JP6795048B2 - Non-treated low yield ratio high-strength thick steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a non-healing low yield ratio high tension thick steel plate suitable for welded structures such as buildings, and is particularly earthquake resistant due to its high strength and plastic deformation ability used for construction of building box columns. It relates to a high-strength thick steel plate with an excellent non-welded low yield ratio. Here, "non-tempered" means a steel sheet manufactured without quenching and tempering.

In recent years, steel sheets having excellent seismic resistance have been required for building steel structures and the like from the viewpoint of ensuring safety in the event of an earthquake. The lower the yield ratio YR (yield strength YS / tensile strength TS), the higher the plastic deformability and the better the earthquake resistance of the steel sheet. Therefore, it is obligatory to use a steel sheet with a YR of 80% or less for the building structure. There is. On the other hand, recently, with the increase in size, height, and span of structures, steel plates having higher strength than before are required.

In order to increase the strength of the steel sheet, it is necessary to harden the structure, but there is a problem that the above-mentioned criteria cannot be satisfied because the YR becomes high due to the hardening.

In response to such a problem, for example, Patent Document 1 proposes a low yield ratio non-healing steel sheet having a tensile strength of 590 MPa or more by forming a structure mainly composed of ferrite which is a soft phase.

Further, Patent Document 2 proposes a method for producing a low yield ratio tempered steel sheet having a tensile strength of 780 MPa or more by performing a multi-step heat treatment in the process of producing the steel sheet.

JP-A-2007-177325 JP-A-2007-177266

However, since the low yield ratio non-healable steel sheet proposed in Patent Document 1 is mainly composed of ferrite which is a soft phase, it is not possible to secure a tensile strength of 690 MPa or more.

Further, in the method proposed in Patent Document 2, since it is necessary to perform multi-step heat treatment in order to achieve both high strength and low yield ratio, the manufacturing cost increases.

Furthermore, the deformability of the steel frame until it breaks into brittleness is affected by the Charpy absorption energy of the steel material. It is known that the Charpy absorption energy tends to decrease with temperature, and in particular, when the temperature falls below the fracture surface transition temperature, the Charpy absorption energy decreases significantly. Therefore, it is also required that the fracture surface transition temperature is low.

The present invention has been made in view of the above circumstances, can be manufactured without performing multi-step heat treatment, has a tensile strength of 690 MPa or more and a yield ratio of 80% or less without tempering, and has a fracture surface transition. An object of the present invention is to provide a non-heat-treated low yield ratio high-tensile thick steel sheet having a temperature of −25 ° C. or lower. Another object of the present invention is to provide a method for producing the non-healing low yield ratio high tension thick steel sheet.

The present inventors have diligently studied and obtained the following findings in order to achieve the above-mentioned object.

(1) Ar @ 3 starts accelerated cooling after the completion of the hot rolling at a transformation point or higher, the accelerated cooling at temperatures steel sheet surface temperature after heat recuperation is equal to or less than the bainite transformation start temperature (B _S point), 300 ° C. or higher By stopping the above, a hard phase such as bainite can be generated, and at the same time, a larger amount of harder island-like martensite (Martensite-Austenite Constituent, MA) structure can be generated in the former austenite grains.

(2) Since movable dislocations remain at high density in MA, YR can be lowered by generating MA in the old austenite grains. By forming the structure of the thick steel sheet into a two-phase structure of a hard phase such as bainite and MA, it is possible to achieve both strength and a low yield ratio even in non-healing.

The present invention has been completed by further studying based on the above findings. That is, the gist of the present invention is as follows.

1. 1. By mass%
C: 0.05 to 0.10%,
Si: 0.15-0.40%,
Mn: 0.6 to 1.8%,
P: 0.010% or less,
S: 0.003% or less,
Al: 0.05% or less,
N: 0.0050% or less,
Ti: 0.005 to 0.020%,
Mo: 0.15-0.50%,
Nb: 0.005 to 0.030%,
Cr: 0.05 to 0.50%, and V: 0.005 to 0.070%,
The balance consists of Fe and unavoidable impurities, and
It has a component composition in which Ti / N defined as the ratio of Ti content (mass%) to N content (mass%) is 2.0 or more and 4.0 or less.
By area ratio,
Island-shaped martensite: 15% or more, of which island-shaped martensite present in the former austenite grain: 4% or more,
Ferrite: 0-10%, including
The balance has a microstructure at a plate thickness of 1/4 consisting of a pearlite phase, a bainite phase, a martensite phase, or a mixed phase thereof.
A non-healing low yield ratio high tension thick steel sheet having a tensile strength of 690 MPa or more, a yield ratio of 80% or less, and a fracture surface transition temperature of -25 ° C or less at a plate thickness 1/4 position.

