JP7338811B1 - Steel plate and its manufacturing method - Google Patents

Abstract

Provided is a steel plate that has high strength with TS≧690MPa, achieves excellent low-temperature toughness and low yield ratio, and has small acoustic anisotropy. The steel plate has a composition in which the steel plate contains a predetermined amount of a predetermined element, and the remainder is Fe and unavoidable impurities, and the microstructure at a depth of 1/4 of the plate thickness from the surface of the steel plate in the thickness direction is as follows: The ferrite fraction is 5 to 95%, the island martensite fraction is 1 to 30%, and the remainder is tempered martensite and/or bainite, and the region surrounded by large-angle grain boundaries with a misorientation of 15 degrees or more is defined as a crystal grain. In this case, the average aspect ratio of the crystal grains is less than 2.0, and the number density of crystal grains having an equivalent circular diameter of more than 30 μm among the crystal grains is 250 pieces/mm 2 or less.

Description

TECHNICAL FIELD The present invention relates to a steel sheet, and more particularly to a steel sheet that can stably ensure excellent strength, low-temperature toughness and a low yield ratio and is suitable for low-temperature applications, and a method for producing the same. The steel plate of the present invention, for example, in liquefied gas storage tanks for ships, etc., can be mechanically removed instead of post-welding heat treatment for stress relief, so it is suitable for large-sized low-temperature liquefied gas storage tanks. be able to.

Among liquefied gas storage tanks for ships, for tanks classified as independent Type-C tanks and having a design temperature lower than -10°C, when using carbon steel and/or carbon manganese steel, stress after welding Removal is required. This stress relief is usually performed by PWHT (Post Weld Heat Treatment), but if the yield ratio of the steel material is 0.8 or less, mechanical stress relief can also be performed. be. However, as the size of the tank increases, the construction of PWHT becomes difficult, so a material with a low yield ratio that allows mechanical stress relief is desired. For example, when carbon steel is used for a large storage tank for liquefied _CO2 , it has a high tensile strength (hereinafter also referred to as TS) of 690 MPa or more while ensuring excellent toughness at a low temperature of -50 ° C. High strength steel is required. That is, a steel material with a low yield ratio, high strength, and excellent low-temperature toughness is required. In addition, since welding defects may be confirmed by ultrasonic testing during welding, materials with small acoustic anisotropy may be desired.

Regarding a steel sheet with low-temperature toughness and a low yield ratio, for example, Patent Document 1 discloses a steel sheet having an impact toughness of 150 J at -75°C, a yield ratio of 0.8 or less, and a tensile strength of 530 MPa or more.

Japanese Patent Publication No. 2016-507649

However, the steel sheet described in Patent Literature 1 has a maximum TS of 620 MPa, and a steel sheet with a TS of 690 MPa or more has not yet been provided. Thus, conventionally, carbon steel having a low yield ratio, excellent low-temperature toughness, and high strength of 690 MPa or more has not been provided, so expensive nickel steel such as 9% Ni steel has to be used. However, there is a problem that the material cost is high.

The present invention has been made in view of the above circumstances, and provides a steel sheet having high strength of TS≧690 MPa, excellent low-temperature toughness and low yield ratio, and small acoustic anisotropy, and a method for producing the same. intended to

In order to solve the above problems, the present inventors conducted intensive research on the chemical composition and structure that achieve low-temperature toughness and low yield ratio on the premise of high-strength steel. Obtained. That is, the ferrite fraction in the microstructure at a depth position of 1/4 of the sheet thickness in the sheet thickness direction from the surface of the steel sheet is 5 to 95%, the island martensite fraction is 1 to 30%, and the remainder is tempered marten. A region surrounded by large-angle grain boundaries having a site and/or bainite structure with an orientation difference of 15 degrees or more is defined as a crystal grain, and the number density of crystal grains with an equivalent circle diameter of more than 30 μm is 250/mm. It has been newly discovered that an average aspect ratio of ² or less and an average aspect ratio of less than 2.0 is effective for improving the desired characteristics.

