged to be carbides.

The bainite referred to here refers to a structure in which upper bainite, lower bainite, bainitic ferrite, etc. are tempered, but it is generally difficult to select these structures including tempered martensite. Since the structure was sufficiently tempered, structures other than polygonal ferrite and MA were designated as bainite and / or martensite. It was also confirmed that none of the test pieces used in this example contained a pearlite structure.

3. 3. Mechanical properties Tensile tests were performed on the obtained samples, YP (YS), TS and EL were measured, and YR and TS × EL were calculated. In addition, a Charpy impact test was performed to measure the Charpy absorption energy and the separation length. The details will be described below.

[Tensile test (evaluation of tensile properties)]
A round bar tensile test piece is collected from a portion of t (plate thickness) / 4 in the direction perpendicular to rolling (plate width direction, C direction), and a tensile test is performed in the manner of JIS Z 2201, yield strength (YP), tensile strength. The strength (TS), elongation (total elongation, EL) and uniform elongation (UE) were measured, and the yield ratio (YR) and TS × EL (product A of tensile strength TS and elongation EL) were calculated. As the test piece, a JIS No. 4 test piece or a JIS No. 5 test piece was used. Yield strength YP of 700 MPa or more, tensile strength TS of 780 to 930 MPa, yield ratio YR of 85% or more, and TS × EL of 15000 MPa% or more are high strength (excellent in tensile properties) and It was evaluated as having excellent elongation characteristics.

[Charpy impact test (evaluation of impact characteristics)]
A full-size V-notch test piece was taken from the portion of t (plate thickness) / 4 in the direction perpendicular to the rolling direction, and a Charpy impact test was performed at a test temperature of -40 ° C in the same manner as JIS Z 2242 to measure the absorbed energy. .. For the absorbed energy, the average value of the three test pieces was adopted. Then, those having an absorption energy of 100 J or more were evaluated as having excellent low temperature toughness (excellent impact characteristics).
Further, the separation length in the C direction in the fracture surface was measured, and those having a separation length of 1 mm or more and 100 mm or less were regarded as acceptable.
These results are shown in Table 3.

Consider the results in Table 3.
Sample No. which is an example sample satisfying the conditions of the present invention. 4 to 8, 10 and 12 to 16 all have a yield strength YP of 700 MPa or more, a tensile strength TS of 780 to 930 MPa, a yield ratio YR of 85% or more, and a product TS of tensile strength TS and elongation EL in the plate width direction. × EL is 15,000 MPa% or more, Charpy absorption energy vE-40 at ₋₄₀ ° C. is 100 J or more, and the separation length in the fracture surface is 1 mm or more and 100 mm or less.

On the other hand, sample No. In Nos. 1 to 3, a steel sheet having both desired strength and toughness could not be obtained because the tempering step was not performed or the tempering temperature was low.
Sample No. In No. 9, since the cumulative rolling reduction in the unrecrystallized rolling step was low, the aspect ratio of the old γ grains was small, and the low temperature toughness was inferior.
Sample No. In No. 11, since the cooling rate in the cooling step was slow, the area ratios of bainite and martensite decreased, and the yield strength YP deviated to a low level, so that the desired strength could not be obtained.
Sample No. In 17 to 19, the C amount, Mn amount, P amount, Cu amount, Ni amount, Ti amount, Ca amount and DI value do not satisfy the specified values, and the first rolling step and the unrecrystallized rolling step are not performed. Further, since the cooling rate in the cooling step was low, the area ratio of the charcoal and the diameter corresponding to the average circle were large, the aspect ratio of the old γ grains was low, and the elongation characteristics were inferior.

Claims (2)

C: 0.035 to 0.070% by mass,
Si: 0.10 to 0.55% by mass,
Mn: 1.55 to 2.20% by mass,
P: 0.0100% by mass or less (not including 0% by mass),
S: 0.0050% by mass or less (not including 0% by mass),
Al: 0.015 to 0.050% by mass,
Ti: 0.005 to 0.030% by mass,
N: 0.0010 to 0.0060% by mass,
Ca: 0.0005 to 0.0040% by mass,
B: 0.0003 to 0.0030% by mass,
Includes Cu: 0.20 to 0.70% by mass, Ni: 1.05 to 2.00% by mass, Cr: 0.55 to 1.00% by mass, and Mo: 0.20 to 0.60 % by mass. , The balance consists of Fe and unavoidable impurities,
Pcm represented by the following formula (1) is 0.30 or less,
The DI represented by the following formula (2) is 7.0 or more, and the Ceq represented by the following formula (3) is more than 0.45.
The metallographic structure
The area ratio of bainite and martensite is 90% or more, and the area ratio of MA (Martensite-Austenite Constituent) is 5% or less.
The area ratio of carbide to the area of bainite and martensite is 5% or less, and the average circle-equivalent diameter of the carbide is 0.15 μm or less.
The aspect ratio, which is the value obtained by dividing the length of the old austenite grains in the rolling direction by the length in the plate thickness direction, is 3 or more and 20 or less.
In the plate width direction
Yield strength YP is 700MPa or more,
Tensile strength TS is 780 to 930 MPa,
Yield ratio YR is 85% or more,
The product TS × EL of tensile strength TS and elongation EL is 15000 MPa% or more, the Charpy absorption energy vE-40 at ₋₄₀ ° C. is 100 J or more, and the separation length in the fracture surface is 1 mm or more and 100 mm or less. High strength and high ductility steel plate.

Pcm = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + 5 × [B] ... (1) )

DI = 1.16 x ([C] / 10) ^0.5 x (0.7 x [Si] + 1) x (5.1 x ([Mn] -1.2) + 5) x (0.35 x) [Cu] +1) x (0.36 x [Ni] +1) x (2.16 x [Cr] +1) x (3 x [Mo] +1) x (1.75 x [V] +1) x (200) × [B] +1) ・ ・ ・ (2)

Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (3)

However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu, Ni, respectively. The content (% by mass) of Cr, Mo, V and B is shown. The steel piece having the chemical composition according to claim 1 is heated so that the surface temperature becomes 1100 ° C to 1400 ° C, then hot-rolled so that the cumulative reduction rate becomes 5% or more, and then the surface is subjected to hot rolling. The first rolling process, which cools the temperature to 300 ° C or lower, and
In the second rolling step, after the steel subjected to the first rolling step is heated so that the surface temperature becomes 950 ° C to 1250 ° C, the surface temperature is in the temperature range of 900 ° C to 1200 ° C. In the recrystallizing rolling step of hot rolling so that the cumulative rolling reduction in the above temperature is 5% or more, and in the temperature range where the surface temperature is 900 ° C. or less, the cumulative rolling reduction in the temperature range is 30% or more, 80% or less, and An unrecrystallized rolling process in which hot rolling is performed so that the rolling completion temperature is 680 to 900 ° C at the surface temperature, and 2 ° C / sec from the cooling start temperature of 500 ° C or higher to the cooling stop temperature of 250 ° C or lower. A cooling step of cooling at the above cooling rate, a second rolling step including this order, and a second rolling step.
A tempering step of tempering the steel subjected to the second rolling step at a tempering temperature of 560 ° C. or higher and 700 ° C. or lower.
The method for producing a high-strength, high-ductility steel sheet according to claim 1.