2. 2. The component composition is further increased by mass%.
Cu: 0.05-1.0%,
The non-conditioned low yield ratio high tension according to 1 above, which contains 1 or 2 or more selected from the group consisting of Ni: 0.05 to 2.0% and B: 0.0003 to 0.0050%. Thick steel plate.

3. 3. The steel material having the component composition according to 1 or 2 above is heated to a heating temperature of 1050 to 1250 ° C.
The steel material is hot-rolled under the conditions that the cumulative reduction rate in the temperature range of 950 ° C. or lower at the surface temperature is 30% or more and the rolling end temperature is the Ar3 transformation point or more at the surface temperature to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet
At the surface temperature, from the cooling start temperature above the Ar3 transformation point to the cooling stop temperature above 300 ° C.
Accelerated cooling at an average cooling rate of 8 ° C./s or higher at the 1/4 position of the plate thickness.
A method for producing a non-healing low yield ratio high-strength thick steel sheet in which the reheat temperature after accelerated cooling is set to be equal to or lower than the bainite transformation start temperature.

According to the present invention, it can be manufactured without performing multi-step heat treatment, has a tensile strength of 690 MPa or more and a yield ratio of 80% or less, and has a fracture surface transition temperature of −25 ° C. or less. It is possible to provide a heat-treated low yield ratio high-tensile thick steel sheet. Further, according to the manufacturing method of the present invention, the non-healing low yield ratio high tension thick steel sheet can be manufactured without performing multi-step heat treatment.

Hereinafter, embodiments of the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited thereto.

[Ingredient composition]
The non-healing low yield ratio high-strength thick steel sheet of the present invention (hereinafter, may be simply referred to as “thick steel sheet”) and the steel material used for producing the thick steel sheet need to have the above-mentioned composition. Hereinafter, each component contained in the component composition will be described. Unless otherwise specified, "%" representing the content of each component means "mass%".

C: 0.05 to 0.10%
C is an element useful for increasing the strength of steel and ensuring the strength required as a structural steel material. Further, C has an action of increasing the volume fraction of the hard phase and lowering the yield ratio. In order to obtain the above effect, a content of 0.05% or more is required. Therefore, the C content is set to 0.05% or more, preferably 0.06% or more. On the other hand, a content of more than 0.10% significantly reduces weldability and toughness. Therefore, the C content is 0.10% or less, preferably 0.09% or less.

Si: 0.15-0.40%
Si acts as an antacid and dissolves in the steel to increase the strength of the steel material. In order to obtain the above effect, a content of 0.15% or more is required. Therefore, the Si content is 0.15% or more, preferably 0.17% or more. On the other hand, if the content exceeds 0.40%, the toughness of the base metal is lowered and the toughness of the weld heat affected zone (also referred to as HAZ) is remarkably lowered. Therefore, the Si content is 0.40% or less, preferably 0.30% or less.

Mn: 0.6 to 1.8%
Mn is an element that has the effect of solid-solving and increasing the strength of steel. In addition, Mn is cheaper than other alloying elements. Therefore, in the present invention, one of the purposes of which is to minimize the content of other expensive alloying elements, the Mn content is set to 0.6 in order to secure the desired high strength (tensile strength of 690 MPa or more). % Or more, preferably 0.8% or more. On the other hand, a content of more than 1.8% significantly reduces the toughness and HAZ toughness of the base metal. Therefore, the Mn content is set to 1.8% or less, preferably 1.6% or less.

P: 0.010% or less P is an element having an action of increasing the strength of steel, but lowers toughness, particularly toughness of welded parts. Therefore, it is desirable to reduce the P content as much as possible. If the content exceeds 0.010%, the above-mentioned adverse effects become remarkable, so the P content is set to 0.010% or less. On the other hand, the lower the P content, the more preferable it is, so that the lower limit of the P content is not particularly limited. However, excessive reduction can lead to higher refining costs. Therefore, from the viewpoint of cost reduction, the P content is preferably 0.0005% or more, and more preferably 0.001% or more.