Here, the above-mentioned ferrite refers to a BCC phase that remains without undergoing reverse transformation even when martensite or bainite is heat-treated at a temperature of Ac ₁ point or more, and inherits the original lath-like structure. A low yield ratio can be achieved by making the relatively soft ferrite phase 5 to 95% and finely dispersing island martensite.
Further, when a region surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is defined as a crystal grain, the number density of crystal grains with an equivalent circle diameter of more than 30 μm is 250/mm ² or less. Low-temperature toughness can be achieved by suppressing the formation of ferrite and the rest of the ferrite and island martensite to be tempered martensite and/or bainite phase.
Furthermore, by setting the average aspect ratio of the crystal grains to less than 2.0, it is possible to simultaneously reduce the acoustic anisotropy.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
1. in % by mass,
C: 0.02% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and 2.50% or less,
Ni: 0.5% or more and less than 5.0%,
P: 0.03% or less,
S: 0.005% or less and N: 0.0010% or more and 0.0080% or less, with the balance being Fe and unavoidable impurities,
The microstructure at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate has a ferrite fraction of 5 to 95%, an island martensite fraction of 1 to 30%, and the remainder being tempered martensite and / Or when a region surrounded by large-angle grain boundaries with a misorientation of 15 degrees or more is bainite, the average aspect ratio of the crystal grain is less than 2.0, and the equivalent circle diameter of the crystal grain is A steel sheet in which the number density of crystal grains of more than 30 μm is 250/mm ² or less.

2. The component composition is further, in mass %,
Al: 0.100% or less Nb: 0.1% or less,
Cr: 2.00% or less,
Mo: 1.0% or less,
Cu: 2.0% or less,
V: 0.05% or less,
Ti: 0.03% or less,
B: 0.0030% or less,
Ca: 0.007% or less,
2. The steel sheet according to 1 above, containing at least one selected from REM: 0.010% or less and Mg: 0.007% or less.

3. The steel material having the chemical composition described in 1 or 2 above is subjected to hot rolling, followed by first heating and holding, then quenching, then second heating and holding, and then cooling treatment. In the steel plate manufacturing method,
In the hot rolling, the finishing temperature is 900 ° C. or higher,
In the first heating and holding, the heating temperature is set to a temperature range of ₃ points to 1000 ° C.,
In the quenching, the average cooling rate from 600 ° C. to 300 ° C. at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate is 3 ° C./s or more, and the cooling end temperature is 300 ° C. or less. ,
In the second heating and holding, the heating temperature is set to a temperature range of Ac ₁ point or more and AC ₃ points or more,
In the cooling treatment, the average cooling rate from 700 ° C. to 500 ° C. at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate is 3 ° C./s or more, and the cooling end temperature is 500 ° C. or less. A method for manufacturing a steel sheet, wherein the temperature is 200°C or higher.

According to the steel plate of the present invention, although the material is carbon steel or carbon manganese steel, steel structures used in low temperature environments such as large cryogenic storage tanks for ships, such as liquefied _CO2 tanks and LPG tanks Since it can be used for steel, and the construction cost can be reduced more than that of nickel steel, it brings about a remarkable effect in industry.

Hereinafter, embodiments of the present invention will be specifically described. In addition, the following description shows a preferred embodiment of the present invention, and the present invention is not limited to this.

[Component composition]
The steel sheet of the present invention has a predetermined chemical composition. Moreover, it is preferable that the steel material used for manufacturing the steel sheet of the present invention also has the above-described predetermined chemical composition. Each element contained in this component composition will be described below. In this specification, unless otherwise specified, "%" as a unit of content of each element means "% by mass".

C: 0.02% or more and 0.15% or less C is an element that has the effect of improving the strength of the steel sheet. To obtain this effect, the C content should be 0.02% or more. Preferably, the C content is 0.03% or more. On the other hand, when the C content exceeds 0.15%, the amount of island-shaped martensite in the steel sheet increases and the low temperature toughness decreases. Therefore, the C content is made 0.15% or less. The C content is preferably 0.12% or less.