S: 0.003% or less S exists as sulfide-based inclusions such as MnS in steel and deteriorates the toughness of the base metal and the welded portion. Further, S is unevenly distributed in a large amount in the central segregation portion of the slab and the like, and tends to cause defects in the slab and the like. Such a tendency becomes remarkable when the content exceeds 0.003%. Therefore, the S content is set to 0.003% or less. Although the lower limit of the S content is not particularly limited, the S content is preferably about 0.001% or more because excessive reduction of S increases the refining cost and is economically disadvantageous.

Al: 0.05% or less Al is an element that acts as a deoxidizing agent, and is most commonly used as a deoxidizing agent in the molten steel deoxidizing process of high-strength steel. However, if the content exceeds 0.05%, the toughness of the base metal is lowered, and the toughness of the weld metal portion is lowered by being mixed with the weld metal during welding. Therefore, the Al content is 0.05% or less, preferably 0.045% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of enhancing the effect of adding Al, the Al content is preferably 0.01% or more.

N: 0.0050% or less If N is solid-solved in steel, strain aging occurs after cold working and the toughness deteriorates. Therefore, it is desirable to reduce the N content as much as possible. If it is contained in excess of 0.0050%, the toughness is significantly deteriorated. Therefore, the N content is set to 0.0050% or less. On the other hand, the lower the N content is, the more preferable it is. Therefore, the lower limit of the N content is not particularly limited. However, excessive reduction can lead to higher refining costs. Therefore, from the viewpoint of cost reduction, the N content is preferably 0.0005% or more, and more preferably 0.0010% or more.

Ti: 0.005 to 0.020%
Ti is an element having a strong affinity for N and precipitates as TiN during solidification. As a result, the solid solution N in the steel is reduced, so that the deterioration of toughness after cold working due to the strain aging of N is suppressed. In addition, the precipitated TiN further refines the structure of HAZ and contributes to the improvement of HAZ toughness. In order to obtain these effects, a content of 0.005% or more is required. Therefore, the Ti content is 0.005% or more, preferably 0.007% or more. On the other hand, if the content exceeds 0.020%, the TiN particles become coarse and the above-mentioned effect cannot be expected. Therefore, the Ti content is 0.020% or less, preferably 0.015% or less.

Mo: 0.15-0.50%
Mo is an element effective for improving toughness and increasing strength. TS: A content of 0.15% or more is required to stably produce a steel sheet of 690 MPa or more. Therefore, the Mo content is 0.15% or more, preferably 0.20% or more. On the other hand, if it is added in excess of 0.50%, the weldability and HIC resistance deteriorate. Therefore, the Mo content is 0.50% or less, preferably 0.40% or less.

Nb: 0.005 to 0.030%
Nb is an element having an effect of improving hardenability. In order to obtain the above effect, a content of 0.005% or more is required. Therefore, the Nb content is 0.005% or more, preferably 0.010% or more. On the other hand, if it is added in excess of 0.030%, the HAZ toughness and the base metal toughness deteriorate. Therefore, the Nb content is 0.030% or less, preferably 0.025% or less.

Cr: 0.05 to 0.50%
Cr has the effect of increasing the strength of the base metal through the improvement of hardenability, and is an element useful for increasing the strength of thick steel sheets. In order to obtain the above effect, a content of 0.05% or more is required. Therefore, the Cr content is set to 0.05% or more, preferably 0.10% or more. On the other hand, a content exceeding 0.50% causes an increase in alloy cost. Therefore, the Cr content is 0.50% or less, preferably 0.40% or less.

V: 0.005 to 0.070%
V is an element having the effect of increasing the strength by strengthening precipitation. In order to obtain the above effect, a content of 0.005% or more is required. Therefore, the V content is 0.005% or more, preferably 0.010% or more. On the other hand, if it is added in excess of 0.070%, the HAZ toughness and the base metal toughness deteriorate. Therefore, the V content is 0.070% or less, preferably 0.060% or less.

In one embodiment of the present invention, the composition may include each of the above elements and the balance may be Fe and unavoidable impurities.

Further, in another embodiment of the present invention, the component composition may further optionally contain 1 or 2 or more selected from the group consisting of Cu, Ni, and B in the amounts described below.

Cu: 0.05-1.0%
Cu is an element having the effect of improving the strength of thick steel sheets by strengthening solid solution and improving hardenability. In order to obtain the above effect, it is preferably contained in an amount of 0.05% or more. Therefore, when Cu is added, the Cu content is 0.05% or more, preferably 0.10% or more. On the other hand, a content exceeding 1.0% causes an increase in alloy cost and deterioration of surface properties due to hot brittleness. Therefore, when Cu is added, the Cu content is 1.0% or less, preferably 0.50% or less.

Ni: 0.05-2.0%
Ni is an element that has the effect of increasing the strength of a steel sheet with almost no deterioration in toughness. Moreover, since it has a small adverse effect on HAZ toughness, it is an extremely useful element for increasing the strength of thick steel sheets. In order to obtain the above effect, it is desirable to contain 0.05% or more. Therefore, when Ni is added, the Ni content is set to 0.05% or more, preferably 0.10% or more. On the other hand, since Ni is an expensive element, a content exceeding 2.0% causes an increase in alloy cost. Therefore, when Ni is added, the Ni content is set to 2.0% or less, preferably 1.0% or less.

B: 0.0003 to 0.0050%
B is an element that contributes to the increase in the strength of steel through the improvement of hardenability. In order to obtain the above effect, it is desirable to contain 0.0003% or more. Therefore, when B is added, the B content is 0.0003% or more, preferably 0.0006% or more. On the other hand, a content exceeding 0.0050% deteriorates the toughness of the base metal and HAZ. Therefore, when B is contained, the B content is set to 0.0050% or less, preferably 0.0030% or less.

Ti / N: 2.0 to 4.0
The component composition further needs to satisfy the condition that Ti / N defined as the ratio of Ti content (mass%) to N content (mass%) is 2.0 or more and 4.0 or less. The reason will be described below.

In the thick steel sheet of the present invention, the solid solution N is fixed as TiN by containing Ti in an amount commensurate with the N content. For that purpose, it is necessary to set Ti / N to 2.0 or more. When Ti / N is less than 2.0, Ti is too small as compared with N, and a large amount of N remains as a solid solution N. As a result, the HAZ toughness is lowered, and the member deformation performance is lowered due to the occurrence of brittle fracture from the welded portion. The Ti / N is preferably 2.5 or more.

On the other hand, when Ti / N exceeds 4.0, the TiN particles become coarse and the desired effect cannot be secured. Therefore, Ti / N needs to be 4.0 or less. The Ti / N is preferably 3.5 or less.

[Micro tissue]
The thick steel sheet of the present invention contains island-shaped martensite: 15% or more and ferrite: 0 to 10% in terms of area ratio, and the balance is composed of a pearlite phase, a bainite phase, a martensite phase, or a mixed phase thereof. It has a microstructure at the position of 1/4 of the plate thickness. Hereinafter, the reason for limiting the microstructure of the thick steel sheet to the above range will be described.

(Island martensite)
Island-shaped martensite (MA) is a tissue that significantly improves TS more than YS, and is a particularly important tissue that contributes to lower YR and higher toughness. In order to obtain this effect, the area ratio of MA is set to 15% or more, and the area ratio of MA existing in the former austenite grains is set to 4% or more. The area ratio of MA is preferably more than 20%. On the other hand, the upper limit of the area ratio of MA is not particularly limited, but if the area ratio of MA becomes excessively high, the toughness may decrease. Therefore, the area ratio of MA is preferably 30% or less. Here, the "area ratio of MA existing in the former austenite grain" refers to the area ratio of MA existing in the former austenite grain to the entire structure. Further, the area ratio of the MA refers to the area ratio of all MA including MA existing in the old austenite grains.

(Ferrite)
Since ferrite is a soft phase, if the area ratio exceeds 10%, the desired strength cannot be satisfied. Therefore, the area ratio of ferrite is set to 10% or less. On the other hand, from the viewpoint of improving the strength, the lower the area ratio of ferrite is, the more preferable it is. Therefore, the lower limit of the ferrite area ratio is set to 0%.