Si: 0.01% to 0.50% Si is an element that acts as a deoxidizing agent. In order to express this effect, the Si content is set to 0.01% or more. Preferably, the Si content is 0.03% or more. On the other hand, when the Si content is excessively high, the toughness is lowered. Therefore, the Si content is set to 0.50% or less. The Si content is preferably 0.30% or less.

Mn: 0.05% or more and 2.50% or less,
Mn is an element effective in increasing the hardenability of steel and increasing the strength of the steel sheet. To obtain this effect, the Mn content is 0.05% or more. Preferably, the Mn content is 0.10% or more. On the other hand, if Mn exceeds 2.50%, the toughness deteriorates, so the Mn content is made 2.50% or less. The Mn content is preferably 2.00% or less.

Ni: 0.5% or more and less than 5.0% Ni is an element effective in improving the low temperature toughness of the steel sheet. For that purpose, the Ni content is set to 0.5% or more. On the other hand, since Ni is an expensive element, the higher the Ni content, the higher the steel sheet cost. Therefore, in the present invention, the Ni content is set to less than 5.0%. The Ni content is preferably 0.8% or more and 3.5% or less.

P: 0.03% or less P is an impurity that can inevitably exist, and is a harmful element that adversely affects the low temperature toughness of the steel sheet. For example, in order to obtain sound base materials and welded joints when steel plates are welded to form a welded structure, it is preferable to reduce the P content as much as possible. Therefore, the P content is suppressed to 0.03% or less. Also, from the viewpoint of low temperature toughness, the lower the P content, the better, so the lower limit is not particularly limited, and may be 0%. On the other hand, excessive reduction causes an increase in cost, so from the viewpoint of cost, it is preferable to set the lower limit of the P content to 0.001%.

S: 0.005% or less S forms MnS in the steel and significantly deteriorates the low temperature toughness. The S content is preferably 0.002% or less. On the other hand, the lower the S content, the better, so the lower limit is not particularly limited, and may be 0%.

N: 0.0010% or more and 0.0080% or less N forms precipitates in steel, and if the N content exceeds 0.0080%, it causes a decrease in the toughness of the base material. However, N is also an element that contributes to grain refinement of the base material by forming AlN, and such an effect can be obtained by setting the N content to 0.0010% or more. Therefore, the N content should be 0.0010% or more and 0.0080% or less. The N content is preferably 0.0020% or more. The N content is preferably 0.0060% or less.

The component composition in one embodiment of the present invention contains the above-described predetermined amount of elements, and the balance is Fe and unavoidable impurities.

In another embodiment of the present invention, the above component composition is optionally one or more selected from Cr, Mo, Al, Cu, Nb, V, Ti, B, Ca, REM and Mg, It can be further contained preferably in the amounts described below.

Cr: 2.00% or less Cr is an element that can improve the strength of the steel sheet without significantly impairing the low temperature toughness. In order to obtain the above effects, the Cr content is preferably 0.01% or more. More preferably, it is 0.30% or more. However, when the Cr content exceeds 2.00%, the low temperature toughness of the steel sheet may deteriorate. Therefore, the Cr content is preferably 2.00% or less. More preferably, it is 0.80% or less.

Mo: 1.0% or less Mo is an element that contributes to the improvement of the strength of steel, and can be arbitrarily contained according to the desired strength. However, if the Mo content exceeds 1.0%, the toughness may deteriorate, so when Mo is contained, the Mo content is preferably 1.0% or less. From the viewpoint of obtaining the strength improvement effect of Mo, it is preferable to set the Mo content to 0.01% or more.

Al: 0.100% or less Al is an element that acts as a deoxidizing agent, and is most commonly used in the molten steel deoxidizing process for high-strength steel. In order to obtain the above effects, the Al content is preferably 0.001% or more. More preferably, the Al content is 0.010% or more. On the other hand, if the Al content exceeds 0.100%, the toughness of the base material may decrease. Therefore, the Al content is preferably 0.100% or less. More preferably, the Al content is 0.07% or less.

Cu: 2.0% or less Cu is an element capable of increasing strength while maintaining high toughness, and can be arbitrarily contained according to the desired strength. However, when the Cu content exceeds 2.0%, hot shortness may occur and the surface properties of the steel sheet may deteriorate. Therefore, when Cu is contained, the Cu content is preferably 2.0% or less. More preferably, the Cu content is 1.0% or less. In order to obtain the above effects, it is preferable to set the Cu content to 0.01% or more. Furthermore, the Cu content is preferably 0.10% or more, more preferably 0.20% or more.

Nb: 0.1% or less Nb is an element that contributes to improving the strength of steel, and can be arbitrarily contained depending on the desired strength. However, if the Nb content exceeds 0.1%, the toughness of the base material may deteriorate. Therefore, when Nb is contained, the Nb content is preferably 0.1% or less. From the viewpoint of obtaining the strength improvement effect of Nb, the Nb content is preferably 0.005% or more.

V: 0.05% or less V is an effective element that increases the strength of the steel sheet by precipitation strengthening. However, if the V content is excessively high, the low-temperature toughness of the steel sheet may deteriorate. Therefore, when V is added, the V content is preferably 0.05% or less. More preferably, the V content is 0.04% or less. On the other hand, the lower limit of the V content is not particularly limited, but the V content is preferably 0.010% or more in order to obtain the above effects.

Ti: 0.03% or less Ti is an element that has the effect of increasing the toughness of the weld zone without degrading the mechanical properties of the base metal when steel sheets are welded to form a welded structure. For that purpose, it is preferable to add 0.003% or more. On the other hand, if the Ti content exceeds 0.03%, the toughness may rather be lowered, so it is preferable to include Ti in the range of 0.03% or less.

B: 0.0030% or less B is an element that enhances hardenability when added in a small amount. In order to effectively exhibit this effect, it is preferable to contain B at 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, the toughness may deteriorate. Therefore, when B is contained, the content is preferably 0.0030% or less.

Ca: 0.007% or less Ca is an element that has the effect of improving the low-temperature toughness of the steel sheet by controlling the form of inclusions in the steel. However, excessive Ca may impair the cleanliness of the steel and reduce the Charpy absorbed energy at low temperatures (hereinafter also referred to as Charpy toughness). Therefore, when Ca is added, the Ca content is preferably 0.007% or less. More preferably, the Ca content is 0.004% or less. On the other hand, although the lower limit of the Ca content is not particularly limited, it is preferably 0.001% or more in order to obtain the above effects.

REM: 0.010% or less REM (rare earth metal), like Ca, is an element that has the effect of improving the low-temperature toughness of the steel sheet by controlling the form of inclusions in the steel. However, excessive REM may impair the cleanliness of the steel and reduce the Charpy toughness. Therefore, when REM is added, the REM content is preferably 0.010% or less. More preferably, the REM content is 0.008% or less. On the other hand, the lower limit of the REM content is not particularly limited, but the REM content is preferably 0.001% or more in order to obtain the above effects.
Here, REM is a general term for 17 elements including Y and Sc in addition to 15 lanthanoid elements, and these elements can be contained singly or in combination. The content of REM means the total content of these elements.

Mg: 0.007% or less Mg, like Ca and REM, is an element that has the effect of improving the low-temperature toughness of the steel sheet by controlling the form of inclusions in the steel. However, excessive Mg may impair the cleanliness of the steel and reduce the Charpy toughness. Therefore, when Mg is added, the Mg content is preferably 0.007% or less. More preferably, the Mg content is 0.004% or less. On the other hand, the lower limit of the Mg content is not particularly limited, but the Mg content is preferably 0.001% or more in order to obtain the above effect.

[Microstructure]
(Ferrite fraction is 5 to 95%, island martensite fraction is 1 to 30%, and the balance is tempered martensite and/or bainite)
The steel sheet of the present invention has a ferrite fraction of 5 to 95% and an island martensite fraction of 1 to 30% in the microstructure at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate. , and the balance is tempered martensite and/or bainite. If the ferrite fraction in the microstructure is less than 5%, the yield ratio does not satisfy 0.80 or less. On the other hand, if the ferrite fraction exceeds 95%, the fraction of hard phases such as island-shaped martensite becomes low, and the yield ratio does not satisfy 0.80 or less. Moreover, when the island-shaped martensite is less than 1%, the yield ratio does not satisfy 0.80 or less. On the other hand, when the island martensite content exceeds 30%, the toughness deteriorates. Furthermore, if the balance is not tempered martensite and/or bainite, such as as-quenched martensite or bainite, the desired toughness is not met.