(Remaining organization)
The remaining structure other than the above shall be any of the following phases (1) to (4), which is a hard phase, from the viewpoint of ensuring strength.
(1) Pearlite phase (2) Bainite phase (3) Martensite phase (4) Two-phase or three-phase mixed phase selected from the above (1) to (3)

As will be described later, the microstructure of the thick steel sheet and its area ratio can be measured by observing the cross section of the thick steel sheet perpendicular to the rolling direction with a 400x optical microscope and a 2000x scanning electron microscope. it can.

[Tensile properties]
The non-healing low yield ratio high-tensile thick steel sheet of the present invention has the tensile properties described below. These characteristics can be measured by performing a tensile test under the conditions described in the examples.

TS: 690 MPa or more The non-healing low yield ratio high-strength thick steel sheet of the present invention has a tensile strength (TS) of 690 MPa or more. On the other hand, the upper limit of TS is not particularly limited, but may be, for example, 800 MPa or less, or 780 MPa or less.

YR: 80% or less The non-treated low yield ratio high-strength thick steel sheet of the present invention has a yield ratio (YR) of 80% or less. Here, the yield ratio YR (%) is (yield strength YS / tensile strength TS) × 100 (%). On the other hand, the lower limit of YR is not particularly limited, but may be, for example, 70% or more.

vTrs: -25 ° C or lower In the present invention, the fracture surface transition temperature (vTrs), which is an index of toughness, is set to -25 ° C or lower. Here, the fracture surface transition temperature is a value at the position where the plate thickness is 1/4. On the other hand, the lower the fracture surface transition temperature, the better the toughness, so the lower limit of the fracture surface transition temperature is not particularly limited. However, it may usually be −45 ° C. or higher. The fracture surface transition temperature can be measured by a Charpy impact test, and specifically, can be obtained by the method described in Examples.

[Plate thickness]
In the present invention, the "thick steel plate" refers to a steel plate having a plate thickness of 19 mm or more. The upper limit of the plate thickness is not particularly limited, but is preferably 50 mm or less.

[Production method]
Next, a method for manufacturing a non-treatable low yield ratio high-strength thick steel sheet according to an embodiment of the present invention will be described.

The thick steel sheet of the present invention can be produced by sequentially performing the following treatments (1) to (3) under specific conditions on a steel material having the above-mentioned composition. Hereinafter, each step will be described.
(1) Heating (2) Hot rolling (3) Accelerated cooling

[Steel material]
As the steel material, any material having the above-mentioned composition can be used. Further, the steel material is not particularly limited and can be produced by any method. For example, molten steel having the above-mentioned composition is melted by a common melting method using a converter, an electric furnace, or the like, and a slab (steel material) having a predetermined size is obtained by a common casting method such as a continuous casting method. be able to. It should be noted that a steel piece obtained by further hot rolling the slab to obtain a desired size and shape can be used as the steel material. Further, a steel piece produced by the ingot-block rolling method can also be used as a steel material.

[heating]
Heating temperature: 1050 to 1250 ° C
Next, the steel material is heated (heating step). In the heating, the steel material is heated to a heating temperature of 1050 to 1250 ° C. or lower. If the heating temperature is less than 1050 ° C., the hardenability is lowered and the desired strength cannot be secured. Therefore, the heating temperature is set to 1050 ° C. or higher, preferably 1080 ° C. or higher. On the other hand, if the heating temperature is higher than 1250 ° C., the crystal grains become coarse and the toughness deteriorates. Therefore, the heating temperature is set to 1250 ° C. or lower, preferably 1150 ° C. or lower.

[Hot rolling]
Next, the heated steel material is hot-rolled to obtain a hot-rolled steel sheet (hot-rolling step). The conditions for hot rolling will be described below. The temperature in the following description represents the surface temperature of the steel sheet unless otherwise specified.

Cumulative rolling reduction in a temperature range of 950 ° C or lower: 30% or more In order to appropriately miniaturize the microstructure, controlled rolling is performed in a temperature range where the surface temperature of the steel sheet is 950 ° C or lower. If the cumulative reduction rate in the temperature range is less than 30%, the structure will not be miniaturized due to insufficient reduction amount, and the hardenability will increase too much, making it impossible to secure the desired toughness. Therefore, the cumulative reduction rate in the temperature range of 950 ° C. or lower at the surface temperature is set to 30% or more. On the other hand, the upper limit of the cumulative reduction rate is not particularly limited, but is preferably 50% or less.