(Average aspect ratio of crystal grains: less than 2.0, number density of crystal grains with an equivalent circle diameter of more than 30 μm: 250/mm ² or less)
In the steel sheet of the present invention, in the microstructure at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate, the crystal grains are regions surrounded by large-angle grain boundaries with a misorientation of 15 degrees or more. , the average aspect ratio of the crystal grains is less than 2.0, and the number density of crystal grains having an equivalent circle diameter of more than 30 μm among the crystal grains is 250/mm ² or less. When the average aspect ratio of the crystal grains is 2.0 or more, the orientation of the structure tends to be high, resulting in large acoustic anisotropy. Further, when the number density of crystal grains having an equivalent circle diameter of more than 30 μm among the crystal grains exceeds 250/mm ² , the toughness is lowered. The number density of crystal grains having an equivalent circle diameter of more than 30 μm is preferably 150 grains/mm ² or less.

The plate thickness of the steel plate is not particularly limited, and may be any thickness. However, the plate thickness of the steel plate is preferably 6 mm or more and 50 mm or less, for example.
stomach.

[Mechanical properties]
(Tensile strength)
The tensile strength of the steel sheet is not particularly limited, but is preferably 690 MPa or more. This is because the plate thickness can be reduced when applied to a tank. The tensile strength of the steel sheet is more preferably 720 MPa or more. On the other hand, the upper limit of the tensile strength is not particularly limited, but it is preferably 1000 MPa or less.
The tensile strength can be measured by the method described in Examples below.

(yield ratio)
The yield ratio of the steel sheet is not particularly limited, but is preferably 0.80 or less. This is because it allows mechanical stress relief instead of post-weld heat treatment.

(low temperature toughness)
The toughness value of the steel sheet is not particularly limited, but the Charpy absorbed energy at -50°C (vE _-50°C ) is preferably 100 J or more in a full size Charpy impact test. The vE _-50°C of the steel sheet is more preferably 150J or more.

[Production method]
Next, a method (manufacturing method) for manufacturing the steel sheet of the present invention will be described. In the following description, the temperature refers to the temperature at the center of the plate thickness unless otherwise specified. The temperature at the center of the plate thickness can be obtained, for example, by heat transfer calculation from the surface temperature of the steel plate measured with a radiation thermometer.
That is, in the manufacturing method, a steel material having the chemical composition described above is subjected to hot rolling, followed by first heating and holding, then quenching, then second heating and holding, and then cooling treatment. , is a method of manufacturing a steel plate. In the hot rolling, the finishing temperature is set to 900 ° C. or higher, in the first heating and holding, the heating temperature is set to a temperature range of _AC3 or higher and 1000 ° C. or lower, and in the quenching, from the surface of the steel sheet in the thickness direction. The average cooling rate from 600 ° C. to 300 ° C. at a depth position of 1/4 of the plate thickness is 3 ° C./s or more, and the cooling end temperature is 300 ° C. or less, and in the second heating and holding, the heating temperature is a temperature range of Ac ₁ point or more and Ac ₃ points, and in the cooling treatment, the average cooling rate from 700 ° C. to 500 ° C. at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate is 3 °C/s or more, and the cooling end temperature is 500°C or less and 200°C or more. By satisfying these conditions, the steel sheet of the present invention can be produced favorably.
Each step will be described in detail below.

First, the steel material having the chemical composition described above may be heated prior to hot rolling. In that case, the heating temperature of the steel material is preferably 900° C. or higher and 1250° C. or lower. The method of manufacturing the steel material is not particularly limited. For example, the steel material can be manufactured by melting and casting molten steel having the above-described chemical composition by a conventional method. This smelting can be performed by any method such as a converter, an electric furnace, an induction furnace, or the like. Casting is preferably carried out by a continuous casting method from the viewpoint of productivity, but it can also be carried out by an ingot casting-decomposition rolling method. For example, a steel slab can be used as the steel material.
Here, the heating of the steel material may be performed after the steel material obtained by a method such as casting is once cooled, or the obtained steel material may be directly heated without being cooled.

If the heating temperature of the steel material is less than 900°C, the deformation resistance of the steel material is high, so the load on the rolling mill in the subsequent hot rolling increases, and hot rolling may be difficult. . Therefore, the heating temperature of the steel material is preferably 900° C. or higher. On the other hand, if the heating temperature of the steel material is higher than 1250° C., the oxidation of the steel becomes significant, and as a result of the loss due to the removal of the oxide film due to the oxidation, the loss may decrease, resulting in a decrease in yield. Therefore, it is preferable to set the heating temperature of the steel material to 1250° C. or less.

(hot rolling)
A steel material having the chemical composition described above is subjected to hot rolling to obtain a hot-rolled steel sheet having a final thickness. In hot rolling, the finishing temperature is 900° C. or higher. If the finishing temperature is less than 900°C, the acoustic anisotropy may deteriorate. The final thickness of the hot-rolled steel sheet is not particularly limited, but is preferably 6 mm or more and 50 mm or less as described above. Moreover, after hot rolling, it can be cooled appropriately. The cooling method is not particularly limited, but may be air cooling, water cooling, or the like.

(First heating and holding)
After the hot rolling, the hot-rolled steel sheet is heated and held in an austenite region (first heated and held). Therefore, in the first heating and holding, the heating temperature is set to _AC3 point or higher. On the other hand, if the heating temperature of the hot-rolled steel sheet exceeds 1000°C, the crystal grains become coarse and the toughness decreases.

(Quenching)
Quenching (that is, accelerated cooling) is performed on the hot-rolled steel sheet after the first heating and holding. In this quenching, the average cooling rate from 600 ° C. to 300 ° C. at a depth of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate (hereinafter also referred to as 1/4t) is 3 ° C./s or more. It is essential that

That is, in quenching, if the average cooling rate is less than 3° C./s, it is difficult to obtain a desired transformed structure and sufficient strength. On the other hand, the upper limit of the average cooling rate is not particularly limited, but if it is higher than 200 ° C./s, it becomes difficult to control the temperature at each position in the steel sheet, and the material quality tends to vary in the width direction and rolling direction. . As a result, material properties such as tensile properties and toughness tend to vary. Therefore, the above average cooling rate is preferably 200° C./s or less.

Also, in quenching, if the cooling stop temperature at the 1/4t position is higher than 300° C., the desired transformation structure cannot be obtained. Therefore, in the above quenching, the cooling stop temperature at the position of 1/4t shall be 300° C. or lower. By performing accelerated cooling under such conditions, the hot-rolled steel sheet is quenched well.

The quenching (accelerated cooling) treatment is not particularly limited and can be performed by any method. For example, one or both of air cooling and water cooling can be used. As water cooling, any cooling method using water (for example, spray cooling, mist cooling, laminar cooling, etc.) can be adopted.

(Second heating and holding)
Next, the quenched hot-rolled steel sheet is heated and held at a two-phase temperature (second heated and held). In the second heating and holding, the heating temperature is set to Ac ₁ point or more and Ac ₃ point or less. If the heating temperature is less than Ac ₁ point, island martensite cannot be obtained and a low yield ratio cannot be achieved. At a heating temperature of Ac ₃ or more, the ferrite phase becomes less than 5% and the tempered martensite phase becomes more than 90%, and a low yield ratio cannot be achieved.

Here, any heating method can be used for heating and holding at the two-phase region temperature as long as the heating temperature can be controlled as described above. An example of a heating method includes furnace heating. The furnace heating is not particularly limited, and a general heat treatment furnace can be used.

After reaching the above heating temperature, the two-phase region temperature may be maintained for an arbitrary period of time before the cooling treatment described later may be started. Although the holding time is not particularly limited, it is preferably 5 minutes or longer.