Rolling end temperature: When rolling is performed at a temperature equal to or higher than the Ar3 transformation point and lower than the Ar3 transformation point, coarse ferrite is generated during rolling, and the desired high strength cannot be secured. Therefore, the rolling end temperature is set to the Ar3 transformation point or higher. On the other hand, the upper limit of the rolling end temperature is not particularly limited, but is preferably 870 ° C. or lower. As the Ar3 transformation point, the value calculated by using the following equation (1) shall be used.
Ar3 transformation point (° C.) = 900-332C + 6Si-77Mn-20Cu-50Ni-18Cr-68Mo ... (1)
Here, the element symbols (C, Si, Mn, Cu, Ni, Cr, and Mo) in the above formula (1) refer to the content (mass%) of each element. The content of elements not contained in steel shall be zero.

[Accelerated cooling]
Next, the hot-rolled steel sheet is accelerated and cooled (accelerated cooling step). In the accelerated cooling, the cooling start temperature, the cooling stop temperature, and the average cooling rate are in the following ranges.

Cooling start temperature: Ar3 transformation point or more If the temperature at which accelerated cooling is started (cooling start temperature) is less than the Ar3 transformation point, coarse ferrite is generated and the desired high strength cannot be secured. Therefore, the cooling start temperature, which is the temperature at which accelerated cooling is started, is set to the Ar3 transformation point or higher.

Cooling stop temperature: 300 ° C or higher If the temperature at which accelerated cooling is stopped (cooling stop temperature) is less than 300 ° C, the area ratio of MA cannot be set to a desired value, and a desired yield ratio cannot be obtained. .. Therefore, the cooling stop temperature is set to 300 ° C. or higher.

Average cooling rate: 8 ° C./s or more If the average cooling rate in accelerated cooling is less than 8 ° C./s, a large amount of ferrite is generated and the desired strength cannot be obtained. Therefore, the average cooling rate is set to 8 ° C./s or higher. The average cooling rate refers to the average cooling rate at the temperature at the plate thickness 1/4 position in the temperature range from the cooling start temperature to the cooling stop temperature.

After the completion of the accelerated cooling, the cooling is usually not limited, but it may be allowed to cool (air cooling). The cooling can be carried out up to room temperature, for example.

[Reheat]
Reheat temperature: B _S point or less The temperature of the steel sheet at the time when accelerated cooling is stopped is higher inside than on the surface. Therefore, after the acceleration cooling is stopped, the surface temperature of the steel sheet rises due to heat transfer from the inside of the steel sheet (recovery heat). When the surface temperature of the recuperator after cooling stop exceeds bainite transformation start temperature (B _S point), ferrite causes a large amount produced, can not satisfy the desired tensile strength. Therefore, the recuperation temperature after the accelerated cooling than bainite transformation starting temperature (B _S point). Here, the "reheat temperature" refers to the temperature at which the surface temperature of the steel sheet becomes the highest due to the reheat after the acceleration cooling is stopped.

As the B _S point, the value calculated by using the following equation (2) shall be used.
B _S (° C.) = 830-270C-90Mn-37Ni-70Cr-83Mo ... (2)
Here, the element symbols (C, Mn, Ni, Cr, and Mo) in the above formula (2) refer to the content (mass%) of each element. The content of elements not contained in steel shall be zero.

After the above-mentioned accelerated cooling and reheating, tempering may be optionally performed for the purpose of adjusting the strength and toughness. The tempering is preferably performed at a tempering temperature of 400 ° C. or higher and 700 ° C. or lower. If the tempering temperature is less than 400 ° C., the desired effect cannot be expected. On the other hand, at a tempering temperature exceeding 700 ° C., the strength is significantly reduced.

Hereinafter, the present invention will be described in more detail based on Examples.

The steel material having the composition shown in Table 1 is sequentially heated, hot-rolled, and accelerated cooled under the conditions shown in Table 2 to produce a non-healable thick steel sheet having the plate thickness shown in Table 2. did. The cumulative reduction rate shown in Table 2 is the cumulative reduction rate in the temperature range of the surface temperature of 950 ° C. or lower. The cooling start temperature and cooling stop temperature shown in Table 2 are the surface temperature, and the average cooling rate is the average cooling rate at the 1/4 position of the plate thickness.