The Ac ₁ point can be obtained by the following formula (1).
A _C1 point (°C) = 750.8 - 26.6 x C + 17.6 x Si - 11.6 x Mn - 22.9 x Cu - 23 x Ni + 24.1 x Cr + 22.5 x Mo - 39.7 x V - 5.7 x Ti + 232.4 x Nb - 169.4 ×Al (1)
Further, Ac ₃ points can be obtained by the following equation (2).
Ac ₃ points (°C) = 937.2 - 436.5 x C + 56 x Si - 19.7 x Mn - 16.3 x Cu - 26.6 x Ni - 4.9 x Cr + 38.1 x Mo + 124.8 x V + 136.3 x Ti - 19.1 x Nb + 198.4 x Al + 3315 ×B (2)

(cooling treatment)
Then, a cooling treatment is performed. In this cooling process, the average cooling rate from 700° C. to 500° C. at the position of 1/4t is 3° C./s or higher, and the cooling end temperature is 500° C. or lower and 200° C. or higher. That is, if the average cooling rate is less than 3° C./s, a desired transformed structure may not be obtained, resulting in a decrease in strength and toughness. Also, if the cooling stop temperature exceeds 500° C., bainite generated after cooling stop becomes the main component, and there is a possibility that the low temperature toughness is not satisfied. Also, if the cooling stop temperature is less than 200° C., the final desired tempering effect cannot be obtained and the toughness deteriorates.

After the cooling is stopped (after the cooling treatment is completed), air cooling can be performed to improve the toughness by self-tempering. In this case, the cooling rate in air cooling is not particularly limited, but is usually 1° C./s or less when the plate thickness is 6 to 50 mm, for example.

A steel plate was produced by the procedure described below, and its properties were evaluated.
First, molten steel having the chemical composition shown in Table 1 was melted in a converter, and a steel slab (thickness: 200 mm) as a steel material was obtained by a continuous casting method. Table 1 (and Table 2) also show the Ac ₁ point (° C.) determined by the above equation (1) and the Ac ₃ point (° C.) determined by the above equation (2).

Next, the obtained steel slabs were heated according to the conditions shown in Table 2 and then hot-rolled to obtain hot-rolled steel sheets having respective thicknesses (final thicknesses). After that, each treatment was performed according to the conditions shown in Table 2 to obtain a steel plate.

The microstructure, tensile strength (TS), yield ratio (YR), low temperature toughness (vE _-50°C ) and acoustic anisotropy (sound velocity ratio) of each of the steel sheets thus obtained were evaluated according to the following methods. . Table 3 shows the evaluation results.

[Microstructure]
A test piece for microstructure observation was taken from each steel plate so that the position of 1/4t was the observation position. This test piece was embedded in resin so that the cross section perpendicular to the rolling direction was the observation surface, and mirror-polished. Then, after performing nital corrosion, the tissue was observed with a scanning electron microscope at a magnification of 5000 times and an image of the tissue was taken. The resulting images were analyzed to identify the microstructure fractions. The identification of each organization is as follows.
・Tempered martensite and/or bainite: a hard phase containing cementite ・Island martensite: a hard phase that does not contain cementite and has an equivalent circle diameter of 1 μm or less ・As-quenched martensite: does not contain cementite and has an equivalent circle diameter that exceeds 1 μm Hard phase/ferrite: Mother phase other than the above

Furthermore, a structure analysis is performed by EBSD, and a region surrounded by large-angle grain boundaries with a misorientation of 15 degrees is defined as a crystal grain for a total area of 1 mm × 1 mm. measured the number density of grains with a diameter greater than 30 μm. Similarly, the average value (average aspect ratio) of the aspect ratios (=length in the rolling direction/length in the plate thickness direction) was determined for crystal grains obtained in an area of 1 mm×1 mm.