A test piece was collected from the obtained thick steel sheet, and a microstructure observation, a tensile test, and an impact test were carried out. The results obtained are shown in Table 3. The test method was as follows.

(1) Structure observation Using a two-step etching method in which a cross section parallel to the rolling direction of a test piece for structure observation with a full thickness is mirror-polished, etched with a nital solution, and then electrolytically etched in a sodium picrinate solution. The MA was revealed. Then, the plate thickness 1/4 position in the cross section was observed using an optical microscope (magnification: 400 times) or a scanning electron microscope (magnification: 2000 times). In the above observation, the microstructure was imaged in three or more fields of view, and the type of tissue, the area ratio of island-shaped martensite, and the area ratio of island-shaped martensite existing in the former austenite (former γ) grain were obtained by image analysis. It was.

(2) Tensile test JIS No. 5 full-thickness tensile test pieces were collected in accordance with JIS Z 2201 so that the tensile direction would be the rolling direction. Using the obtained test piece, a tensile test was carried out in accordance with the provisions of JIS Z 2241 to determine tensile properties (yield strength YS, tensile strength TS). Moreover, the yield ratio YR (= YS / TS × 100%) was calculated from the obtained measured values.

(3) Impact test A V-notch impact test piece was sampled from a plate thickness of 1/4 in accordance with JIS Z 2242. A Charpy impact test was carried out using the obtained test piece to determine the fracture surface transition temperature vTrs (° C.). When vTrs was -25 ° C or lower, the toughness was considered to be excellent.

The non-treated low yield ratio high-strength thick steel sheets satisfying the conditions of the present invention all have a tensile strength of 690 MPa or more, a yield ratio of 80% or less, a fracture surface transition temperature of -25 ° C or less, and high strength and low. It had an excellent yield ratio and toughness. On the other hand, the thick steel sheet that does not satisfy the conditions of the present invention is inferior in at least one of the properties of strength, yield ratio, and toughness.

Claims (3)

By mass%
C: 0.05 to 0.10%,
Si: 0.15-0.40%,
Mn: 0.6 to 1.8%,
P: 0.010% or less,
S: 0.003% or less,
Al: 0.05% or less,
N: 0.0050% or less,
Ti: 0.005 to 0.020%,
Mo: 0.15-0.50%,
Nb: 0.005 to 0.030%,
Cr: 0.05 to 0.50%, and V: 0.005 to 0.070%,
The balance consists of Fe and unavoidable impurities, and
It has a component composition in which Ti / N defined as the ratio of Ti content (mass%) to N content (mass%) is 2.0 or more and 4.0 or less.
By area ratio,
Island-shaped martensite: 15% or more, of which island-shaped martensite present in the former austenite grain: 4% or more,
Ferrite: 0-10%, including
The balance has a microstructure at a plate thickness of 1/4 consisting of a pearlite phase, a bainite phase, a martensite phase, or a mixed phase thereof.
A non-healing low yield ratio high tension thick steel sheet having a tensile strength of 690 MPa or more, a yield ratio of 80% or less, and a fracture surface transition temperature of -25 ° C or less at a plate thickness 1/4 position. The component composition is further increased by mass%.
Cu: 0.05-1.0%,
The non-prepared low yield ratio according to claim 1, which contains 1 or 2 or more selected from the group consisting of Ni: 0.05 to 2.0% and B: 0.0003 to 0.0050%. Tension thick steel plate. The steel material having the component composition according to claim 1 or 2 is heated to a heating temperature of 1050 to 1250 ° C.
The steel material is hot-rolled under the conditions that the cumulative reduction rate in the temperature range of 950 ° C. or lower at the surface temperature is 30% or more and the rolling end temperature is the Ar3 transformation point or more at the surface temperature to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet
At the surface temperature, from the cooling start temperature above the Ar3 transformation point to the cooling stop temperature above 300 ° C.
Accelerated cooling at an average cooling rate of 8 ° C./s or higher at the 1/4 position of the plate thickness.
The method for producing a non-healing low yield ratio high-strength thick steel sheet according to claim 1 or 2, wherein the reheating temperature after accelerated cooling is set to be equal to or lower than the bainite transformation start temperature.