[Tensile strength (TS) and yield ratio (YR)]
A JIS No. 4 tensile test piece was taken perpendicularly to the rolling direction from the position of 1/4t of the steel plate. Using this tensile test piece, a tensile test was carried out in accordance with JIS Z2241 to evaluate the tensile strength (TS) of the steel plate. If the tensile strength was 690 MPa or more, the strength was high and it was considered acceptable. Similarly, the yield ratio (YR) was also evaluated from the tensile test results, and if it was 0.80 or less, it was regarded as acceptable.

[Low temperature toughness (vE _-50°C )]
A V-notch test piece was taken from a position of 1/4t of the steel plate parallel to the rolling direction in accordance with JIS Z2202. Using this V-notch test piece, a Charpy impact test was carried out in accordance with JIS Z2242 to determine the Charpy absorbed energy at -50°C (vE _-50°C ). The Charpy absorbed energy can be regarded as an index of the low temperature toughness of the steel sheet. In the Charpy impact test, three test pieces (A, B, C) were taken from each steel plate and measured. Table 3 shows individual measurement results. In this full-size Charpy impact test, if vE _-50°C of each test piece was 100 J or more, it was evaluated as having excellent Charpy toughness and was accepted.

[Acoustic anisotropy (sound velocity ratio)]
In order to evaluate the acoustic anisotropy of the steel sheet, the shear wave speed ratio specified in JIS Z3060 was evaluated. Here, the shear wave sound velocity ratio is the sound speed CSL when the transverse wave vibration direction is the rolling direction (L direction) with respect to the sound speed CSC (m/sec) when the transverse wave vibration direction is the direction perpendicular to the rolling direction (C direction). (m/s) ratio, a value defined as CSL/CSC. Table 3 shows the measurement results. If the CSL/CSC was 1.02 or less, it was evaluated as having little acoustic anisotropy and was judged as acceptable.

As can be seen from the above tables, all of the invention examples provided steel sheets having a predetermined chemical composition and microstructure, high strength, low-temperature toughness, low yield ratio, and good acoustic anisotropy. there is

Claims (3)

in % by mass,
C: 0.02% or more and 0.15% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 0.05% or more and 2.50% or less,
Ni: 0.5% or more and less than 5.0%,
P: 0.03% or less,
S: 0.005% or less and N: 0.0010% or more and 0.0080% or less, with the balance being Fe and unavoidable impurities,
The microstructure at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate has a ferrite fraction of 5 to 95%, an island martensite fraction of 1 to 30%, and the remainder being tempered martensite and / Or when a region surrounded by large-angle grain boundaries with a misorientation of 15 degrees or more is bainite, the average aspect ratio of the crystal grain is less than 2.0, and the equivalent circle diameter of the crystal grain is A steel sheet in which the number density of crystal grains of more than 30 μm is 250/mm ² or less. The component composition is further, in mass %,
Nb: 0.05% or less,
Al: 0.100% or less Cr: 2.00% or less,
Mo: 1.0% or less,
Cu: 2.0% or less,
V: 0.05% or less,
Ti: 0.03% or less,
B: 0.0030% or less,
Ca: 0.007% or less,
The steel sheet according to claim 1, containing at least one selected from REM: 0.010% or less and Mg: 0.007% or less. A method for manufacturing a steel plate according to claim 1 or 2,
A steel material having the chemical composition according to claim 1 or 2 is subjected to hot rolling, followed by first heating and holding, then quenching, then second heating and holding, and then cooling treatment. , in the steel plate manufacturing method,
In the hot rolling, the finishing temperature is 900 ° C. or higher,
In the first heating and holding, the heating temperature is set to a temperature range of Ac ₃ or more and 1000° C. or less,
In the quenching, the average cooling rate from 600 ° C. to 300 ° C. at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate is 3 ° C./s or more, and the cooling end temperature is 300 ° C. or less. ,
In the second heating and holding, the heating temperature is set to a temperature range of Ac ₁ point or more and Ac ₃ points or more,
In the cooling treatment, the average cooling rate from 700 ° C. to 500 ° C. at a depth position of 1/4 of the plate thickness in the plate thickness direction from the surface of the steel plate is 3 ° C./s or more, and the cooling end temperature is 500 ° C. or less. A method for manufacturing a steel sheet, wherein the temperature is 200°C or higher.